linux-kernel - Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to nr

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Message-ID: <YVwdrh5pg0zSv2/b@hirez.programming.kicks-ass.net>
Date:   Tue, 5 Oct 2021 11:41:02 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Mike Galbraith <efault@....de>, Ingo Molnar <mingo@...nel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Aubrey Li <aubrey.li@...ux.intel.com>,
        Barry Song <song.bao.hua@...ilicon.com>,
        Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] sched/fair: Scale wakeup granularity relative to
 nr_running

On Wed, Sep 22, 2021 at 04:04:57PM +0100, Mel Gorman wrote:
> On Wed, Sep 22, 2021 at 04:15:27PM +0200, Vincent Guittot wrote:

> The reason I used SCHED_TUNABLESCALING_NONE for the changelog is that
> the exact values depend on the number of CPUs so values are not even
> the same across the range of machines I'm using. sysctl_sched_latency,
> sysctl_sched_min_granularity sysctl_sched_wakeup_granularity are all
> scaled but the ratios remain constant.

It might make sense to reconsider the whole scaling thing FWIW. It used
to be that desktop systems had 'small' number of CPUs (<=8) and servers
had more.

But today it's not uncommon to have 32-64 CPUs in a desktop system. And
even LOG scaling gets us into stupid numbers for the latencies.