Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex_wake()

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Nov 26, 2013 at 12:12:31AM -0800, Davidlohr Bueso wrote:
> 
> > I am becoming hesitant about this approach. The following are some
> > results, from my quad-core laptop, measuring the latency of nthread
> > wakeups (1 at a time). In addition, failed wait calls never occur -- so
> > we don't end up including the (otherwise minimal) overhead of the list
> > queue+dequeue, only measuring the smp_mb() usage when !empty list never
> > occurs.
> > 
> > +---------+--------------------+--------+-------------------+--------+----------+
> > | threads | baseline time (ms) | stddev | patched time (ms) | stddev | overhead |
> > +---------+--------------------+--------+-------------------+--------+----------+
> > |     512 | 4.2410             | 0.9762 | 12.3660           | 5.1020 | +191.58% |
> > |     256 | 2.7750             | 0.3997 | 7.0220            | 2.9436 | +153.04% |
> > |     128 | 1.4910             | 0.4188 | 3.7430            | 0.8223 | +151.03% |
> > |      64 | 0.8970             | 0.3455 | 2.5570            | 0.3710 | +185.06% |
> > |      32 | 0.3620             | 0.2242 | 1.1300            | 0.4716 | +212.15% |
> > +---------+--------------------+--------+-------------------+--------+----------+
> > 
> 
> Whee, this is far more overhead than I would have expected... pretty
> impressive really for a simple mfence ;-)

I'm somewhat reluctant to chalk it up to a single mfence - maybe 
timings/behavior changed in some substantial way?

Thanks,

	Ingo