Issue #21950 has been updated by headius (Charles Nutter).

JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability

The other issue with real CPU timing is the heavy responsibility for accurately representing individual methods' CPU times without influencing those results and returning results in a reasonable amount of time. That's a big reason why sampling profilers are used more often for JVM applications. Any CPU timing in Ruby must also consider JIT improvements over time, and be able to decode things like inlined method calls so that the reported execution time is associated with the correct body of code. Many of the CPU profiling tools for JVM either disable aggressive optimizations (giving a poor view of optimized code CPU time) or indirectly break those optimizations (by triggering more safepoints and more stack traces that change influence inlining heuristics. I'd also caution against making this easily enabled by plain old Ruby code, because: 1. People are definitely going to accidentally commit that stuff. 2. JVM-based Ruby implementations generally won't support this, since for security and integrity reasons the low-level JVM CPU profilers usually have to be specified at boot time (or via service-loading mechanisms that are sometimes restricted). Could the old dtrace hooks help this at all? That's an approach that would avoid users stumbling into deoptimization accidentally while also letting the kernel do most of the heavy lifting. ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116662 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/

Issue #21950 has been updated by osyoyu (Daisuke Aritomo).

Any CPU timing in Ruby must also consider JIT improvements over time, and be able to decode things like inlined method calls so that the reported execution time is associated with the correct body of code.

Yes, ZJIT probabaly would invalidate assumptions supporting the design of `rb_profile_frames()`. At the moment I'm unsure if some degree of deoptimization has to happen for profiling JITed sites, but even if so, I believe that kind of logic should live inside the runtime, not in external libraries.

People are definitely going to accidentally commit that stuff.

Are you mentioning APIs like `RubyVM::Profile.enable` ? This kind of APIs is what existing Ruby profilers have provided and people are used to, so I don't think there is a real risk here. Still though, I am OK with providing a option like `--profile`. ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116667 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/

Issue #21950 has been updated by osyoyu (Daisuke Aritomo). Thank you for your comments and input! Eregon (Benoit Daloze) wrote in #note-4:

The API should not be under RubyVM, that's CRuby-specific

`RubyVM` being CRuby-specific is actually the exact reason I choosed the place to locate a `Profile` feature. I am rather doubtful that CRuby/JRuby/TruffleRuby can share any profiler implementation at all. They are too different runtimes, and they can emit very different information. For example, a useful CRuby profiler would want to emit GVL contention status, which is a CRuby-specific concept. Given this, my idea was that each Ruby implementation have its own profiling API (especially output). Perhaps they can share a high-level `Ruby::Profiler.start/stop` API, but I'm not sure how useful that would be. headius (Charles Nutter) wrote in #note-1:

Could the old dtrace hooks help this at all? That's an approach that would avoid users stumbling into deoptimization accidentally while also letting the kernel do most of the heavy lifting.

For notifying when to collect samples, perhaps yes. For avoiding deoptimization, I think that is more of how sample collection is implemented within the VM, not the notification path. (I'm not sure if I have understood your question correctly) ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116677 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/

Issue #21950 has been updated by Eregon (Benoit Daloze). osyoyu (Daisuke Aritomo) wrote in #note-5:

I am rather doubtful that CRuby/JRuby/TruffleRuby can share any profiler implementation at all.

They can't share the implementation of gathering backtraces (that's implementation-specific), sure, but the API should be shared, so whatever gems/Visualization on top can be shared too. ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116679 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/

Issue #21950 has been updated by headius (Charles Nutter).

conceptually it's like collecting data equivalent to caller_locations efficiently every X milliseconds.

In fact Truffle CPUSampler

Unless I've read this issue completely wrong, it is not about adding a sampling profiler, it is about adding a CPU time profiler that does not sample. A CPU time profiler is expected to show actual CPU time spent per method. That means accurately tracking execution time of every invocation, not periodically sampling to see what methods are on the stack. JRuby does ship such a profiler but it is of limited use due to the high overhead of tracking ever method call and return. Of course, if this is not actually a true CPU time profiler and simply a built-in sampling profiler, that's a much easier (albeit less accurate) feature to implement. ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116681 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/

Issue #21950 has been updated by byroot (Jean Boussier).

it is not about adding a sampling profiler, it is about adding a CPU time profiler that does not sample.

Well, the issue description talk of CPU-time, but it also talk about sampling (`timer_create(3)` etc). So yes, it's a CPU-time *sampling* profiler, not a tracing one. Which is an interesting choice, because maybe it's my bias, but I find that CPU-time profiling is really not as commonly useful as a WALL-time one. For purely computational workloads it doesn't matter much, but as soon as you have a bit of IO it's really not that useful. I almost exclusively use WALL profiling with vernier/stackprof etc. But I suspect supporting both isn't that much extra work? ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116684 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/

Issue #21950 has been updated by Eregon (Benoit Daloze). byroot (Jean Boussier) wrote in #note-11:

But I suspect supporting both isn't that much extra work?

I think it'd be good for this API to be able to say which one it wants (like `StackProf`'s `mode:` kwarg). ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116686 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/

Issue #21950 has been updated by byroot (Jean Boussier).

Reducing CPU load can be useful even in tasks containing IO.

Oh absolutely, this is a huge part of my day to day work. But even for this I tend to prefer looking at a WALL report as it include both information. I see no need to ignore IO to find CPU hotspot, and having both offer perspective.

CPU time profiling can be easily expanded to wall-time profiling (just use CLOCK_MONOTONIC kernel timer).

Right, that's what I suspected, as long as WALL is readily available then I'm sold. ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116688 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/

Issue #21950 has been updated by cfis (Charlie Savage). There are quite a few existing tracing and sampling ruby profilers - see the list I compiled here: https://ruby-prof.github.io/alternatives/ How would this new proposed profiler improve upon what already exists? And why not just improve what already exists instead of starting from scratch? ---------------------------------------- Feature #21950: Add a built-in CPU-time profiler https://bugs.ruby-lang.org/issues/21950#change-116780 * Author: osyoyu (Daisuke Aritomo) * Status: Open ---------------------------------------- Modern CRuby workloads can consume CPU concurrently across multiple native threads, especially with multiple Ractors and C exts which release the GVL. I'd like to propose the idea of integrating a built-in CPU-time profiler CRuby to enable more accurate and stable profiling in such situations. ## Motivation & Background CPU profilers indicate how much CPU time were consumed by different methods. Most CPU-time profilers rely on the kernel to track consumed CPU time. `setitimer(3)` and `timer_create(3)` are APIs to configure kernel timers. The process receives a profiling signal (SIGPROF) every given time of CPU time consumed (e.g. 10 ms). In general, a profiler needs to know which _thread_ consumed how much CPU time to attribute work to the correct thread. This wasn't a real requirement for Ruby in the pre-Ractor age, since only one native thread could consume CPU time at any given moment due to GVL limitations. Using process-wide CPU-time timers provided by `setitimer()` effectively did the job. It was safe to assume that the active thread was doing some work using the CPU when a profiling signal arrived. Of course, this assumption does not stand in all situations. One is the case where C extensions release the GVL. Another is the multi-Ractor situation. In these cases, multiple native threads may simultaneously consume CPU time. Linux 2.6.12+ provides a per-thread timer to address this task. Profilers such as [Pf2](https://github.com/osyoyu/pf2) and [dd-trace-rb](https://github.com/DataDog/dd-trace-rb) use this feature to keep track of CPU time. However, utilizing this API requires information that CRuby does not necessarily expose. Both carry copies of CRuby headers in order to access internal structs such as `rb_thread_t`. This is quite fragile and possibly unsustainable in the age where CRuby evolves towards Ractors and M:N threading. ## Proposal Implement a built-in CPU-time profiler as `ext/profile`, and add some information extraction points to the VM built exclusively for it. ```ruby require 'profile' RubyVM::Profile.start # your code here RubyVM::Profile.stop #=> results ``` `ext/profile` will take care of the following in coordination with some VM helpers: - Tracking creation and destruction of native thread (NT) s - Management of kernel timers for those threads - i.e. calling `pthread_getcpuclockid(3)` and `timer_create(3)` - This will require iterating over all Ractors and shared NTs on profiler init - Handling of SIGPROF signals which those timers will generate - Walking the stack (calling `rb_profile_frames()`) - I'm not going to make this part of this ticket, but I'm thinking we can make `rb_profile_frames()` even more granular by following VM instructions, which is probably something we don't want to expose as an API We would need to add some APIs to the VM for `ext/profile`: - An API returning all alive NTs - Event hooks notifying creation and destruction of NTs - Event hooks notifying assign/unassign of RTs to NTs Since only Linux provides the full set of required kernel features, the initial implementation will be limited to Linux systems. I can expand support to other POSIX systems by employing `setitimer()` later, but they will receive limited granularity (process-level CPU-time timers). ### Output interface One thing to consider is the output format. I think we have a few practical choices here: - Adopt pprof's `profile.proto` format. - This is a widely accepted format across tools including visualizers. - The format itself is pretty flexible, so it shouldn't be hard to add custom fields. - Just return a Hash containing profile results. - We'd need to design some good format. ### Things out of scope - Visualization - Interaction with external visualizers / trackers / etc These can be better left to normal RubyGems. ### Why not an external gem? Through maintaining [Pf2](https://github.com/osyoyu/pf2), an CPU-time profiler library, I have encountered many difficulties obtaining information required for accurate profiling. The rule of thumb is that more internal information a profiler can access, the more accuracy it can achieve. However, from the CRuby maintenance perspective, I suppose not too many APIs exposing implementation details are wanted. Locating a profiler under `ext/` looks like nice middle ground. Placing core 'profiling' logic (sampler scheduling, sampling itself) in CRuby and abstracting it as 'RubyVM::Profile' should work cleanly. It should be noted that existing profilers have their own unique features, such as markers, unwinding of C stacks and integration with external APMs. I don't want to make this a tradeoff between accuracy and feature; instead, I'd like to design an API where both could live. ### Study on other languages A handful of VM-based languages carry profiler implementation in their runtime. - Go: runtime/pprof https://github.com/golang/go/tree/master/src/runtime/pprof - OpenJDK: JFR https://github.com/openjdk/jdk/tree/master/src/hotspot/share/jfr - Node.js: --cpu-prof https://nodejs.org/en/learn/getting-started/profiling - And more Among these, OpenJDK is a notable entry. JVM profilers have configured used `AsyncGetCallTrace()`, which is just like `rb_profile_frames()`, to obtain stack traces from signal handlers. The signal originates from kernel timers installed by the profilers, configured to fire every specified interval of CPU time (e.g. 10 ms). Even though `AsyncGetCallTrace()` and async-profiler (its primary user) are very sophisticated and battle-tested, JFR folks have decided to control sampling timing within the runtime to improve accuracy and stability. For more information on JVM, see: - [JEP 509](https://openjdk.org/jeps/509) - [JEP 518](https://openjdk.org/jeps/518) - [Taming the Bias: Unbiased* Safepoint-Based Stack Walking in JFR](https://mostlynerdless.de/blog/2025/05/20/taming-the-bias-unbiased-safepoint...) -- https://bugs.ruby-lang.org/