linux-kernel - Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

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Date:   Tue, 18 Jul 2017 20:55:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Arjan van de Ven <arjan@...ux.intel.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Christoph Lameter <cl@...ux.com>,
        Aubrey Li <aubrey.li@...el.com>, tglx@...utronix.de,
        len.brown@...el.com, rjw@...ysocki.net, tim.c.chen@...ux.intel.com,
        paulmck@...ux.vnet.ibm.com, yang.zhang.wz@...il.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        daniel.lezcano@...aro.org
Subject: Re: [RFC PATCH v1 00/11] Create fast idle path for short idle periods

On Mon, Jul 17, 2017 at 12:48:38PM -0700, Arjan van de Ven wrote:
> On 7/17/2017 12:23 PM, Peter Zijlstra wrote:
> > Of course, this all assumes a Gaussian distribution to begin with, if we
> > get bimodal (or worse) distributions we can still get it wrong. To fix
> > that, we'd need to do something better than what we currently have.
> > 
> 
> fwiw some time ago I made a chart for predicted vs actual so you can sort
> of judge the distribution of things visually
> 
> http://git.fenrus.org/tmp/linux2.png

That shows we get it wrong a lot of times (about 50%, as per the
average) and moving the line has benefit. Since for performance you
really don't want to pick the deeper idle state, so you want to bias
your pick towards a shallower state.

Using the CDF approach you can quantify by how much you want it moved.