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    SubjectRe: [RFC PATCH v1 00/11] Create fast idle path for short idle periods
    On Fri, Jul 14, 2017 at 05:58:53PM +0200, Peter Zijlstra wrote:
    > On Fri, Jul 14, 2017 at 08:52:28AM -0700, Arjan van de Ven wrote:
    > > On 7/14/2017 8:38 AM, Peter Zijlstra wrote:
    > > > No, that's wrong. We want to fix the normal C state selection process to
    > > > pick the right C state.
    > > >
    > > > The fast-idle criteria could cut off a whole bunch of available C
    > > > states. We need to understand why our current C state pick is wrong and
    > > > amend the algorithm to do better. Not just bolt something on the side.
    > >
    > > I can see a fast path through selection if you know the upper bound of any
    > > selection is just 1 state.

    fast idle doesn't have an upper bound.

    If the prediction exceeds the fast idle threshold any C state can be used.

    It's just another state (fast C1), but right now it has an own threshold
    which may be different from standard C1.

    > >
    > > But also, how much of this is about "C1 be fast" versus "selecting C1 is slow"
    >
    > I got the impression its about we need to select C1 for longer. But the
    > fact that the patches don't in fact answer any of these questions,
    > they're wrong in principle ;-)

    For that workload. Tuning idle thresholds is a complex trade off.

    -Andi

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