Re: [PATCH 1/1] mutex: prevent optimistic spinning from spinning longer than neccessary (Repost)

[Ingo Molnar <mingo@elte.hu>]

* Tim Chen <tim.c.chen@linux.intel.com> wrote:

> I didn't get any feedback on this post sent a while back.  So I'm
> reposting it to see if I can get some comments back this time.
> 
> There is a scalability issue for current implementation of optimistic
> mutex spin in the kernel.  It is found on a 8 node 64 core Nehalem-EX
> system (HT mode).
> 
> The intention of the optimistic mutex spin is to busy wait and spin on a 
> mutex if the owner of the mutex is running, in the hope that the mutex 
> will be released soon and be acquired, without the thread
> trying to acquire mutex going to sleep.  However, 
> when we have a large number of threads, contending for the mutex, we could 
> have the mutex grabbed by other thread, and then another ……, and we will keep 
> spinning, wasting cpu cycles and adding to the contention.  One
> possible fix is to quit spinning and put the current thread on wait-list
> if mutex lock switch to a new owner while we spin, indicating heavy
> contention (see the patch included).   
> 
> I did some testing on a 8 socket Nehalem-EX system with a total of 64
> cores. Using Ingo's test-mutex program that creates/delete files with
> 256 threads (http://lkml.org/lkml/2006/1/8/50) , I see the following
> speed up after putting in the mutex spin fix:
> 
> ./mutex-test V 256 10
>                 Ops/sec
> 2.6.34          62864
> With fix        197200
> 
> Repeating the test with Aim7 fserver workload, again there is a speed up
> with the fix:
> 
>                 Jobs/min
> 2.6.34          91657
> With fix        149325
> 
> To look at the impact on the distribution of mutex acquisition time, I
> collected the mutex acquisition time on Aim7 fserver workload with some
> instrumentation.  The average acquisition time is reduced by 48% and
> number of contentions reduced by 32%.
> 
>                 #contentions    Time to acquire mutex (cycles)
> 2.6.34          72973           44765791
> With fix        49210           23067129 
> 
> The histogram of mutex acquisition time is listed below.  The
> acquisition time is in 2^bin cycles.  We see that without the fix, the
> acquisition time is mostly around 2^26 cycles.  With the fix, we the
> distribution get spread out a lot more towards the lower cycles,
> starting from 2^13.  However, there is an increase of the tail
> distribution with the fix at 2^28 and 2^29 cycles.  It seems a
> small price to pay for the reduced average acquisition time and also
> getting the cpu to do useful work.
> 
> Mutex acquisition time distribution (acq time = 2^bin cycles):
>         2.6.34                  With Fix
> bin     #occurrence     %       #occurrence     %
> 11      2               0.00%   120             0.24%
> 12      10              0.01%   790             1.61%
> 13      14              0.02%   2058            4.18%
> 14      86              0.12%   3378            6.86%
> 15      393             0.54%   4831            9.82%
> 16      710             0.97%   4893            9.94%
> 17      815             1.12%   4667            9.48%
> 18      790             1.08%   5147            10.46%
> 19      580             0.80%   6250            12.70%
> 20      429             0.59%   6870            13.96%
> 21      311             0.43%   1809            3.68%
> 22      255             0.35%   2305            4.68%
> 23      317             0.44%   916             1.86%
> 24      610             0.84%   233             0.47%
> 25      3128            4.29%   95              0.19%
> 26      63902           87.69%  122             0.25%
> 27      619             0.85%   286             0.58%
> 28      0               0.00%   3536            7.19%
> 29      0               0.00%   903             1.83%
> 30      0               0.00%   0               0.00%
> 
> Regards,
> Tim
> 
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> diff -ur linux-2.6.34/kernel/sched.c linux-2.6.34-fix/kernel/sched.c
> --- linux-2.6.34/kernel/sched.c 2010-05-16 14:17:36.000000000 -0700
> +++ linux-2.6.34-fix/kernel/sched.c     2010-06-04 10:28:33.564777030 -0700
> @@ -3815,8 +3815,11 @@
>                 /*
>                  * Owner changed, break to re-assess state.
>                  */
> -               if (lock->owner != owner)
> +               if (lock->owner != owner) {
> +                       if (lock->owner)
> +                               return 0;
>                         break;
> +               }
>  
>                 /*
>                  * Is that owner really running on that cpu?

These are some rather impressive speedups!

Have you tried to see what performance effects this change has on smaller 
boxes? Just to see what flip side (if any) this change has.

Thanks,

	Ingo