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aio: Add io_method=worker
The previous commit introduced the infrastructure to start io_workers. This commit actually makes the workers execute IOs. IO workers consume IOs from a shared memory submission queue, run traditional synchronous system calls, and perform the shared completion handling immediately. Client code submits most requests by pushing IOs into the submission queue, and waits (if necessary) using condition variables. Some IOs cannot be performed in another process due to lack of infrastructure for reopening the file, and must processed synchronously by the client code when submitted. For now the default io_method is changed to "worker". We should re-evaluate that around beta1, we might want to be careful and set the default to "sync" for 18. Reviewed-by: Noah Misch <noah@leadboat.com> Co-authored-by: Thomas Munro <thomas.munro@gmail.com> Co-authored-by: Andres Freund <andres@anarazel.de> Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt Discussion: https://postgr.es/m/20210223100344.llw5an2aklengrmn@alap3.anarazel.de Discussion: https://postgr.es/m/stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf@gcnactj4z56m
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‎doc/src/sgml/config.sgml

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@@ -2676,6 +2676,11 @@ include_dir 'conf.d'
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Selects the method for executing asynchronous I/O.
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Possible values are:
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<itemizedlist>
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<listitem>
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<para>
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<literal>worker</literal> (execute asynchronous I/O using worker processes)
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</para>
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</listitem>
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<listitem>
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<para>
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<literal>sync</literal> (execute asynchronous-eligible I/O synchronously)

‎src/backend/storage/aio/aio.c

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@@ -64,6 +64,7 @@ static void pgaio_io_wait(PgAioHandle *ioh, uint64 ref_generation);
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/* Options for io_method. */
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const struct config_enum_entry io_method_options[] = {
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{"sync", IOMETHOD_SYNC, false},
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{"worker", IOMETHOD_WORKER, false},
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{NULL, 0, false}
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};
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@@ -80,6 +81,7 @@ PgAioBackend *pgaio_my_backend;
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static const IoMethodOps *const pgaio_method_ops_table[] = {
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[IOMETHOD_SYNC] = &pgaio_sync_ops,
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[IOMETHOD_WORKER] = &pgaio_worker_ops,
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};
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/* callbacks for the configured io_method, set by assign_io_method */

‎src/backend/storage/aio/aio_init.c

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@@ -18,6 +18,7 @@
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#include "storage/aio.h"
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#include "storage/aio_internal.h"
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#include "storage/aio_subsys.h"
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#include "storage/io_worker.h"
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#include "storage/ipc.h"
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#include "storage/proc.h"
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#include "storage/shmem.h"
@@ -39,6 +40,11 @@ AioCtlShmemSize(void)
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static uint32
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AioProcs(void)
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{
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/*
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* While AIO workers don't need their own AIO context, we can't currently
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* guarantee nothing gets assigned to the a ProcNumber for an IO worker if
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* we just subtracted MAX_IO_WORKERS.
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*/
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return MaxBackends + NUM_AUXILIARY_PROCS;
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}
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@@ -223,6 +229,9 @@ pgaio_init_backend(void)
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/* shouldn't be initialized twice */
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Assert(!pgaio_my_backend);
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if (MyBackendType == B_IO_WORKER)
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return;
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if (MyProc == NULL || MyProcNumber >= AioProcs())
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elog(ERROR, "aio requires a normal PGPROC");
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