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help / color / mirror / Atom feedFrom: Andres Freund <andres@anarazel.de>
Subject: [PATCH v2.8 26/38] aio: Basic read_stream adjustments for real AIO
Date: Fri, 14 Mar 2025 11:39:34 -0400
Adapt the read stream logic for real AIO:
- If AIO is enabled, we shouldn't issue advice, but if it isn't, we should
continue issuing advice
- AIO benefits from reading ahead with direct IO
- While in read_stream_look_ahead(), we can use AIO batch submission mode for
increased efficiency
There is one comment talking about max_ios logic with "real asynchronous I/O"
that I am not sure about, so I left it alone for now.
There are further improvements we should consider, e.g. waiting to issue IOs
until we can issue multiple IOs at once. But that's left for a future change,
since it would involve additional heuristics.
---
src/backend/storage/aio/read_stream.c | 39 ++++++++++++++++++++-------
1 file changed, 29 insertions(+), 10 deletions(-)
diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c
index 0f1525f46c5..b44cc358f29 100644
--- a/src/backend/storage/aio/read_stream.c
+++ b/src/backend/storage/aio/read_stream.c
@@ -72,6 +72,7 @@
#include "postgres.h"
#include "miscadmin.h"
+#include "storage/aio.h"
#include "storage/fd.h"
#include "storage/smgr.h"
#include "storage/read_stream.h"
@@ -99,6 +100,7 @@ struct ReadStream
int16 pinned_buffers;
int16 distance;
int16 initialized_buffers;
+ bool sync_mode;
bool advice_enabled;
bool temporary;
@@ -420,6 +422,13 @@ read_stream_start_pending_read(ReadStream *stream)
static void
read_stream_look_ahead(ReadStream *stream)
{
+ /*
+ * Allow amortizing the cost of submitting IO over multiple IOs. This
+ * requires that we don't do any operations that could lead to a deadlock
+ * with staged-but-unsubmitted IO.
+ */
+ pgaio_enter_batchmode();
+
while (stream->ios_in_progress < stream->max_ios &&
stream->pinned_buffers + stream->pending_read_nblocks < stream->distance)
{
@@ -467,6 +476,7 @@ read_stream_look_ahead(ReadStream *stream)
{
/* We've hit the buffer or I/O limit. Rewind and stop here. */
read_stream_unget_block(stream, blocknum);
+ pgaio_exit_batchmode();
return;
}
}
@@ -501,6 +511,8 @@ read_stream_look_ahead(ReadStream *stream)
* time.
*/
Assert(stream->pinned_buffers > 0 || stream->distance == 0);
+
+ pgaio_exit_batchmode();
}
/*
@@ -556,12 +568,12 @@ read_stream_begin_impl(int flags,
max_ios = get_tablespace_io_concurrency(tablespace_id);
/*
- * XXX Since we don't have asynchronous I/O yet, if direct I/O is enabled
- * then just behave as though I/O concurrency is set to 0. Otherwise we
- * would look ahead pinning many buffers for no benefit, for lack of
- * advice and AIO.
+ * If real asynchronous I/O is disabled, and direct I/O is enabled, just
+ * behave as though I/O concurrency is set to 0. Otherwise we would look
+ * ahead pinning many buffers for no benefit, as the advice-based
+ * readahead doesn't support direct I/O.
*/
- if (io_direct_flags & IO_DIRECT_DATA)
+ if (io_method == IOMETHOD_SYNC && (io_direct_flags & IO_DIRECT_DATA))
max_ios = 0;
/* Cap to INT16_MAX to avoid overflowing below */
@@ -641,15 +653,19 @@ read_stream_begin_impl(int flags,
stream->per_buffer_data = (void *)
MAXALIGN(&stream->ios[Max(1, max_ios)]);
+ stream->sync_mode = io_method == IOMETHOD_SYNC;
+
#ifdef USE_PREFETCH
/*
- * This system supports prefetching advice. We can use it as long as
- * direct I/O isn't enabled, the caller hasn't promised sequential access
- * (overriding our detection heuristics), and max_ios hasn't been set to
- * zero.
+ * This system supports prefetching advice.
+ *
+ * Issue advice only if AIO is not used, direct I/O isn't enabled, the
+ * caller hasn't promised sequential access (overriding our detection
+ * heuristics), and max_ios hasn't been set to zero.
*/
- if ((io_direct_flags & IO_DIRECT_DATA) == 0 &&
+ if (stream->sync_mode &&
+ (io_direct_flags & IO_DIRECT_DATA) == 0 &&
(flags & READ_STREAM_SEQUENTIAL) == 0 &&
max_ios > 0)
stream->advice_enabled = true;
@@ -659,6 +675,9 @@ read_stream_begin_impl(int flags,
* For now, max_ios = 0 is interpreted as max_ios = 1 with advice disabled
* above. If we had real asynchronous I/O we might need a slightly
* different definition.
+ *
+ * FIXME: Not sure what different definition we would need? I guess we
+ * could add the READ_BUFFERS_SYNCHRONOUSLY flag automatically?
*/
if (max_ios == 0)
max_ios = 1;
--
2.48.1.76.g4e746b1a31.dirty
--ow5flh3n247znjrs
Content-Type: text/x-diff; charset=us-ascii
Content-Disposition: attachment;
filename="v2.8-0027-aio-Experimental-heuristics-to-increase-batchin.patch"
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Subject: Re: [PATCH v2.8 26/38] aio: Basic read_stream adjustments for real AIO
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