Adjust snt < max_cpu calculation #14280
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[Bug #20146] Previously we dealt with the main Ractor not being enabled for M:N by incrementing snt_cnt++. This worked for comparing against ractor count, but meant that we always had one less SNT than was specified by RUBY_MAX_CPU. This was notably a problem for RUBY_MAX_CPU=1, which would cause Ractors to hang. This commit instead of adjusting snt, adjusts a "schedulable_ractor_cnt". This way snt_cnt will actually reach RUBY_MAX_CPU.
luke-gruber
reviewed
Aug 20, 2025
| @@ -397,11 +397,15 @@ native_thread_check_and_create_shared(rb_vm_t *vm) | |||
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| 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 | |||
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Is it possible to not increase snt_cnt here and have it work?
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That and no other changes? It would work, but we'd spawn one more SNT than we need until we hit vm->ractor.sched.max_cpu (because we'd be spawning one "for" the main Ractor).
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[Bug #20146]
Previously we dealt with the main Ractor not being enabled for M:N by incrementing
snt_cnt++. This worked for comparing against ractor count, but meant that we always had one less SNT than was specified byRUBY_MAX_CPU.This was notably a problem for
RUBY_MAX_CPU=1, which would cause Ractors to hang.This commit instead of adjusting snt, adjusts a "
schedulable_ractor_cnt". This waysnt_cntwill actually reachRUBY_MAX_CPU.