linux-kernel - Re: [RFC PATCH] mm: don't reclaim inodes with many attached pages

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Message-ID: <20181026155652.GA7647@tower.DHCP.thefacebook.com>
Date:   Fri, 26 Oct 2018 15:56:55 +0000
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Kernel Team <Kernel-team@...com>,
        "Rik van Riel" <riel@...riel.com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "dairinin@...il.com" <dairinin@...il.com>
Subject: Re: [RFC PATCH] mm: don't reclaim inodes with many attached pages

On Fri, Oct 26, 2018 at 10:57:35AM +0200, Michal Hocko wrote:
> Spock doesn't seem to be cced here - fixed now
> 
> On Tue 23-10-18 16:43:29, Roman Gushchin wrote:
> > Spock reported that the commit 172b06c32b94 ("mm: slowly shrink slabs
> > with a relatively small number of objects") leads to a regression on
> > his setup: periodically the majority of the pagecache is evicted
> > without an obvious reason, while before the change the amount of free
> > memory was balancing around the watermark.
> > 
> > The reason behind is that the mentioned above change created some
> > minimal background pressure on the inode cache. The problem is that
> > if an inode is considered to be reclaimed, all belonging pagecache
> > page are stripped, no matter how many of them are there. So, if a huge
> > multi-gigabyte file is cached in the memory, and the goal is to
> > reclaim only few slab objects (unused inodes), we still can eventually
> > evict all gigabytes of the pagecache at once.
> > 
> > The workload described by Spock has few large non-mapped files in the
> > pagecache, so it's especially noticeable.
> > 
> > To solve the problem let's postpone the reclaim of inodes, which have
> > more than 1 attached page. Let's wait until the pagecache pages will
> > be evicted naturally by scanning the corresponding LRU lists, and only
> > then reclaim the inode structure.
> 
> Has this actually fixed/worked around the issue?

Spock wrote this earlier to me directly. I believe I can quote it here:

"Patch applied, looks good so far. System behaves like it was with
pre-4.18.15 kernels.
Also tried to add some user-level tests to the geneic background activity, like
- stat'ing a bunch of files
- streamed read several large files at once on ext4 and XFS
- random reads on the whole collection with a read size of 16K

I will be monitoring while fragmentation stacks up and report back if
something bad happens."

Spock, please let me know if you have any new results.

Thanks!