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Commitf4d0730

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anarazelmacdice
andcommitted
aio: Basic read_stream adjustments for real AIO
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- If effective_io_concurrency=0, pass READ_BUFFERS_SYNCHRONOUSLY to StartReadBuffers() to ensure synchronous IO executionThere are further improvements we should consider:- While in read_stream_look_ahead(), we can use AIO batch submission mode for increased efficiency. That however requires care to avoid deadlocks and thus done separately.- It can be beneficial to defer starting new IOs until we can issue multiple IOs at once. That however requires non-trivial heuristics to decide when to do so.Reviewed-by: Noah Misch <noah@leadboat.com>Co-authored-by: Andres Freund <andres@anarazel.de>Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
1 parentb27f863 commitf4d0730

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‎src/backend/storage/aio/read_stream.c‎

Lines changed: 26 additions & 13 deletions
Original file line numberDiff line numberDiff line change
@@ -72,6 +72,7 @@
7272
#include"postgres.h"
7373

7474
#include"miscadmin.h"
75+
#include"storage/aio.h"
7576
#include"storage/fd.h"
7677
#include"storage/smgr.h"
7778
#include"storage/read_stream.h"
@@ -99,6 +100,8 @@ struct ReadStream
99100
int16pinned_buffers;
100101
int16distance;
101102
int16initialized_buffers;
103+
intread_buffers_flags;
104+
boolsync_mode;/* using io_method=sync */
102105
booladvice_enabled;
103106
booltemporary;
104107

@@ -250,7 +253,7 @@ read_stream_start_pending_read(ReadStream *stream)
250253
Assert(stream->next_buffer_index==stream->oldest_buffer_index);
251254

252255
/* Do we need to issue read-ahead advice? */
253-
flags=0;
256+
flags=stream->read_buffers_flags;
254257
if (stream->advice_enabled)
255258
{
256259
if (stream->pending_read_blocknum==stream->seq_blocknum)
@@ -261,7 +264,7 @@ read_stream_start_pending_read(ReadStream *stream)
261264
* then stay of the way of the kernel's own read-ahead.
262265
*/
263266
if (stream->seq_until_processed!=InvalidBlockNumber)
264-
flags=READ_BUFFERS_ISSUE_ADVICE;
267+
flags|=READ_BUFFERS_ISSUE_ADVICE;
265268
}
266269
else
267270
{
@@ -272,7 +275,7 @@ read_stream_start_pending_read(ReadStream *stream)
272275
*/
273276
stream->seq_until_processed=stream->pending_read_blocknum;
274277
if (stream->pinned_buffers>0)
275-
flags=READ_BUFFERS_ISSUE_ADVICE;
278+
flags|=READ_BUFFERS_ISSUE_ADVICE;
276279
}
277280
}
278281

@@ -613,27 +616,33 @@ read_stream_begin_impl(int flags,
613616
stream->per_buffer_data= (void*)
614617
MAXALIGN(&stream->ios[Max(1,max_ios)]);
615618

619+
stream->sync_mode=io_method==IOMETHOD_SYNC;
620+
616621
#ifdefUSE_PREFETCH
617622

618623
/*
619-
*This system supports prefetchingadvice. We can use it as long as
620-
* direct I/O isn't enabled, the caller hasn't promised sequential access
621-
*(overriding our detection heuristics), and max_ioshasn'tbeen set to
622-
* zero.
624+
*Read-aheadadvice simulating asynchronous I/O with synchronous calls.
625+
*Issue advice only if AIO is not used,direct I/O isn't enabled, the
626+
*callerhasn'tpromised sequential access (overriding our detection
627+
*heuristics), and max_ios hasn't been set tozero.
623628
*/
624-
if ((io_direct_flags&IO_DIRECT_DATA)==0&&
629+
if (stream->sync_mode&&
630+
(io_direct_flags&IO_DIRECT_DATA)==0&&
625631
(flags&READ_STREAM_SEQUENTIAL)==0&&
626632
max_ios>0)
627633
stream->advice_enabled= true;
628634
#endif
629635

630636
/*
631-
*For now,max_ios= 0 is interpreted as max_ios = 1 with advice disabled
632-
*above. If we had real asynchronousI/O we might need a slightly
633-
*different definition.
637+
*Settingmax_iosto zero disables AIO and advice-based pseudo AIO, but
638+
*we still need to allocate space to combine and run oneI/O. Bump it up
639+
*to one, and remember to ask for synchronous I/O only.
634640
*/
635641
if (max_ios==0)
642+
{
636643
max_ios=1;
644+
stream->read_buffers_flags=READ_BUFFERS_SYNCHRONOUSLY;
645+
}
637646

638647
/*
639648
* Capture stable values for these two GUC-derived numbers for the
@@ -777,6 +786,11 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
777786

778787
if (likely(next_blocknum!=InvalidBlockNumber))
779788
{
789+
intflags=stream->read_buffers_flags;
790+
791+
if (stream->advice_enabled)
792+
flags |=READ_BUFFERS_ISSUE_ADVICE;
793+
780794
/*
781795
* Pin a buffer for the next call. Same buffer entry, and
782796
* arbitrary I/O entry (they're all free). We don't have to
@@ -792,8 +806,7 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
792806
if (likely(!StartReadBuffer(&stream->ios[0].op,
793807
&stream->buffers[oldest_buffer_index],
794808
next_blocknum,
795-
stream->advice_enabled ?
796-
READ_BUFFERS_ISSUE_ADVICE :0)))
809+
flags)))
797810
{
798811
/* Fast return. */
799812
returnbuffer;

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