Am Donnerstag, den 30.07.2015, 10:07 +0200 schrieb Jens Gustedt:
> Am Mittwoch, den 29.07.2015, 20:10 -0400 schrieb Rich Felker:
> > On Thu, Jul 30, 2015 at 01:49:20AM +0200, Jens Gustedt wrote:
> > > Hm, could you be more specific about where this hurts?
> > > 
> > > In the code I have there is
> > > 
> > >         for (;val & lockbit;) {
> > >           __syscall(SYS_futex, loc, FUTEX_WAIT, val, 0);
> > >           val = atomic_load_explicit(loc, memory_order_consume);
> > >         }
> > > 
> > > so this should be robust against spurious wakeups, no?
> > 
> > The problem is that futex_wait returns immediately with EAGAIN if
> > *loc!=val, which happens very often if *loc is incremented or
> > otherwise changed on each arriving waiter.
> 
> Yes, sure, it may change. Whether or not this is often may depend, I
> don't think we can easily make a quantitative statement, here.
> 
> In the case of atomics the critical section is extremely short, and
> the count, if it varies so much, should have a bit stabilized during
> the spinlock phase before coming to the futex part. That futex part is
> really only a last resort for the rare case that the thread that is
> holding the lock has been descheduled in the middle.
> 
> My current test case is having X threads hammer on one single
> location, X being up to some hundred. On my 2x2 hyperthreaded CPU for
> a reasonable number of threads (X = 16 or 32) I have an overall
> performance improvement of 30%, say, when using my version of the lock
> instead of the original musl one. The point of inversion where the
> original musl lock is better is at about 200 threads.
> 
> I'll see how I can get hold on occurrence statistics of the different
> phases without being too intrusive (which would change the
> scheduling).

So I tested briefly varying the number of threads from 2 up to 2048.

Out of the loop iterations on the slow path, less than 0.1 % try to go
into futex wait, and out of these about 20 % come back with EGAIN.

In particular the figure of only 20-30 % of the futex calls failing
with EAGAIN, is quite stable.

For me these figures show that the futex phase is really neglectable
for performance and only serves as a last resort that protects us from
an attack.

Jens


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