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Date:   Sat, 24 Jul 2021 11:45:26 -0700
From:   "Andres Freund" <andres@anarazel.de>
To:     "James Bottomley" <James.Bottomley@HansenPartnership.com>,
        "Matthew Wilcox" <willy@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        linux-fsdevel@vger.kernel.org,
        "Linus Torvalds" <torvalds@linux-foundation.org>,
        "Andrew Morton" <akpm@linux-foundation.org>,
        "Darrick J. Wong" <djwong@kernel.org>,
        "Christoph Hellwig" <hch@lst.de>,
        "Michael Larabel" <Michael@michaellarabel.com>
Subject: Re: Folios give an 80% performance win
Content-Type: text/plain
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Hi,

On Sat, Jul 24, 2021, at 11:23, 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. 
> 
> > Regardless of whether postgres is "doing something wrong" or not,
> > do you not think that an 80% performance win would exert a certain
> > amount of pressure on distros to do the backport?
> 
> Well, I cut the previous question deliberately, but if you're going to
> force me to answer, my experience with storage tells me that one test
> being 10x different from all the others usually indicates a problem
> with the benchmark test itself rather than a baseline improvement, so
> I'd wait for more data.

I have a similar reaction - the large improvements are for a read/write pgbench benchmark at a scale that fits in memory. That's typically purely bound by the speed at which the WAL can be synced to disk. As far as I recall mariadb also uses buffered IO for WAL (but there was recent work in the area).

Is there a reason fdatasync() of 16MB files to have got a lot faster? Or a chance that could be broken?

Some improvement for read-only wouldn't surprise me, particularly if the os/pg weren't configured for explicit huge pages. Pgbench has a uniform distribution so its *very* tlb miss heavy with 4k pages.

Regards,
Andres