Re: Default cache_hot_time value back to 10ms

[Ingo Molnar]

* Chen, Kenneth W <kenneth.w.chen@xxxxxxxxx> wrote:

> Here is a patch that revert default cache_hot_time value back to the
> equivalence of pre-domain scheduler, which determins task's cache
> affinity via architecture defined variable cache_decay_ticks.

i could agree with this oneliner patch for 2.6.9, it only affects the
SMP balancer and there for the most common boxes it likely results in a
similar migration cutoff as the 2.5 msec we currently have. Here are the
changes that occur on a couple of x86 boxes:

 2-way celeron, 128K cache:         2.5 msec -> 1.0 msec 
 2-way/4-way P4 Xeon 1MB cache:     2.5 msec -> 2.0 msec
 8-way P3 Xeon 2MB cache:           2.5 msec -> 6.0 msec

each of these changes is sane and not too drastic.

(on ia64 there is no auto-tuning of cache_decay_ticks, there you've got
a decay=<x> boot parameter anyway, to fix things up.)

there was one particular DB test that was quite sensitive to idle time
introduced by too large migration cutoff: dbt2-pgsql. Could you try that
one too and compare -rc3 performance to -rc3+migration-patches?

> This is a mere request that we go back to what *was* before, *NOT* as
> a new scheduler tweak (Whatever tweak done for domain scheduler broke
> traditional/ industry recognized workload).
> 
> As a side note, I'd like to get involved on future scheduler tuning
> experiments, we have fair amount of benchmark environments where we
> can validate things across various kind of workload, i.e., db, java,
> cpu, etc.  Thanks.

yeah, it would be nice to test the following 3 kernels:

 2.6.9-rc3 vanilla,
 2.6.9-rc3 + cache_hot_fix + use-cache_decay_ticks
 2.6.9-rc3 + cache_hot_fixes + autotune-patch

using as many different CPU types (and # of CPUs) as possible.

The most important factor in these measurements is statistical stability
of the result - if noise is too high then it's hard to judge. (the
numbers you posted in previous mails are quite stable, but not all
benchmarks are like that.)

> Signed-off-by: Ken Chen <kenneth.w.chen@xxxxxxxxx>

Signed-off-by: Ingo Molnar <mingo@xxxxxxx>

	Ingo
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