linux-kernel - Re: Reclaim regression after 1c30844d2dfe

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Date:   Fri, 07 Feb 2020 18:05:08 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Ivan Babrou <ivan@...udflare.com>, linux-mm@...ck.org,
        Mel Gorman <mgorman@...hsingularity.net>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...udflare.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Vlastimil Babka <vbabka@...e.cz>
Subject: Re: Reclaim regression after 1c30844d2dfe

On Fri, 2020-02-07 at 14:54 -0800, Ivan Babrou wrote:
> This change from 5.5 times:
> 
> * https://github.com/torvalds/linux/commit/1c30844d2dfe
> 
> > mm: reclaim small amounts of memory when an external fragmentation
> > event occurs
> 
> Introduced undesired effects in our environment.
> 
> * NUMA with 2 x CPU
> * 128GB of RAM
> * THP disabled
> * Upgraded from 4.19 to 5.4
> 
> Before we saw free memory hover at around 1.4GB with no spikes. After
> the upgrade we saw some machines decide that they need a lot more
> than
> that, with frequent spikes above 10GB, often only on a single numa
> node.
> 
> We can see kswapd quite active in balance_pgdat (it didn't look like
> it slept at all):
> 
> $ ps uax | fgrep kswapd
> root       1850 23.0  0.0      0     0 ?        R    Jan30 1902:24
> [kswapd0]
> root       1851  1.8  0.0      0     0 ?        S    Jan30 152:16
> [kswapd1]
> 
> This in turn massively increased pressure on page cache, which did
> not
> go well to services that depend on having a quick response from a
> local cache backed by solid storage.
> 
> Here's how it looked like when I zeroed vm.watermark_boost_factor:

We have observed the same thing, even on single node systems.

I have some hacky patches to apply the watermark_boost thing on
a per pgdat basis, which seems to resolve the issue, but I have
not yet found the time to get the locking for that correct.

Given how rare the watermark boosting is, maybe the answer is
just to use atomics? Not sure :)

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