Issue #20301 has been updated by shyouhei (Shyouhei Urabe). Yes. `add(o) unless include?(o)` isn't thread safe already. My implementation just doesn't care to improve that. ---------------------------------------- Bug #20301: `Set#add?` does two hash look-ups https://bugs.ruby-lang.org/issues/20301#change-107279 * Author: AMomchilov (Alexander Momchilov) * Status: Open * Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- A common usage of `Set`s is to keep track of seen objects, and do something different whenever an object is seen for the first time, e.g.: ```ruby SEEN_VALUES = Set.new def receive_value(value) if SEEN_VALUES.add?(value) puts "Saw #{value} for the first time." else puts "Already seen #{value}, ignoring." end end receive_value(1) # Saw 1 for the first time. receive_value(2) # Saw 2 for the first time. receive_value(3) # Saw 3 for the first time. receive_value(1) # Already seen 1, ignoring. ``` Readers might reasonably assume that `add?` is only looking up into the set a single time, but it's actually doing two separate look-ups! ([source](https://github.com/ruby/ruby/blob/c976cb5/lib/set.rb#L517-L525)) ```rb class Set def add?(o # 1. `include?(o)` looks up into `@hash` # 2. if the value isn't there, `add(o)` does a second look-up into `@hash` add(o) unless include?(o) end end ``` This gets especially expensive if the values are large hash/arrays/objects, whose `#hash` is expensive to compute. We can optimize this if it was possible to set a value in hash, *and* retrieve the value that was already there, in a single go. I propose adding `Hash#exchange_value` to do exactly that. If that existed, we can re-implement `#add?` as: ```rb class Set def add?(o) # Only requires a single look-up into `@hash`! self unless @hash.exchange_value(o, true) end ``` Here's a proof-of-concept implementation: https://github.com/ruby/ruby/pull/10093 # Theory How much of a benefit this has depends on 2 factors: 1. How much `#hash` is called, which depends on how many **new** objects are added to the set. * If every object is new, then `#hash` used to be called twice on every `#add?`. * This is where this improvement makes the biggest (2x!) change. * If every object has already been seen, then `#hash` was never being called twice before anyway, so there would be no improvement. * It's important to not regress in this case, because many use cases of sets don't deal with many distinct objects, but just need to do quick checks against an existing set. * Every other case lies somewhere in between those two, depending on the % of objects which are new. 2. How slow `#hash` is to compute for the key * If the hash is slow to compute, this change will make a bigger improvement * If the hash value is fast to compute, then it won't matter as much. Even if we called it half as much, it's a minority of the total time, so it won't have much net impact. # Benchmark summary | | All objects are new | All objects are preexisting | |---------------------------|-------:|------:| | objects with slow `#hash` | 100.0% | ~0.0% | | objects with fast `#hash` | 24.5% | 4.6% | As we see, this change makes a huge improvement the cases where it helps, and crucially, doesn't slow down the cases where it can't. For the complete benchmark source code and results, see the PR: https://github.com/ruby/ruby/pull/10093 -- https://bugs.ruby-lang.org/