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    SubjectRe: [PATCH v2] cpuidle: Fix the menu governor to boost IO performance
    On Wed, 4 Nov 2009 10:39:13 +0100
    Corrado Zoccolo <czoccolo@gmail.com> wrote:

    > Hi Arjan,
    > On Tue, Sep 15, 2009 at 4:42 AM, Arjan van de Ven
    > <arjan@infradead.org> wrote:
    > > From: Arjan van de Ven <arjan@linux.intel.com>
    > > 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:
    > > +       /*
    > > +        * We want to default to C1 (hlt), not to busy polling
    > > +        * unless the timer is happening really really soon.
    > > +        */
    > > +       if (data->expected_us > 5)
    > > +               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
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    visit http://www.lesswatts.org
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