Issue #20146 has been updated by shia (Sangyong Sim). It seems any shared thread created when max cpu = 1 without enabled_mn_threads. I suspect native_thread_check_and_create_shared, which always reject create shared thread. Ref: https://github.com/ruby/ruby/blob/e4a9a73931072458f2bc13b29aeb33efb2eb9ae9/t... As a result, scheduled Ractors won't be executed, and also won't be clean up as Ractor started. I observed this bug fixed with this patch: ```diff thread_pthread_mn.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/thread_pthread_mn.c b/thread_pthread_mn.c index c8c7d9f173..572d5e49a1 100644 --- a/thread_pthread_mn.c +++ b/thread_pthread_mn.c @@ -379,7 +379,8 @@ native_thread_check_and_create_shared(rb_vm_t *vm) rb_native_mutex_lock(&vm->ractor.sched.lock); { unsigned int snt_cnt = vm->ractor.sched.snt_cnt; - if (!vm->ractor.main_ractor->threads.sched.enable_mn_threads) snt_cnt++; // do not need snt for main ractor + // create at least one shared thread with max_cpu 1 case. + if (!vm->ractor.main_ractor->threads.sched.enable_mn_threads && vm->ractor.sched.max_cpu > 1) snt_cnt++; if (((int)snt_cnt < MINIMUM_SNT) || (snt_cnt < vm->ractor.cnt && -- 2.25.1 ``` (I don't think this patch is acceptable, which actually use one more cpu for shared thread, so just explanation purpose) Furthermore, I'm not sure expected behavior in this case. For instance, I think here are some choices to create ractor with max cpu=1: - always use dedicated thread as same as Thread class - respect enable_mn_threads flag(but I think this flag might be for Thread class, not for Ractor) - always use shared thread - Let max_cpu=1 be exceptional case(like above patch) - Let max cpu num = shared thread num to make things simpler - prohibit max_cpu=1 mode ;) ---------------------------------------- Bug #20146: Code using Ractor with env `RUBY_MAX_CPU=1` ends with unreachable https://bugs.ruby-lang.org/issues/20146#change-106057 * Author: shia (Sangyong Sim) * Status: Assigned * Priority: Normal * Assignee: ko1 (Koichi Sasada) * ruby -v: ruby 3.4.0dev * Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN ---------------------------------------- ## Reproducible code ```rb # sample-code.rb Ractor.new { 1 } ``` ```bash RUBY_MAX_CPU=1 ruby sample-code.rb # This will not end with exit code 0 RUBY_MAX_CPU=2 ruby sample-code.rb # This ends with exit code 0 as expected ``` ## Expected process with RUBY_MAX_CPU=1 exits successfully as same as RUBY_MAX_CPU more than 1. -- https://bugs.ruby-lang.org/