Messages in this thread |  | | Date | Wed, 18 Apr 2007 11:53:34 +0200 | | From | Ingo Molnar <> | | Subject | Re: [Announce] [patch] Modular Scheduler Core and Completely Fair Scheduler [CFS] |
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* Nick Piggin <npiggin@suse.de> wrote:
> > > 535.43user 30.62system 2:23.72elapsed 393%CPU > > > > Thanks for testing this! Could you please try this also with: > > > > echo 100000000 > /proc/sys/kernel/sched_granularity > > 507.68user 31.87system 2:18.05elapsed 390%CPU > 507.99user 31.93system 2:18.09elapsed 390%CPU
> > could you maybe even try a more extreme setting of: > > > > echo 500000000 > /proc/sys/kernel/sched_granularity
> 506.69user 31.96system 2:17.82elapsed 390%CPU > 505.70user 31.84system 2:17.90elapsed 389%CPU
> Again, for comparison 2.6.21-rc7 mainline: > > 508.87user 32.47system 2:17.82elapsed 392%CPU > 509.05user 32.25system 2:17.84elapsed 392%CPU
thanks for testing this!
> So looking at elapsed time, a granularity of 100ms is just behind the > mainline score. However it is using slightly less user time and > slightly more idle time, which indicates that balancing might have got > a bit less aggressive. > > But anyway, it conclusively shows the efficiency impact of such tiny > timeslices.
yeah, the 4% drop in a CPU-cache-sensitive workload like kernbench is not unexpected when going to really frequent preemption. Clearly, the default preemption granularity needs to be tuned up.
I think you said you measured ~3msec average preemption rate per CPU? That would suggest the average cache-trashing cost was 120 usecs per every 3 msec window. Taking that as a ballpark figure, to get the difference back into the noise range we'd have to either use ~5 msec:
echo 5000000 > /proc/sys/kernel/sched_granularity
or 15 msec:
echo 15000000 > /proc/sys/kernel/sched_granularity
(depending on whether it's 5x 3msec or 5x 1msec - i'm still not sure i correctly understood your 3msec value. I'd have to know your kernbench workload's approximate 'steady state' context-switch rate to do a more accurate calculation.)
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