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    SubjectRe: Dynamic configure max_cstate

    On Fri, Jul 31, 2009 at 03:06:46PM +0800, Zhang, Yanmin wrote:
    > On Thu, 2009-07-30 at 21:43 -0600, Robert Hancock wrote:
    > > On 07/28/2009 04:11 AM, Andreas Mohr wrote:
    > > > Oh, and about the places which submit I/O requests where one would have to
    > > > flag this: are they in any way correlated with the scheduler I/O wait
    > > > value? Would the I/O wait mechanism be a place to more easily and centrally
    > > > indicate that we're waiting for a request to come back in "very soon"?
    > > > OTOH I/O requests may have vastly differing delay expectations,
    > > > thus specifically only short-term expected I/O replies should be flagged,
    > > > otherwise we're wasting lots of ACPI deep idle opportunities.
    > >
    > > Did the results show a big difference in performance between maximum C2
    > > and maximum C3?
    > No big difference. I tried different max cstate by processor.max_cstate.
    > Mostly, processor.max_cstate=1 could get the similiar result like idle=poll.

    OK, but I'd say that this doesn't mean that we should implement a
    hard-coded mechanism which simply says "in such cases, don't do anything > C1".
    Instead we should strive for a far-reaching _generic_ mechanism
    which gathers average latencies of various I/O activities/devices
    and then uses some formula to determine the maximum (not necessarily ACPI)
    idle latency that we're willing to endure (e.g. average device I/O reply latency
    divided by 10 or so).

    And in addition to this, we should also take into account (read: skip)
    any idle states which kill busmaster DMA completely
    (in case of busmaster DMA I/O activities, that is).

    _Lots_ of very nice opportunities for improvement here, I'd say...
    (in the 5, 10 or even 40% range in the case of certain network I/O)


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