Re: Folios give an 80% performance win

Matthew Wilcox
2021-07-24T18:50:30Z

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/

: The first and most obvious type of IO are pages reads and writes from
: the tablespaces. The pages are most often read one at a time, as 16KB
: random read operations. Writes to the tablespaces are also typically
: 16KB random operations, but they are done in batches. After every batch,
: fsync is called on the tablespace file handle.

(the current folio patch set does not create multi-page folios for
writes, only for reads)

I downloaded the mariadb source package that's in Debian, and from
what I can glean, it does indeed set O_DIRECT on data files in Linux,
through os_file_set_nocache().

> > 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.

... or the two benchmarks use Linux in completely different ways such
that one sees a huge benefit while the other sees none. Which is what
you'd expect for a patchset that improves the page cache and using a
benchmark that doesn't use the page cache.