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Date:   Sat, 24 Jul 2021 19:14:14 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     James Bottomley <James.Bottomley@hansenpartnership.com>
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>,
        Andres Freund <andres@anarazel.de>,
        Michael Larabel <Michael@michaellarabel.com>
Subject: Re: Folios give an 80% performance win
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References: <20210715033704.692967-1-willy@infradead.org>
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 <1e48f7edcb6d9a67e8b78823660939007e14bae1.camel@HansenPartnership.com>
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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 test 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.

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?