linux-kernel - Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO

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Date:   Tue, 28 Aug 2018 16:56:25 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Daniel Drake <drake@...lessm.com>,
        Vinayak Menon <vinmenon@...eaurora.org>,
        Christopher Lameter <cl@...ux.com>,
        Peter Enderborg <peter.enderborg@...y.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Mike Galbraith <efault@....de>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and
 IO

On Tue, Aug 28, 2018 at 01:11:11PM -0700, Randy Dunlap wrote:
> On 08/28/2018 10:22 AM, Johannes Weiner wrote:
> > diff --git a/Documentation/accounting/psi.txt b/Documentation/accounting/psi.txt
> > new file mode 100644
> > index 000000000000..51e7ef14142e
> > --- /dev/null
> > +++ b/Documentation/accounting/psi.txt
> > @@ -0,0 +1,64 @@
> > +================================
> > +PSI - Pressure Stall Information
> > +================================
> > +
> > +:Date: April, 2018
> > +:Author: Johannes Weiner <hannes@...xchg.org>
> > +
> > +When CPU, memory or IO devices are contended, workloads experience
> > +latency spikes, throughput losses, and run the risk of OOM kills.
> > +
> > +Without an accurate measure of such contention, users are forced to
> > +either play it safe and under-utilize their hardware resources, or
> > +roll the dice and frequently suffer the disruptions resulting from
> > +excessive overcommit.
> > +
> > +The psi feature identifies and quantifies the disruptions caused by
> > +such resource crunches and the time impact it has on complex workloads
> > +or even entire systems.
> > +
> > +Having an accurate measure of productivity losses caused by resource
> > +scarcity aids users in sizing workloads to hardware--or provisioning
> > +hardware according to workload demand.
> > +
> > +As psi aggregates this information in realtime, systems can be managed
> > +dynamically using techniques such as load shedding, migrating jobs to
> > +other systems or data centers, or strategically pausing or killing low
> > +priority or restartable batch jobs.
> > +
> > +This allows maximizing hardware utilization without sacrificing
> > +workload health or risking major disruptions such as OOM kills.
> > +
> > +Pressure interface
> > +==================
> > +
> > +Pressure information for each resource is exported through the
> > +respective file in /proc/pressure/ -- cpu, memory, and io.
> > +
> 
> Hi,
> 
> > +In both cases, the format for CPU is as such:
> 
> I don't see what "In both cases" refers to here.  It seems that you could
> just remove it.

You're right, that must be a left-over from when I described CPU
separately; "both cases" referred to memory and IO which have
identical formats. It needs to be removed:

diff --git a/Documentation/accounting/psi.txt b/Documentation/accounting/psi.txt
index e051810d5127..b8ca28b60215 100644
--- a/Documentation/accounting/psi.txt
+++ b/Documentation/accounting/psi.txt
@@ -35,7 +35,7 @@ Pressure interface
 Pressure information for each resource is exported through the
 respective file in /proc/pressure/ -- cpu, memory, and io.
 
-In both cases, the format for CPU is as such:
+The format for CPU is as such:
 
 some avg10=0.00 avg60=0.00 avg300=0.00 total=0