linux-kernel - Re: [PATCH 1/2] Revert "mm: don't reclaim inodes with many attached pages"

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Date:   Sat, 9 Feb 2019 03:42:30 +0000
From:   Roman Gushchin <guro@...com>
To:     Andrew Morton <akpm@...ux-foundation.org>
CC:     Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
        Michal Hocko <mhocko@...nel.org>, Chris Mason <clm@...com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-xfs@...r.kernel.org" <linux-xfs@...r.kernel.org>,
        "vdavydov.dev@...il.com" <vdavydov.dev@...il.com>
Subject: Re: [PATCH 1/2] Revert "mm: don't reclaim inodes with many attached
 pages"

On Fri, Feb 08, 2019 at 02:49:44PM -0800, Andrew Morton wrote:
> On Fri, 8 Feb 2019 13:50:49 +0100 Jan Kara <jack@...e.cz> wrote:
> 
> > > > Has anyone done significant testing with Rik's maybe-fix?
> > > 
> > > I will give it a spin with bonnie++ today. We'll see what comes out.
> > 
> > OK, I did a bonnie++ run with Rik's patch (on top of 4.20 to rule out other
> > differences). This machine does not show so big differences in bonnie++
> > numbers but the difference is still clearly visible. The results are
> > (averages of 5 runs):
> > 
> > 		 Revert			Base			Rik
> > SeqCreate del    78.04 (   0.00%)	98.18 ( -25.81%)	90.90 ( -16.48%)
> > RandCreate del   87.68 (   0.00%)	95.01 (  -8.36%)	87.66 (   0.03%)
> > 
> > 'Revert' is 4.20 with "mm: don't reclaim inodes with many attached pages"
> > and "mm: slowly shrink slabs with a relatively small number of objects"
> > reverted. 'Base' is the kernel without any reverts. 'Rik' is a 4.20 with
> > Rik's patch applied.
> > 
> > The numbers are time to do a batch of deletes so lower is better. You can see
> > that the patch did help somewhat but it was not enough to close the gap
> > when files are deleted in 'readdir' order.
> 
> OK, thanks.
> 
> I guess we need a rethink on Roman's fixes.   I'll queued the reverts.

Agree.

I still believe that we should cause the machine-wide memory pressure
to clean up any remains of dead cgroups, and Rik's patch is a step into
the right direction. But we need to make some experiments and probably
some code changes here to guarantee that we don't regress on performance.

> 
> 
> BTW, one thing I don't think has been discussed (or noticed) is the
> effect of "mm: don't reclaim inodes with many attached pages" on 32-bit
> highmem machines.  Look why someone added that code in the first place:
> 
> : commit f9a316fa9099053a299851762aedbf12881cff42
> : Author: Andrew Morton <akpm@...eo.com>
> : Date:   Thu Oct 31 04:09:37 2002 -0800
> : 
> :     [PATCH] strip pagecache from to-be-reaped inodes
> :     
> :     With large highmem machines and many small cached files it is possible
> :     to encounter ZONE_NORMAL allocation failures.  This can be demonstrated
> :     with a large number of one-byte files on a 7G machine.
> :     
> :     All lowmem is filled with icache and all those inodes have a small
> :     amount of highmem pagecache which makes them unfreeable.
> :     
> :     The patch strips the pagecache from inodes as they come off the tail of
> :     the inode_unused list.
> :     
> :     I play tricks in there peeking at the head of the inode_unused list to
> :     pick up the inode again after running iput().  The alternatives seemed
> :     to involve more widespread changes.
> :     
> :     Or running invalidate_inode_pages() under inode_lock which would be a
> :     bad thing from a scheduling latency and lock contention point of view.
> 
> I guess I shold have added a comment.  Doh.
> 

It's a very useful link.

Thanks!