Re: Performance issue since 3.2.6

From: Srivatsa S. Bhat
Date: Sat May 18 2013 - 01:08:13 EST

On 05/18/2013 05:21 AM, Rafael J. Wysocki wrote:
> On Saturday, May 18, 2013 01:20:10 AM Srivatsa S. Bhat wrote:
>> On 05/17/2013 11:47 PM, Olivier Doucet wrote:
>>> Hello,
>>> This performance penalty is still present in kernel 3.9.2. And
>>> CONFIG_PM cannot be deactivated anymore.
>>> I was able to make a working 3.9.2 (meaning with no penalty) with
>>> following config and patch :
>>> Patch :
>> So, to summarize my thoughts:
>> - IMHO there is no regression here, you just depended on a bug included
>> in 3.2.0 (which made it behave like idle=poll with CONFIG_PM=n) and
>> started your comparisons from there. The later kernels (3.2.6+) got
>> that bug fixed which is why you saw "performance drops".
>> - As much as we would like to do it, we can't set the value of
>> CONFIG_PM doesn't encompass all power-management features (which is
>> a pity). Doing that would need a big overhaul of all the relevant
>> Kconfigs, which might or might not be worth the effort. (Because, who
>> says that CONFIG_PM=n kernels are supposed to eat power like crazy??)
> I think it *is* worth the effort. We could drop some CONFIG_PM* options in the
> process which would simplify things quite a bit too.

Ah, ok..

>> So here is my suggestion - use the interfaces provided by the kernel to
>> fix your problem:
>> - you can give idle=poll in the kernel command line,
>> - OR you can echo 0 > /dev/cpu_dma_latency
>> Irrespective of your kernel configuration options (CONFIG_PM=y/n), the
>> CPUs will not enter deep idle states, giving you the performance
>> improvement that you are looking for.
> Thanks a lot for the very clear explanation of this!

No problem! :-)

Srivatsa S. Bhat

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