linux-kernel - Re: Folios give an 80% performance win

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Message-ID: <f1d0d61e9528c011f25ee1dfdf6c0b14fb3166b1.camel@HansenPartnership.com>
Date:   Sat, 24 Jul 2021 12:21:16 -0700
From:   James Bottomley <James.Bottomley@...senPartnership.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Darrick J. Wong" <djwong@...nel.org>,
        Christoph Hellwig <hch@....de>,
        Andres Freund <andres@...razel.de>,
        Michael Larabel <Michael@...haellarabel.com>
Subject: Re: Folios give an 80% performance win

On Sat, 2021-07-24 at 19:50 +0100, Matthew Wilcox wrote:
> On Sat, Jul 24, 2021 at 11:23:25AM -0700, James Bottomley wrote:
> > On Sat, 2021-07-24 at 19:14 +0100, Matthew Wilcox wrote:
> > > On Sat, Jul 24, 2021 at 11:09:02AM -0700, James Bottomley wrote:
> > > > On Sat, 2021-07-24 at 18:27 +0100, Matthew Wilcox wrote:
> > > > > What blows me away is the 80% performance improvement for
> > > > > PostgreSQL. I know they use the page cache extensively, so
> > > > > it's
> > > > > plausibly real. I'm a bit surprised that it has such good
> > > > > locality, and the size of the win far exceeds my
> > > > > expectations.  We should probably dive into it and figure out
> > > > > exactly what's going on.
> > > > 
> > > > Since none of the other tested databases showed more than a 3%
> > > > improvement, this looks like an anomalous result specific to
> > > > something in postgres ... although the next biggest db: mariadb
> > > > wasn't part of the tests so I'm not sure that's
> > > > definitive.  Perhaps the next step should be to t
> > > > est mariadb?  Since they're fairly similar in domain (both full
> > > > SQL) if mariadb shows this type of improvement, you can
> > > > safely assume it's something in the way SQL databases handle
> > > > paging and if it doesn't, it's likely fixing a postgres
> > > > inefficiency.
> > > 
> > > I think the thing that's specific to PostgreSQL is that it's a
> > > heavy user of the page cache.  My understanding is that most
> > > databases use direct IO and manage their own page cache, while
> > > PostgreSQL trusts the kernel to get it right.
> > 
> > That's testable with mariadb, at least for the innodb engine since
> > the flush_method is settable. 
> 
> We're still not communicating well.  I'm not talking about writes,
> I'm talking about reads.  Postgres uses the page cache for reads.
> InnoDB uses O_DIRECT (afaict).  See articles like this one:
> https://www.percona.com/blog/2018/02/08/fsync-performance-storage-devices/

If it were all about reads, wouldn't the Phoronix pgbench read only
test have shown a better improvement than 7%?  I think the Phoronix
data shows that whatever it is it's to do with writes ... that does
imply something in the way the log syncs data.

James