Re: Folios give an 80% performance win

Andres Freund
2021-07-24T18:46:15Z

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