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    SubjectRe: Dynamic configure max_cstate
    > When running a fio workload, I found sometimes cpu C state has
    > big impact on the result. Mostly, fio is a disk I/O workload
    > which doesn't spend much time with cpu, so cpu switch to C2/C3
    > freqently and the latency is big.
    > If I start kernel with idle=poll or processor.max_cstate=1,
    > the result is quite good. Consider a scenario that machine is
    > busy at daytime and free at night. Could we add a dynamic
    > configuration interface for processor.max_cstate or something
    > similiar with sysfs? So user applications could change the
    > max_cstate dynamically? For example, we could add a new
    > parameter to function cpuidle_governor->select to mark the
    > highest c state.

    max_cstate is a debug param. It isn't a run-time API and never will be.
    User-space shouldn't need to know or care about C-states,
    and if it appears it needs to, then we have a bug we need to fix.

    The interface in Documentation/power/pm_qos_interface.txt
    is supposed to handle this. Though if the underlying code
    is not noticing IO interrupts, then it can't help.

    Another thing to look at is processor.latency_factor
    which you can change at run-time in

    We multiply the advertised exit latency by this
    before deciding to enter a C-state. The concept
    is that ACPI reports a performance number, but what
    we really want is a power break-even. Anyway,
    we know the default mulitple is too low, and will be
    raising it shortly.

    Of course if the current code is not predicting any
    IO interrupts on your IO-only workload, this, like
    pm_qos, will not help.

    -Len Brown, Intel Open Source Technology Center

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