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This finally introduces the first actual use of AIO. StartReadBuffers() now uses the AIO routines to issue IO. As the implementation of StartReadBuffers() is also used by the functions for reading individual blocks (StartReadBuffer() and through that ReadBufferExtended()) this means all buffered read IO passes through the AIO paths. However, as those are synchronous reads, actually performing the IO asynchronously would be rarely beneficial. Instead such IOs are flagged to always be executed synchronously. This way we don't have to duplicate a fair bit of code. When io_method=sync is used, the IO patterns generated after this change are the same as before, i.e. actual reads are only issued in WaitReadBuffers() and StartReadBuffers() may issue prefetch requests. This allows to bypass most of the actual asynchronicity, which is important to make a change as big as this less risky. One thing worth calling out is that, if IO is actually executed asynchronously, the precise meaning of what track_io_timing is measuring has changed. Previously it tracked the time for each IO, but that does not make sense when multiple IOs are executed concurrently. Now it only measures the time actually spent waiting for IO. A subsequent commit will adjust the docs for this. While AIO is now actually used, the logic in read_stream.c will often prevent using sufficiently many concurrent IOs. That will be addressed in the next commit. Reviewed-by: Noah Misch <noah@leadboat.com> Reviewed-by: Nazir Bilal Yavuz <byavuz81@gmail.com> Co-authored-by: Andres Freund <andres@anarazel.de> Co-authored-by: Thomas Munro <thomas.munro@gmail.com> Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt Discussion: https://postgr.es/m/20210223100344.llw5an2aklengrmn@alap3.anarazel.de Discussion: https://postgr.es/m/stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf@gcnactj4z56m
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/.