linux-kernel - Re: [PATCH 0/7] psi: pressure stall information for CPU, memory, and IO

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Date:   Mon, 14 May 2018 10:35:37 -0700
From:   Bart Van Assche <bart.vanassche@....com>
To:     Christopher Lameter <cl@...ux.com>,
        Johannes Weiner <hannes@...xchg.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org, cgroups@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...uxfoundation.org>,
        Tejun Heo <tj@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Mike Galbraith <efault@....de>,
        Oliver Yang <yangoliver@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        xxx xxx <x.qendo@...il.com>,
        Taras Kondratiuk <takondra@...co.com>,
        Daniel Walker <danielwa@...co.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Ruslan Ruslichenko <rruslich@...co.com>, kernel-team@...com
Subject: Re: [PATCH 0/7] psi: pressure stall information for CPU, memory, and
 IO

On 05/14/18 08:39, Christopher Lameter wrote:
> On Mon, 7 May 2018, Johannes Weiner wrote:
>> What to make of this number? If CPU utilization is at 100% and CPU
>> pressure is 0, it means the system is perfectly utilized, with one
>> runnable thread per CPU and nobody waiting. At two or more runnable
>> tasks per CPU, the system is 100% overcommitted and the pressure
>> average will indicate as much. From a utilization perspective this is
>> a great state of course: no CPU cycles are being wasted, even when 50%
>> of the threads were to go idle (and most workloads do vary). From the
>> perspective of the individual job it's not great, however, and they
>> might do better with more resources. Depending on what your priority
>> is, an elevated "some" number may or may not require action.
> 
> This looks awfully similar to loadavg. Problem is that loadavg gets
> screwed up by tasks blocked waiting for I/O. Isnt there some way to fix
> loadavg instead?

The following article explains why it probably made sense in 1993 to 
include TASK_UNINTERRUPTIBLE in loadavg and also why this no longer 
makes sense today:

http://www.brendangregg.com/blog/2017-08-08/linux-load-averages.html

Bart.