Re: [PATCH 7/7] ondemand: Solve the big performance issue with ondemand during disk IO

[Mike Chan]

On Tue, Apr 20, 2010 at 2:24 AM, Thomas Renninger <trenn@xxxxxxx> wrote:
> On Monday 19 April 2010 15:43:25 Arjan van de Ven wrote:
>> On Mon, 19 Apr 2010 10:29:47 +0200
>> Éric Piel <eric.piel@xxxxxxxxxxxxxxxx> wrote:
>> > >
>> > > The problem and fix are both verified with the "perf timechar" tool.
>> > Hi,
>> > I don't doubt that keeping the cpu full frequency during IO can
>> > improve some specific workloads, however in your log message you
>> > don't explain how much we are loosing.
>>
>> first of all, it's so bad that people will just turn the whole power
>> management off... at which point fixing the really bad bug is actually
>> quite a win
> Not sure you fix a bug, I expect this was done on purpose.
> The ondemand governor disadvantages processes with alternating short CPU
> load peaks and idle sequences.
> IO bound processes typically show up with such a behavior.
>
> But I follow Eric and agree that if it costs that much, changing
> above sounds sane.
> Still, I could imagine some people might want to not raise freq on IO bound
> process activity, therefore this should get another ondemand param, similar
> to ignore_nice_load.
>

I agree with Thomas here. Some of these assumptions on IO / FSB
performance with cpu speed do not hold true on various ARM platforms.

Perhaps we could have a min_io_freq value? Which is the min speed for
the cpu to run at for IO bound activity. In the original patch,
min_io_freq = scaling_max_freq. For various arm devices I can happily
set min_io_freq to the lowest cpu speed that satisfies bus speeds.

-- Mike

>    Thomas
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