f63ca33790
This avoids increasing the distance to the maximum in cases where the I/O
subsystem is already keeping up. This turns out to be important for
performance for two reasons:
- Pinning a lot of buffers is not cheap. If additional pins allow us to avoid
IO waits, it's definitely worth it, but if we can already do all the
necessary readahead at a distance of 16, reading ahead 512 buffers can
increase the CPU overhead substantially. This is particularly noticeable
when the to-be-read blocks are already in the kernel page cache.
- If the read stream is read to completion, reading in data earlier than
needed is of limited consequences, leaving aside the CPU costs mentioned
above. But if the read stream will not be fully consumed, e.g. because it is
on the inner side of a nested loop join, the additional IO can be a serious
performance issue. This is not that commonly a problem for current read
stream users, but the upcoming work, to use a read stream to fetch table
pages as part of an index scan, frequently encounters this.
Note that this commit would have substantial performance downsides without
earlier commits:
- Commit 6e36930f9a, which avoids decreasing the readahead distance when
there was recent IO, is crucial, as otherwise we very often would end up not
reading ahead aggressively enough anymore with this commit, due to
increasing the distance less often.
- "read stream: Split decision about look ahead for AIO and combining" is
important as we would otherwise not perform IO combining when the IO
subsystem can keep up.
- "aio: io_uring: Trigger async processing for large IOs" is important to
continue to benefit from memory copy parallelism when using fewer IOs.
Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com>
Tested-by: Tomas Vondra <tomas@vondra.me>
Discussion: https://postgr.es/m/f3xxfrkafjxpyqxywcxricxgyizjirfceychyxsgn7bwjp5eda@kwbduhy7tfmu
Discussion: https://postgr.es/m/CA+hUKGL2PhFyDoqrHefqasOnaXhSg48t1phs3VM8BAdrZqKZkw@mail.gmail.com
PostgreSQL Database Management System
This directory contains the source code distribution of the PostgreSQL database management system.
PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings.
Copyright and license information can be found in the file COPYRIGHT.
General documentation about this version of PostgreSQL can be found at https://www.postgresql.org/docs/devel/. In particular, information about building PostgreSQL from the source code can be found at https://www.postgresql.org/docs/devel/installation.html.
The latest version of this software, and related software, may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.