Poor scheduling when not loaded at 100% (Was: [PATCH] sched.c: Be a bit more conservative in SMP)

[Al Boldi <a1426z@gawab.com>]

Ludovic Drolez wrote:
> Ludovic Drolez <ldrolez <at> linbox.com> writes:
> > In fact, I tested the 1st patch on our cluster (Finite elements
> > computing on 8 CPUs):
> > - Under Windows: 875 seconds
> > - Linux 2.6.16 : 1019 s
> > - Linux 2.6.16 + manual taskset : 842 s
> > - Linux 2.6.16 + Vincent's patch : 1373 s
>
> Anyone has an idea why the scheduling is poor when processes don't use all
> CPU ?
>
> In the above example, we have 4 processes on 4 processors which use about
> 40% of the CPU (computing and waiting for network packets).
> 1- If taskset is not used : CPU0 is used at 80%, and the 3 others at 30%.
> The tasks are constantly migrated between cores -> poor performance
> (1019s), Windows does better :-(
> 2- If taskset is used : All CPUs have 1 process and are used at 40%. No
> migration -> high performance (842s), better than Windows :-)
>
> I tried to play with the 'migration_cost' kernel parameter but it did not
> help. By default, on the Bi-Xeon Dual Core MB (Dell 1855),
> migration_cost=1600, and trying values up to 200000, did not improve
> performance...
>
> Any Ideas ?

Did you try PlugSched?  The spa scheds work wonders, once tuned properly.


Thanks!


--
Al