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

From: Johannes Weiner <hannes@cmpxchg.org>
To: Randy Dunlap <rdunlap@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Tejun Heo <tj@kernel.org>, Suren Baghdasaryan <surenb@google.com>,
	Daniel Drake <drake@endlessm.com>,
	Vinayak Menon <vinmenon@codeaurora.org>,
	Christopher Lameter <cl@linux.com>,
	Peter Enderborg <peter.enderborg@sony.com>,
	Shakeel Butt <shakeelb@google.com>,
	Mike Galbraith <efault@gmx.de>,
	linux-mm@kvack.org, cgroups@vger.kernel.org,
	linux-kernel@vger.kernel.org, kernel-team@fb.com
Subject: Re: [PATCH 8/9] psi: pressure stall information for CPU, memory, and IO
Date: Tue, 28 Aug 2018 16:56:25 -0400	[thread overview]
Message-ID: <20180828205625.GA14030@cmpxchg.org> (raw)
In-Reply-To: <6ff71c29-3b6a-4849-6f2a-3d829bbd43e2@infradead.org>

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@cmpxchg.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
 
