https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106772
--- Comment #24 from Mkkt Bkkt <valera.mironow at gmail dot com> --- (In reply to Thomas Rodgers from comment #22) > Your example of '100+ core' systems especially on NUMA is certainly a valid > one. I would ask, at what point do those collisions and the resulting cache > invalidation traffic swamp the cost of just making the syscall? I do plan to > put these tests together, because there is another algorithm that I am > exploring, that I believe will reduce the likelihood of spurious wakeups, > and achieves the same result as this particular approach, without generating > the same invalidation traffic. At this point, I don't anticipate doing that > work until after GCC13 stage1 closes. I try to explain: syscall overhead is some constant commonly like 10-30ns (futex syscall can be more expensive like 100ns in your example) But numbers of cores are grow, arm also makes more popular (fetch_add/sub have more cost on it compares to x86) And people already faced with situation where fetch_add have a bigger cost than syscall overhead: https://pkolaczk.github.io/server-slower-than-a-laptop/ https://travisdowns.github.io/blog/2020/07/06/concurrency-costs.html I don't think we will faced with problem like in these links in atomic::wait/notify in real code, but I'm pretty sure in some cases it can be more expansive than syscall part of atomic::wait/notify Of course better to prove it, maybe someday I will do it :(
