https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106772
--- Comment #25 from Thomas Rodgers <rodgertq at gcc dot gnu.org> --- (In reply to Mkkt Bkkt from comment #24) > (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 :( So to your previous comment, I don't the discussion is at all pointless. i plan to raise some of these issues at the next SG1 meeting in November. Sure, that doesn't help *you* or any developer with your specific intent until C++26, and maybe Boost's implementation is a better choice, I also get how unsatisfying of an aswer that is. I'm well aware of the potential scalability problems, and I have a longer term plan to get concrete data on how different implementation choices impact scalability. The barrier implementation (which is the same algorithm as in libc++), for example spreads this traffic over 64 individual atomic_refs, for this very reason, and that implementation has been shown to scale quite well on ORNL's Summit. But not all users of libstdc++ have those sorts of problems either.
