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help / color / mirror / Atom feedFrom: Andres Freund <andres@anarazel.de>
To: pgsql-committers@lists.postgresql.org
Subject: pgsql: read stream: Split decision about look ahead for AIO and combini
Date: Sun, 05 Apr 2026 05:22:59 +0000
Message-ID: <E1w9Fwo-0031cY-38@gemulon.postgresql.org> (raw)
read stream: Split decision about look ahead for AIO and combining
In a subsequent commit the read-ahead distance will only be increased when
waiting for IO. Without further work that would cause a regression: As IO
combining and read-ahead are currently controlled by the same mechanism, we
would end up not allowing IO combining when never needing to wait for IO (as
the distance ends up too small to allow for full sized IOs), which can
increase CPU overhead. A typical reason to not have to wait for IO completion
at a low look-ahead distance is use of io_uring with the to-be-read data in
the page cache. But even with worker the IO submission rate may be low enough
for the worker to keep up.
One might think that we could just always perform IO combining, but doing so
at the start of a scan can cause performance regressions:
1) Performing a large IO commonly has a higher latency than smaller IOs. That
is not a problem once reading ahead far enough, but at the start of a stream
it can lead to longer waits for IO completion.
2) Sometimes read streams will not be read to completion. Immediately starting
with full sized IOs leads to more wasted effort. This is not commonly an
issue with existing read stream users, but the upcoming use of read streams
to fetch table pages as part of an index scan frequently encounters this.
Solve this issue by splitting ReadStream->distance into ->combine_distance and
->readahead_distance. Right now they are increased/decreased at the same time,
but that will change in the next commit.
One of the comments in read_stream_should_look_ahead() refers to a motivation
that only really exists as of the next commit, but without it the code doesn't
make sense on its own.
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu
Discussion: https://postgr.es/m/CA+hUKGL2PhFyDoqrHefqasOnaXhSg48t1phs3VM8BAdrZqKZkw@mail.gmail.com
Branch
------
master
Details
-------
https://git.postgresql.org/pg/commitdiff/8ca147d582a5a9f3345478654408c46314758b50
Modified Files
--------------
src/backend/storage/aio/read_stream.c | 164 ++++++++++++++++++++++++++--------
1 file changed, 129 insertions(+), 35 deletions(-)
Message-ID: <E1w9Fwo-0031cY-38@gemulon.postgresql.org>
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To: pgsql-committers@postgresql.org
Cc: andres@anarazel.de, pgsql-committers@lists.postgresql.org
Subject: Re: pgsql: read stream: Split decision about look ahead for AIO and combini
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