Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex

Thomas Gleixner
2014-07-21T21:41:27Z

On Mon, 21 Jul 2014, Ingo Molnar wrote:
> * Waiman Long <Waiman.Long@hp.com> wrote:
>
> > Testing done on a 4-socket Westmere-EX boxes with 40 cores (HT off)
> > showed the following performance data (average kops/s) with various
> > load factor (number of pause instructions) used in the critical
> > section using an userspace mutex microbenchmark.
> >
> > Threads Load Waiting Futex Spinning Futex %Change
> > ------- ---- ------------- -------------- -------
> > 256 1 6894 8883 +29%
> > 256 10 3656 4912 +34%
> > 256 50 1332 4358 +227%
> > 256 100 792 2753 +248%
> > 10 1 6382 4838 -24%
> > 10 10 3614 4748 +31%
> > 10 50 1319 3900 +196%
> > 10 100 782 2459 +214%
> > 2 1 7905 7194 -9.0%
> > 2 10 4556 4717 +3.5%
> > 2 50 2191 4167 +90%
> > 2 100 1767 2407 +36%
>
> So the numbers look interesting - but it would be _really_ important
> to provide noise/sttdev figures in a sixth column as well (denoted in
> percentage units, not in benchmark units), so that we know how
> significant a particular speedup (or slowdown) is.

We care about that, once we have something which

- Has a proper design

- Covers all the corner cases of futexes

- Does not introduce a gazillions of new lines of codes in futex.c

- Does not create another set of subtle security issues. I'm so not
interested to do the same exercise again - my brain still suffers.

The numbers provided are completely irrelevant as the implementation
is just the most idiotic approach to avoid all corner cases of
futexes, error handling and the proper treatment of detached kernel
state for the price of adding a completely unreviewable clusterfuck to
futex.c.

So, no. Don't encourage that number wankery any further. It's going
nowhere, period.

Thanks,

tglx