Re: [PATCH v2] cpuidle: Fix the menu governor to boost IOperformance

[Arjan van de Ven]

On Wed, 4 Nov 2009 10:39:13 +0100
Corrado Zoccolo <czoccolo@xxxxxxxxx> wrote:

> Hi Arjan,
> On Tue, Sep 15, 2009 at 4:42 AM, Arjan van de Ven
> <arjan@xxxxxxxxxxxxx> wrote:
> > From: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> > Subject: cpuidle: Fix the menu governor to boost IO performance
> >
> > Fix the menu idle governor which balances power savings, energy
> > efficiency and performance impact.
> 
> I've tested this patch on an Atom based netbook with SSD, and I see
> 10% improvement in latencies for reading a single 4k block from disk.

great!

> 
> During this test, while looking at powertop, I found that my CPU was
> sitting in polling mode for milliseconds (percentage was however
> negligible).
> I never recalled seeing a non-zero time spent polling, so I looked at
> the patch and found:
> > + Â Â Â /*
> > + Â Â Â Â* We want to default to C1 (hlt), not to busy polling
> > + Â Â Â Â* unless the timer is happening really really soon.
> > + Â Â Â Â*/
> > + Â Â Â if (data->expected_us > 5)
> > + Â Â Â Â Â Â Â data->last_state_idx = CPUIDLE_DRIVER_STATE_START;
> Commenting the if, (the previous behaviour), I no longer see the
> polling, while I still get the performance improvement.
> 
> I wonder if that '5' is a bit too much. According to my BIOS ACPI
> table, the Atom latency for C1 is ~ 1us, so there is very little
> payback in polling on such processors. Should the check use the ACPI
> declared C1 latency to decide whether we should poll or go to C1?

the exit latency is +/- 1 us, the entry latency is similar, and then
you're pretty close to 5 already (esp if you keep in mind that to break
even on energy you also need to be in the C state for a little bit)...

> 
> An other consideration is that sometimes, even if we expect to idle
> for a short time, we end up idling for more (otherwise I would never
> have seen ms polling, when expecting at most 5us). Should we set up a
> timer, that would fire when switching to an higher C state would
> conserve more energy?

this check is supposed to catch the known timer cases; those
are rather accurate in prediction


-- 
Arjan van de Ven 	Intel Open Source Technology Centre
For development, discussion and tips for power savings, 
visit http://www.lesswatts.org
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