Subject: pgsql: aio: Add core asynchronous I/O infrastructure

From: Andres Freund <[email-masked]>
Date: Mon, 17 Mar 2025 23:23:42 +0000

aio: Add core asynchronous I/O infrastructure

The main motivations to use AIO in PostgreSQL are:

a) Reduce the time spent waiting for IO by issuing IO sufficiently early.

   In a few places we have approximated this using posix_fadvise() based
   prefetching, but that is fairly limited (no completion feedback, double the
   syscalls, only works with buffered IO, only works on some OSs).

b) Allow to use Direct-I/O (DIO).

   DIO can offload most of the work for IO to hardware and thus increase
   throughput / decrease CPU utilization, as well as reduce latency.  While we
   have gained the ability to configure DIO in d4e71df6, it is not yet usable
   for real world workloads, as every IO is executed synchronously.

For portability, the new AIO infrastructure allows to implement AIO using
different methods. The choice of the AIO method is controlled by the new
io_method GUC. As of this commit, the only implemented method is "sync",
i.e. AIO is not actually executed asynchronously. The "sync" method exists to
allow to bypass most of the new code initially.

Subsequent commits will introduce additional IO methods, including a
cross-platform method implemented using worker processes and a linux specific
method using io_uring.

To allow different parts of postgres to use AIO, the core AIO infrastructure
does not need to know what kind of files it is operating on. The necessary
behavioral differences for different files are abstracted as "AIO
Targets". One example target would be smgr. For boring portability reasons,
all targets currently need to be added to an array in aio_target.c.  This
commit does not implement any AIO targets, just the infrastructure for
them. The smgr target will be added in a later commit.

Completion (and other events) of IOs for one type of file (i.e. one AIO
target) need to be reacted to differently, based on the IO operation and the
callsite. This is made possible by callbacks that can be registered on
IOs. E.g. an smgr read into a local buffer does not need to update the
corresponding BufferDesc (as there is none), but a read into shared buffers
does.  This commit does not contain any callbacks, they will be added in
subsequent commits.

For now the AIO infrastructure only understands READV and WRITEV operations,
but it is expected that more operations will be added. E.g. fsync/fdatasync,
flush_range and network operations like send/recv.

As of this commit, nothing uses the AIO infrastructure. Later commits will add
an smgr target, md.c and bufmgr.c callbacks and then finally use AIO for
read_stream.c IO, which, in one fell swoop, will convert all read stream users
to AIO.

The goal is to use AIO in many more places. There are patches to use AIO for
checkpointer and bgwriter that are reasonably close to being ready. There also
are prototypes to use it for WAL, relation extension, backend writes and many
more. Those prototypes were important to ensure the design of the AIO
subsystem is not too limiting (e.g. WAL writes need to happen in critical
sections, which influenced a lot of the design).

A future commit will add an AIO README explaining the AIO architecture and how
to use the AIO subsystem. The README is added later, as it references details
only added in later commits.

Many many more people than the folks named below have contributed with
feedback, work on semi-independent patches etc. E.g. various folks have
contributed patches to use the read stream infrastructure (added by Thomas in
b5a9b18cd0b) in more places. Similarly, a *lot* of folks have contributed to
the CI infrastructure, which I had started to work on to make adding AIO
feasible.

Some of the work by contributors has gone into the "v1" prototype of AIO,
which heavily influenced the current design of the AIO subsystem. None of the
code from that directly survives, but without the prototype, the current
version of the AIO infrastructure would not exist.

Similarly, the reviewers below have not necessarily looked at the current
design or the whole infrastructure, but have provided very valuable input. I
am to blame for problems, not they.

Author: Andres Freund <[email-masked]>
Co-authored-by: Thomas Munro <[email-masked]>
Co-authored-by: Nazir Bilal Yavuz <[email-masked]>
Co-authored-by: Melanie Plageman <[email-masked]>
Reviewed-by: Heikki Linnakangas <[email-masked]>
Reviewed-by: Noah Misch <[email-masked]>
Reviewed-by: Jakub Wartak <[email-masked]>
Reviewed-by: Melanie Plageman <[email-masked]>
Reviewed-by: Robert Haas <[email-masked]>
Reviewed-by: Dmitry Dolgov <[email-masked]>
Reviewed-by: Antonin Houska <[email-masked]>
Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt
Discussion: https://postgr.es/m/[email-masked]
Discussion: https://postgr.es/m/stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf@gcnactj4z56m

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/da7226993fd4b73d8b40abb7167d124eada97f2e

Modified Files
--------------
src/backend/storage/aio/Makefile                |    4 +
src/backend/storage/aio/aio.c                   | 1130 +++++++++++++++++++++++
src/backend/storage/aio/aio_callback.c          |  308 ++++++
src/backend/storage/aio/aio_init.c              |  198 ++++
src/backend/storage/aio/aio_io.c                |  184 ++++
src/backend/storage/aio/aio_target.c            |  114 +++
src/backend/storage/aio/meson.build             |    4 +
src/backend/storage/aio/method_sync.c           |   47 +
src/backend/utils/activity/wait_event_names.txt |    1 +
src/include/storage/aio.h                       |  311 +++++++
src/include/storage/aio_internal.h              |  395 ++++++++
src/include/storage/aio_types.h                 |  117 +++
src/tools/pgindent/typedefs.list                |   21 +
13 files changed, 2834 insertions(+)

---

From: Tom Lane <[email-masked]>
Date: Tue, 18 Mar 2025 11:03:36 -0400

> aio: Add core asynchronous I/O infrastructure

Some of the buildfarm is mildly unhappy with this.
So far I see

 ayu           | 2025-03-18 13:08:04 | aio_callback.c:83:12: warning: comparison of constant 1 with expression of type 'PgAioHandleCallbackID' (aka 'enum PgAioHandleCallbackID') is always false [-Wtautological-constant-out-of-range-compare]
 ayu           | 2025-03-18 13:08:04 | aio_callback.c:190:56: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 ayu           | 2025-03-18 13:08:04 | aio_callback.c:220:56: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 ayu           | 2025-03-18 13:08:04 | aio_callback.c:274:56: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 ayu           | 2025-03-18 13:08:04 | aio_target.c:50:41: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 ayu           | 2025-03-18 13:08:04 | aio_target.c:110:41: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 demoiselle    | 2025-03-18 03:43:46 | aio_callback.c:83:12: warning: comparison of constant 1 with expression of type 'PgAioHandleCallbackID' (aka 'enum PgAioHandleCallbackID') is always false [-Wtautological-constant-out-of-range-compare]
 demoiselle    | 2025-03-18 03:43:46 | aio_callback.c:190:56: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 demoiselle    | 2025-03-18 03:43:46 | aio_callback.c:220:56: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 demoiselle    | 2025-03-18 03:43:46 | aio_callback.c:274:56: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 demoiselle    | 2025-03-18 03:43:46 | aio_target.c:50:41: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]
 demoiselle    | 2025-03-18 03:43:46 | aio_target.c:110:41: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-constant-out-of-range-compare]

I don't have that compiler handy to check, but maybe changes like

-	if (cb_id >= lengthof(aio_handle_cbs))
+	if ((size_t) cb_id >= lengthof(aio_handle_cbs))

would silence these?  Or you could just drop all those tests
and have faith in the type-safety of the enums.

			regards, tom lane

---

From: Andres Freund <[email-masked]>
Date: Tue, 18 Mar 2025 11:14:12 -0400

Hi,

>  demoiselle    | 2025-03-18 03:43:46 | aio_target.c:110:41: warning: comparison of constant 1 with expression of type 'PgAioTargetID' (aka 'enum PgAioTargetID') is always true [-Wtautological-const…

Thanks for noticing that.

> would silence these?

I wonder if we should instead either ask those buildfarm animals to be
disabled or have the warning manually disabled. I don't think it's a good
investment on our part to work towards warning cleanliness on clang 4 and 5,
on distros from 2017 and and 2018 respectively.

> Or you could just drop all those tests and have faith in the type-safety of
> the enums.

The issue isn't really the type-safety of the enums (not that I would have a
lot of trust in that), it's whether the relevant callback/target actually have
been added to the the callback/target arrays.

Greetings,

Andres Freund

---

From: Tom Lane <[email-masked]>
Date: Tue, 18 Mar 2025 11:26:15 -0400

> on distros from 2017 and and 2018 respectively.

Well, if we don't want to fix it I can just adjust my warning-scraping
script to ignore these.  Let's wait a bit and see if any newer
compilers warn similarly before deciding.

			regards, tom lane

---

From: Andres Freund <[email-masked]>
Date: Wed, 19 Mar 2025 10:29:19 -0400

Hi,

> compilers warn similarly before deciding.

FWIW, adding a few casts does silence the warnings, at least on clang-3.9,
which I had around.

But there also are other warnings on that compiler, mainly stuff like:

../../home/andres/src/postgresql/src/interfaces/libpq/fe-auth-oauth-curl.c:1437:28: warning: suggest braces around initialization of subobject [-Wmissing-braces]
        struct itimerspec spec = {0};

I lean towards not fixing this stuff, but if you would like me to, I can.

Greetings,

Andres Freund

---

From: Tom Lane <[email-masked]>
Date: Wed, 19 Mar 2025 12:42:28 -0400

> FWIW, adding a few casts does silence the warnings, at least on clang-3.9,

As of now, the only BF members showing these warnings are ayu (clang
version 4.0.1-10~deb9u2), batfish (clang version 5.0.0-3~16.04.1),
and demoiselle (clang version 5.0.1).  None of those seem like
versions that anyone would use for code development anymore (clang 6.0
was released in March 2018), so I'm not feeling like these warnings
are important to silence.  Let's just ignore them.

>         struct itimerspec spec = {0};

And even more so that, if it's so old it's not even in the buildfarm
anymore.  I do care about -Wmissing-braces on a version developers
might use, but ...

			regards, tom lane

---

From: Aleksander Alekseev <[email-masked]>
Date: Mon, 31 Mar 2025 19:23:57 +0300

Hi Andres,

> [...]

I noticed that aio.c references README.md:

```
 * aio.c
 *    AIO - Core Logic
 *
 * For documentation about how AIO works on a higher level, including a
 * schematic example, see README.md.
```

However the file seems to be missing in this commit or in the
following ones. Any chance it was excluded by mistake?

-- 
Best regards,
Aleksander Alekseev

---

From: Andres Freund <[email-masked]>
Date: Mon, 31 Mar 2025 12:38:46 -0400

Hi,

> following ones. Any chance it was excluded by mistake?

It'll be committed soon. There's one pending change in the README.md that I'd
like to make / get comments on...

Sorry for the temporarily "hanging" reference...

Greetings,

Andres