Re: Scheduler on C2D CPU and latest 2.6.27 kernel

[Zdenek Kabelac]

2008/10/21 Chris Snook <csnook@xxxxxxxxxx>:
> Zdenek Kabelac wrote:
>>
>> Hi
>>
>> Recently I'm noticing bad behavior of the CPU scheduler on my T61 (2GB,
>> C2D)
>>
>> It looks like Linux concentrates all running tasks on one CPU and the
>> second cpu is sleeping.
>>
>> With recent changes to DRI - glxgears went up to 840FPS but also takes
>> 100% (with Xorg) and when I run 'while :; do true; done' loop in
>> parallel frame rate drops to 300FPS.
>>
>> But as I have C2D CPU I would expect that there should be no such
>> dramatic slowdown.
>>
>> Xosview shows that only one CPU is fully loaded.
>>
>> Here are my .config scheduler options:
>>
>> CONFIG_HAVE_UNSTABLE_SCHED_CLOCK=y
>> # CONFIG_GROUP_SCHED is not set
>> CONFIG_IOSCHED_NOOP=y
>> CONFIG_IOSCHED_AS=y
>> CONFIG_IOSCHED_DEADLINE=y
>> CONFIG_IOSCHED_CFQ=y
>> CONFIG_DEFAULT_IOSCHED="cfq"
>> CONFIG_SCHED_SMT=y
>> # CONFIG_SCHED_MC is not set

I've also tested   SMT=n  MC=y - with same result

>> CONFIG_SCHED_HRTICK=y
>> # CONFIG_NET_SCHED is not set
>> CONFIG_USB_EHCI_TT_NEWSCHED=y
>> CONFIG_SCHED_DEBUG=y
>> CONFIG_SCHEDSTATS=y
>> CONFIG_SCHED_TRACER=y
>>
>>
>> Am I missing something?
>
> You're running a loop that does nothing except create new tasks that have no
> scheduling history, and then disappear before the scheduler can migrate
> them.
>
> Try running 'openssl speed' to chew up CPU.  I promise you the scheduler
> will behave very nicely.

Well -  sorry you really shouldn't promise things you cannot guarantee.

It's obviously showing exactly same problem on my box.

And I should add that with  2.6.27-rc8 I do not experience this behavior.
Gears are showning 80% speed - but consumes only 25%
and bash loop or openssl speed task do not influence its speed.
(as expected with 2CPU machine)

Zdenek.
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