linux-kernel - Re: [PATCH v2 0/5] mm: support parallel free of memory

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Date:   Thu, 16 Mar 2017 21:51:22 +0800
From:   Aaron Lu <aaron.lu@...el.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        Tim Chen <tim.c.chen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ying Huang <ying.huang@...el.com>
Subject: Re: [PATCH v2 0/5] mm: support parallel free of memory

On Thu, Mar 16, 2017 at 03:34:03PM +0800, Aaron Lu wrote:
> On Wed, Mar 15, 2017 at 05:28:43PM +0100, Michal Hocko wrote:
> ... ...
> > After all the amount of the work to be done is the same we just risk
> > more lock contentions, unexpected CPU usage etc.
> 
> I start to realize this is a good question.
> 
> I guess max_active=4 produced almost the best result(max_active=8 is
> only slightly better) is due to the test box is a 4 node machine and
> therefore, there are 4 zone->lock to contend(let's ignore those tiny
> zones only available in node 0).
> 
> I'm going to test on a EP to see if max_active=2 will suffice to produce
> a good enough result. If so, the proper default number should be the
> number of nodes.

Here are test results on 2 nodes EP with 128GiB memory, test size 100GiB.

max_active           time
vanilla              2.971s ±3.8%
2                    1.699s ±13.7%
4                    1.616s ±3.1%
8                    1.642s ±0.9%

So 4 gives best result but 2 is probably good enough.

If the size each worker deals with is changed from 1G to 2G:

max_active           time
2                    1.605s ±1.7%
4                    1.639s ±1.2%
8                    1.626s ±1.8%

Considering that we are mostly improving for memory intensive apps, the
default setting should probably be: max_active = node_number with each
worker freeing 2G memory.