aio: Add io_method=io_uring

Performing AIO using io_uring can be considerably faster than
io_method=worker, particularly when lots of small IOs are issued, as
a) the context-switch overhead for worker based AIO becomes more significant
b) the number of IO workers can become limiting

io_uring, however, is linux specific and requires an additional compile-time
dependency (liburing).

This implementation is fairly simple and there are substantial optimization
opportunities.

The description of the existing AIO_IO_COMPLETION wait event is updated to
make the difference between it and the new AIO_IO_URING_EXECUTION clearer.

Reviewed-by: Noah Misch <noah@leadboat.com>
Reviewed-by: Jakub Wartak <jakub.wartak@enterprisedb.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

src/backend/storage/aio/Makefile

@@ -14,6 +14,7 @@ OBJS = \
 	aio_init.o \
 	aio_io.o \
 	aio_target.o \
+	method_io_uring.o \
 	method_sync.o \
 	method_worker.o \
 	read_stream.o

src/backend/storage/aio/aio.c

@@ -65,6 +65,9 @@ static void pgaio_io_wait(PgAioHandle *ioh, uint64 ref_generation);
 const struct config_enum_entry io_method_options[] = {
 	{"sync", IOMETHOD_SYNC, false},
 	{"worker", IOMETHOD_WORKER, false},
+#ifdef IOMETHOD_IO_URING_ENABLED
+	{"io_uring", IOMETHOD_IO_URING, false},
+#endif
 	{NULL, 0, false}
 };
 
@@ -82,6 +85,9 @@ PgAioBackend *pgaio_my_backend;
 static const IoMethodOps *const pgaio_method_ops_table[] = {
 	[IOMETHOD_SYNC] = &pgaio_sync_ops,
 	[IOMETHOD_WORKER] = &pgaio_worker_ops,
+#ifdef IOMETHOD_IO_URING_ENABLED
+	[IOMETHOD_IO_URING] = &pgaio_uring_ops,
+#endif
 };
 
 /* callbacks for the configured io_method, set by assign_io_method */
@@ -1118,6 +1124,41 @@ pgaio_closing_fd(int fd)
 	 * it's probably not worth it.
 	 */
 	pgaio_submit_staged();
+
+	/*
+	 * If requested by the IO method, wait for all IOs that use the
+	 * to-be-closed FD.
+	 */
+	if (pgaio_method_ops->wait_on_fd_before_close)
+	{
+		/*
+		 * As waiting for one IO to complete may complete multiple IOs, we
+		 * can't just use a mutable list iterator. The maximum number of
+		 * in-flight IOs is fairly small, so just restart the loop after
+		 * waiting for an IO.
+		 */
+		while (!dclist_is_empty(&pgaio_my_backend->in_flight_ios))
+		{
+			dlist_iter	iter;
+			PgAioHandle *ioh = NULL;
+
+			dclist_foreach(iter, &pgaio_my_backend->in_flight_ios)
+			{
+				ioh = dclist_container(PgAioHandle, node, iter.cur);
+
+				if (pgaio_io_uses_fd(ioh, fd))
+					break;
+				else
+					ioh = NULL;
+			}
+
+			if (!ioh)
+				break;
+
+			/* see comment in pgaio_io_wait_for_free() about raciness */
+			pgaio_io_wait(ioh, ioh->generation);
+		}
+	}
 }
 
 /*

src/backend/storage/aio/aio_io.c

@@ -188,3 +188,25 @@ pgaio_io_get_op_name(PgAioHandle *ioh)
 
 	return NULL;				/* silence compiler */
 }
+
+/*
+ * Used to determine if an IO needs to be waited upon before the file
+ * descriptor can be closed.
+ */
+bool
+pgaio_io_uses_fd(PgAioHandle *ioh, int fd)
+{
+	Assert(ioh->state >= PGAIO_HS_DEFINED);
+
+	switch (ioh->op)
+	{
+		case PGAIO_OP_READV:
+			return ioh->op_data.read.fd == fd;
+		case PGAIO_OP_WRITEV:
+			return ioh->op_data.write.fd == fd;
+		case PGAIO_OP_INVALID:
+			return false;
+	}
+
+	return false;				/* silence compiler */
+}

src/backend/storage/aio/meson.build

@@ -6,6 +6,7 @@ backend_sources += files(
   'aio_init.c',
   'aio_io.c',
   'aio_target.c',
+  'method_io_uring.c',
   'method_sync.c',
   'method_worker.c',
   'read_stream.c',

src/backend/storage/aio/method_io_uring.c

@@ -0,0 +1,484 @@
+/*-------------------------------------------------------------------------
+ *
+ * method_io_uring.c
+ *    AIO - perform AIO using Linux' io_uring
+ *
+ * For now we create one io_uring instance for each backend. These io_uring
+ * instances have to be created in postmaster, during startup, to allow other
+ * backends to process IO completions, if the issuing backend is currently
+ * busy doing other things. Other backends may not use another backend's
+ * io_uring instance to submit IO, that'd require additional locking that
+ * would likely be harmful for performance.
+ *
+ * We likely will want to introduce a backend-local io_uring instance in the
+ * future, e.g. for FE/BE network IO.
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *    src/backend/storage/aio/method_io_uring.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+/* included early, for IOMETHOD_IO_URING_ENABLED */
+#include "storage/aio.h"
+
+#ifdef IOMETHOD_IO_URING_ENABLED
+
+#include <liburing.h>
+
+#include "miscadmin.h"
+#include "storage/aio_internal.h"
+#include "storage/fd.h"
+#include "storage/proc.h"
+#include "storage/shmem.h"
+#include "storage/lwlock.h"
+#include "storage/procnumber.h"
+#include "utils/wait_event.h"
+
+
+/* number of completions processed at once */
+#define PGAIO_MAX_LOCAL_COMPLETED_IO 32
+
+
+/* Entry points for IoMethodOps. */
+static size_t pgaio_uring_shmem_size(void);
+static void pgaio_uring_shmem_init(bool first_time);
+static void pgaio_uring_init_backend(void);
+static int	pgaio_uring_submit(uint16 num_staged_ios, PgAioHandle **staged_ios);
+static void pgaio_uring_wait_one(PgAioHandle *ioh, uint64 ref_generation);
+
+/* helper functions */
+static void pgaio_uring_sq_from_io(PgAioHandle *ioh, struct io_uring_sqe *sqe);
+
+
+const IoMethodOps pgaio_uring_ops = {
+	/*
+	 * While io_uring mostly is OK with FDs getting closed while the IO is in
+	 * flight, that is not true for IOs submitted with IOSQE_ASYNC.
+	 *
+	 * See
+	 * https://postgr.es/m/5ons2rtmwarqqhhexb3dnqulw5rjgwgoct57vpdau4rujlrffj%403fls6d2mkiwc
+	 */
+	.wait_on_fd_before_close = true,
+
+	.shmem_size = pgaio_uring_shmem_size,
+	.shmem_init = pgaio_uring_shmem_init,
+	.init_backend = pgaio_uring_init_backend,
+
+	.submit = pgaio_uring_submit,
+	.wait_one = pgaio_uring_wait_one,
+};
+
+/*
+ * Per-backend state when using io_method=io_uring
+ *
+ * Align the whole struct to a cacheline boundary, to prevent false sharing
+ * between completion_lock and prior backend's io_uring_ring.
+ */
+typedef struct pg_attribute_aligned (PG_CACHE_LINE_SIZE)
+PgAioUringContext
+{
+	/*
+	 * Multiple backends can process completions for this backend's io_uring
+	 * instance (e.g. when the backend issuing IO is busy doing something
+	 * else).  To make that safe we have to ensure that only a single backend
+	 * gets io completions from the io_uring instance at a time.
+	 */
+	LWLock		completion_lock;
+
+	struct io_uring io_uring_ring;
+} PgAioUringContext;
+
+/* PgAioUringContexts for all backends */
+static PgAioUringContext *pgaio_uring_contexts;
+
+/* the current backend's context */
+static PgAioUringContext *pgaio_my_uring_context;
+
+
+static uint32
+pgaio_uring_procs(void)
+{
+	/*
+	 * We can subtract MAX_IO_WORKERS here as io workers are never used at the
+	 * same time as io_method=io_uring.
+	 */
+	return MaxBackends + NUM_AUXILIARY_PROCS - MAX_IO_WORKERS;
+}
+
+static Size
+pgaio_uring_context_shmem_size(void)
+{
+	return mul_size(pgaio_uring_procs(), sizeof(PgAioUringContext));
+}
+
+static size_t
+pgaio_uring_shmem_size(void)
+{
+	return pgaio_uring_context_shmem_size();
+}
+
+static void
+pgaio_uring_shmem_init(bool first_time)
+{
+	int			TotalProcs = MaxBackends + NUM_AUXILIARY_PROCS - MAX_IO_WORKERS;
+	bool		found;
+
+	pgaio_uring_contexts = (PgAioUringContext *)
+		ShmemInitStruct("AioUring", pgaio_uring_shmem_size(), &found);
+
+	if (found)
+		return;
+
+	for (int contextno = 0; contextno < TotalProcs; contextno++)
+	{
+		PgAioUringContext *context = &pgaio_uring_contexts[contextno];
+		int			ret;
+
+		/*
+		 * Right now a high TotalProcs will cause problems in two ways:
+		 *
+		 * - RLIMIT_NOFILE needs to be big enough to allow all
+		 * io_uring_queue_init() calls to succeed.
+		 *
+		 * - RLIMIT_NOFILE needs to be big enough to still have enough file
+		 * descriptors to satisfy set_max_safe_fds() left over. Or, even
+		 * better, have max_files_per_process left over FDs.
+		 *
+		 * We probably should adjust the soft RLIMIT_NOFILE to ensure that.
+		 *
+		 *
+		 * XXX: Newer versions of io_uring support sharing the workers that
+		 * execute some asynchronous IOs between io_uring instances. It might
+		 * be worth using that - also need to evaluate if that causes
+		 * noticeable additional contention?
+		 */
+		ret = io_uring_queue_init(io_max_concurrency, &context->io_uring_ring, 0);
+		if (ret < 0)
+		{
+			char	   *hint = NULL;
+			int			err = ERRCODE_INTERNAL_ERROR;
+
+			/* add hints for some failures that errno explains sufficiently */
+			if (-ret == EPERM)
+			{
+				err = ERRCODE_INSUFFICIENT_PRIVILEGE;
+				hint = _("Check if io_uring is disabled via /proc/sys/kernel/io_uring_disabled.");
+			}
+			else if (-ret == EMFILE)
+			{
+				err = ERRCODE_INSUFFICIENT_RESOURCES;
+				hint = psprintf(_("Consider increasing \"ulimit -n\" to at least %d."),
+								TotalProcs + max_files_per_process);
+			}
+			else if (-ret == ENOSYS)
+			{
+				err = ERRCODE_FEATURE_NOT_SUPPORTED;
+				hint = _("Kernel does not support io_uring.");
+			}
+
+			/* update errno to allow %m to work */
+			errno = -ret;
+
+			ereport(ERROR,
+					errcode(err),
+					errmsg("could not setup io_uring queue: %m"),
+					hint != NULL ? errhint("%s", hint) : 0);
+		}
+
+		LWLockInitialize(&context->completion_lock, LWTRANCHE_AIO_URING_COMPLETION);
+	}
+}
+
+static void
+pgaio_uring_init_backend(void)
+{
+	Assert(MyProcNumber < pgaio_uring_procs());
+
+	pgaio_my_uring_context = &pgaio_uring_contexts[MyProcNumber];
+}
+
+static int
+pgaio_uring_submit(uint16 num_staged_ios, PgAioHandle **staged_ios)
+{
+	struct io_uring *uring_instance = &pgaio_my_uring_context->io_uring_ring;
+	int			in_flight_before = dclist_count(&pgaio_my_backend->in_flight_ios);
+
+	Assert(num_staged_ios <= PGAIO_SUBMIT_BATCH_SIZE);
+
+	for (int i = 0; i < num_staged_ios; i++)
+	{
+		PgAioHandle *ioh = staged_ios[i];
+		struct io_uring_sqe *sqe;
+
+		sqe = io_uring_get_sqe(uring_instance);
+
+		if (!sqe)
+			elog(ERROR, "io_uring submission queue is unexpectedly full");
+
+		pgaio_io_prepare_submit(ioh);
+		pgaio_uring_sq_from_io(ioh, sqe);
+
+		/*
+		 * io_uring executes IO in process context if possible. That's
+		 * generally good, as it reduces context switching. When performing a
+		 * lot of buffered IO that means that copying between page cache and
+		 * userspace memory happens in the foreground, as it can't be
+		 * offloaded to DMA hardware as is possible when using direct IO. When
+		 * executing a lot of buffered IO this causes io_uring to be slower
+		 * than worker mode, as worker mode parallelizes the copying. io_uring
+		 * can be told to offload work to worker threads instead.
+		 *
+		 * If an IO is buffered IO and we already have IOs in flight or
+		 * multiple IOs are being submitted, we thus tell io_uring to execute
+		 * the IO in the background. We don't do so for the first few IOs
+		 * being submitted as executing in this process' context has lower
+		 * latency.
+		 */
+		if (in_flight_before > 4 && (ioh->flags & PGAIO_HF_BUFFERED))
+			io_uring_sqe_set_flags(sqe, IOSQE_ASYNC);
+
+		in_flight_before++;
+	}
+
+	while (true)
+	{
+		int			ret;
+
+		pgstat_report_wait_start(WAIT_EVENT_AIO_IO_URING_SUBMIT);
+		ret = io_uring_submit(uring_instance);
+		pgstat_report_wait_end();
+
+		if (ret == -EINTR)
+		{
+			pgaio_debug(DEBUG3,
+						"aio method uring: submit EINTR, nios: %d",
+						num_staged_ios);
+		}
+		else if (ret < 0)
+		{
+			/*
+			 * The io_uring_enter() manpage suggests that the appropriate
+			 * reaction to EAGAIN is:
+			 *
+			 * "The application should wait for some completions and try
+			 * again"
+			 *
+			 * However, it seems unlikely that that would help in our case, as
+			 * we apply a low limit to the number of outstanding IOs and thus
+			 * also outstanding completions, making it unlikely that we'd get
+			 * EAGAIN while the OS is in good working order.
+			 *
+			 * Additionally, it would be problematic to just wait here, our
+			 * caller might hold critical locks. It'd possibly lead to
+			 * delaying the crash-restart that seems likely to occur when the
+			 * kernel is under such heavy memory pressure.
+			 *
+			 * Update errno to allow %m to work.
+			 */
+			errno = -ret;
+			elog(PANIC, "io_uring submit failed: %m");
+		}
+		else if (ret != num_staged_ios)
+		{
+			/* likely unreachable, but if it is, we would need to re-submit */
+			elog(PANIC, "io_uring submit submitted only %d of %d",
+				 ret, num_staged_ios);
+		}
+		else
+		{
+			pgaio_debug(DEBUG4,
+						"aio method uring: submitted %d IOs",
+						num_staged_ios);
+			break;
+		}
+	}
+
+	return num_staged_ios;
+}
+
+static void
+pgaio_uring_drain_locked(PgAioUringContext *context)
+{
+	int			ready;
+	int			orig_ready;
+
+	Assert(LWLockHeldByMeInMode(&context->completion_lock, LW_EXCLUSIVE));
+
+	/*
+	 * Don't drain more events than available right now. Otherwise it's
+	 * plausible that one backend could get stuck, for a while, receiving CQEs
+	 * without actually processing them.
+	 */
+	orig_ready = ready = io_uring_cq_ready(&context->io_uring_ring);
+
+	while (ready > 0)
+	{
+		struct io_uring_cqe *cqes[PGAIO_MAX_LOCAL_COMPLETED_IO];
+		uint32		ncqes;
+
+		START_CRIT_SECTION();
+		ncqes =
+			io_uring_peek_batch_cqe(&context->io_uring_ring,
+									cqes,
+									Min(PGAIO_MAX_LOCAL_COMPLETED_IO, ready));
+		Assert(ncqes <= ready);
+
+		ready -= ncqes;
+
+		for (int i = 0; i < ncqes; i++)
+		{
+			struct io_uring_cqe *cqe = cqes[i];
+			PgAioHandle *ioh;
+
+			ioh = io_uring_cqe_get_data(cqe);
+			io_uring_cqe_seen(&context->io_uring_ring, cqe);
+
+			pgaio_io_process_completion(ioh, cqe->res);
+		}
+
+		END_CRIT_SECTION();
+
+		pgaio_debug(DEBUG3,
+					"drained %d/%d, now expecting %d",
+					ncqes, orig_ready, io_uring_cq_ready(&context->io_uring_ring));
+	}
+}
+
+static void
+pgaio_uring_wait_one(PgAioHandle *ioh, uint64 ref_generation)
+{
+	PgAioHandleState state;
+	ProcNumber	owner_procno = ioh->owner_procno;
+	PgAioUringContext *owner_context = &pgaio_uring_contexts[owner_procno];
+	bool		expect_cqe;
+	int			waited = 0;
+
+	/*
+	 * XXX: It would be nice to have a smarter locking scheme, nearly all the
+	 * time the backend owning the ring will consume the completions, making
+	 * the locking unnecessarily expensive.
+	 */
+	LWLockAcquire(&owner_context->completion_lock, LW_EXCLUSIVE);
+
+	while (true)
+	{
+		pgaio_debug_io(DEBUG3, ioh,
+					   "wait_one io_gen: %llu, ref_gen: %llu, cycle %d",
+					   (long long unsigned) ioh->generation,
+					   (long long unsigned) ref_generation,
+					   waited);
+
+		if (pgaio_io_was_recycled(ioh, ref_generation, &state) ||
+			state != PGAIO_HS_SUBMITTED)
+		{
+			/* the IO was completed by another backend */
+			break;
+		}
+		else if (io_uring_cq_ready(&owner_context->io_uring_ring))
+		{
+			/* no need to wait in the kernel, io_uring has a completion */
+			expect_cqe = true;
+		}
+		else
+		{
+			int			ret;
+			struct io_uring_cqe *cqes;
+
+			/* need to wait in the kernel */
+			pgstat_report_wait_start(WAIT_EVENT_AIO_IO_URING_EXECUTION);
+			ret = io_uring_wait_cqes(&owner_context->io_uring_ring, &cqes, 1, NULL, NULL);
+			pgstat_report_wait_end();
+
+			if (ret == -EINTR)
+			{
+				continue;
+			}
+			else if (ret != 0)
+			{
+				/* see comment after io_uring_submit() */
+				errno = -ret;
+				elog(PANIC, "io_uring wait failed: %m");
+			}
+			else
+			{
+				Assert(cqes != NULL);
+				expect_cqe = true;
+				waited++;
+			}
+		}
+
+		if (expect_cqe)
+		{
+			pgaio_uring_drain_locked(owner_context);
+		}
+	}
+
+	LWLockRelease(&owner_context->completion_lock);
+
+	pgaio_debug(DEBUG3,
+				"wait_one with %d sleeps",
+				waited);
+}
+
+static void
+pgaio_uring_sq_from_io(PgAioHandle *ioh, struct io_uring_sqe *sqe)
+{
+	struct iovec *iov;
+
+	switch (ioh->op)
+	{
+		case PGAIO_OP_READV:
+			iov = &pgaio_ctl->iovecs[ioh->iovec_off];
+			if (ioh->op_data.read.iov_length == 1)
+			{
+				io_uring_prep_read(sqe,
+								   ioh->op_data.read.fd,
+								   iov->iov_base,
+								   iov->iov_len,
+								   ioh->op_data.read.offset);
+			}
+			else
+			{
+				io_uring_prep_readv(sqe,
+									ioh->op_data.read.fd,
+									iov,
+									ioh->op_data.read.iov_length,
+									ioh->op_data.read.offset);
+
+			}
+			break;
+
+		case PGAIO_OP_WRITEV:
+			iov = &pgaio_ctl->iovecs[ioh->iovec_off];
+			if (ioh->op_data.write.iov_length == 1)
+			{
+				io_uring_prep_write(sqe,
+									ioh->op_data.write.fd,
+									iov->iov_base,
+									iov->iov_len,
+									ioh->op_data.write.offset);
+			}
+			else
+			{
+				io_uring_prep_writev(sqe,
+									 ioh->op_data.write.fd,
+									 iov,
+									 ioh->op_data.write.iov_length,
+									 ioh->op_data.write.offset);
+			}
+			break;
+
+		case PGAIO_OP_INVALID:
+			elog(ERROR, "trying to prepare invalid IO operation for execution");
+	}
+
+	io_uring_sqe_set_data(sqe, ioh);
+}
+
+#endif							/* IOMETHOD_IO_URING_ENABLED */

src/backend/storage/lmgr/lwlock.c

@@ -177,6 +177,7 @@ static const char *const BuiltinTrancheNames[] = {
 	[LWTRANCHE_SUBTRANS_SLRU] = "SubtransSLRU",
 	[LWTRANCHE_XACT_SLRU] = "XactSLRU",
 	[LWTRANCHE_PARALLEL_VACUUM_DSA] = "ParallelVacuumDSA",
+	[LWTRANCHE_AIO_URING_COMPLETION] = "AioUringCompletion",
 };
 
 StaticAssertDecl(lengthof(BuiltinTrancheNames) ==