[libcxx-commits] [PATCH] D148681: [libc++] Make std::mutex use ADAPTIVE_NP intializer if available on gnu

[Noah Goldstein via Phabricator via libcxx-commits]

goldstein.w.n added a comment.

In D148681#4279310 <https://reviews.llvm.org/D148681#4279310>, @philnik wrote:

> Looking at https://stackoverflow.com/questions/19863734/what-is-pthread-mutex-adaptive-np I don't think we want to do this change. If someone needs such a special mutex, I guess they have to implement it themselves. We have to provide a useful mutex for all cases, not just for low contention cases that only have short times during which the mutex is locked.

I don't think the SO link really does justice to the reality of the implementation.
At the very least since the SO post was been written the following have been
added.

1. expo backoff to avoid memory traffic
2. Read guards on the CAS (same as above as exclusive state in mind)

It still hits the `futex`, just not immediately as doing so has a serious
latency cost.

Spinning ~100 times with a dozen or so memory accesses (with likely 
all of them read-only) has a minimal cost in the high contention case 
(its +maybe 100-200 cycles but the futex alone is 10-100x that) and 
helps ALOT in the lower contention case (avoids really expensive syscalls).


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https://reviews.llvm.org/D148681