linux-kernel - Re: [PATCH 1/2] mm: page_counter: relayout structure to reduce false sharing

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Date:   Mon, 4 Jan 2021 15:11:40 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, andi.kleen@...el.com,
        tim.c.chen@...el.com, dave.hansen@...el.com, ying.huang@...el.com,
        Roman Gushchin <guro@...com>
Subject: Re: [PATCH 1/2] mm: page_counter: relayout structure to reduce false
 sharing

On Mon 04-01-21 21:34:45, Feng Tang wrote:
> Hi Michal,
> 
> On Mon, Jan 04, 2021 at 02:03:57PM +0100, Michal Hocko wrote:
> > On Tue 29-12-20 22:35:13, Feng Tang wrote:
> > > When checking a memory cgroup related performance regression [1],
> > > from the perf c2c profiling data, we found high false sharing for
> > > accessing 'usage' and 'parent'.
> > > 
> > > On 64 bit system, the 'usage' and 'parent' are close to each other,
> > > and easy to be in one cacheline (for cacheline size == 64+ B). 'usage'
> > > is usally written, while 'parent' is usually read as the cgroup's
> > > hierarchical counting nature.
> > > 
> > > So move the 'parent' to the end of the structure to make sure they
> > > are in different cache lines.
> > 
> > Yes, parent is write-once field so having it away from other heavy RW
> > fields makes sense to me.
> >  
> > > Following are some performance data with the patch, against
> > > v5.11-rc1, on several generations of Xeon platforms. Most of the
> > > results are improvements, with only one malloc case on one platform
> > > shows a -4.0% regression. Each category below has several subcases
> > > run on different platform, and only the worst and best scores are
> > > listed:
> > > 
> > > fio:				 +1.8% ~  +8.3%
> > > will-it-scale/malloc1:		 -4.0% ~  +8.9%
> > > will-it-scale/page_fault1:	 no change
> > > will-it-scale/page_fault2:	 +2.4% ~  +20.2%
> > 
> > What is the second number? Std?
> 
> For each case like 'page_fault2', I run several subcases on different
> generations of Xeon, and only listed the lowest (first number) and
> highest (second number) scores.
> 
> There are 5 runs and the result are: +3.6%, +2.4%, +10.4%, +20.2%,
> +4.7%, and +2.4% and +20.2% are listed.

This should be really explained in the changelog and ideally mention the
model as well. Seeing a std would be appreciated as well.

-- 
Michal Hocko
SUSE Labs