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

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

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