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Commitaa1e8c2

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Improve read_stream.c's fast path.
The "fast path" for well cached scans that don't do any I/O wasaccidentally coded in a way that could only be triggered by pg_prewarm'susage pattern, which starts out with a higher distance because of theflags it passes in. We want it to work for streaming sequential scanstoo, once that patch is committed. Adjust.Reviewed-by: Melanie Plageman <melanieplageman@gmail.com>Discussion:https://postgr.es/m/CA%2BhUKGKXZALJ%3D6aArUsXRJzBm%3Dqvc4AWp7%3DiJNXJQqpbRLnD_w%40mail.gmail.com
1 parent9e73869 commitaa1e8c2

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1 file changed

+31
-44
lines changed

1 file changed

+31
-44
lines changed

‎src/backend/storage/aio/read_stream.c

Lines changed: 31 additions & 44 deletions
Original file line numberDiff line numberDiff line change
@@ -169,7 +169,7 @@ get_per_buffer_data(ReadStream *stream, int16 buffer_index)
169169
/*
170170
* Ask the callback which block it would like us to read next, with a small
171171
* buffer in front to allow read_stream_unget_block() to work and to allow the
172-
* fast path to workin batches.
172+
* fast path toskip this function andworkdirectly from the array.
173173
*/
174174
staticinlineBlockNumber
175175
read_stream_get_block(ReadStream*stream,void*per_buffer_data)
@@ -578,13 +578,12 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
578578
if (likely(stream->fast_path))
579579
{
580580
BlockNumbernext_blocknum;
581-
boolneed_wait;
582581

583582
/* Fast path assumptions. */
584583
Assert(stream->ios_in_progress==0);
585584
Assert(stream->pinned_buffers==1);
586585
Assert(stream->distance==1);
587-
Assert(stream->pending_read_nblocks==1);
586+
Assert(stream->pending_read_nblocks==0);
588587
Assert(stream->per_buffer_data_size==0);
589588

590589
/* We're going to return the buffer we pinned last time. */
@@ -594,58 +593,45 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
594593
buffer=stream->buffers[oldest_buffer_index];
595594
Assert(buffer!=InvalidBuffer);
596595

597-
/*
598-
* Pin a buffer for the next call. Same buffer entry, and arbitrary
599-
* I/O entry (they're all free).
600-
*/
601-
need_wait=StartReadBuffer(&stream->ios[0].op,
602-
&stream->buffers[oldest_buffer_index],
603-
stream->pending_read_blocknum,
604-
stream->advice_enabled ?
605-
READ_BUFFERS_ISSUE_ADVICE :0);
606-
607-
/* Choose the block the next call will pin. */
596+
/* Choose the next block to pin. */
608597
if (unlikely(stream->blocknums_next==stream->blocknums_count))
609598
read_stream_fill_blocknums(stream);
610599
next_blocknum=stream->blocknums[stream->blocknums_next++];
611600

612-
/*
613-
* Fast return if the next call doesn't require I/O for the buffer we
614-
* just pinned, and we have a block number to give it as a pending
615-
* read.
616-
*/
617-
if (likely(!need_wait&&next_blocknum!=InvalidBlockNumber))
601+
if (likely(next_blocknum!=InvalidBlockNumber))
618602
{
619-
stream->pending_read_blocknum=next_blocknum;
620-
returnbuffer;
621-
}
622-
623-
/*
624-
* For anything more complex, set up some more state and take the slow
625-
* path next time.
626-
*/
627-
stream->fast_path= false;
603+
/*
604+
* Pin a buffer for the next call. Same buffer entry, and
605+
* arbitrary I/O entry (they're all free). We don't have to
606+
* adjust pinned_buffers because we're transferring one to caller
607+
* but pinning one more.
608+
*/
609+
if (likely(!StartReadBuffer(&stream->ios[0].op,
610+
&stream->buffers[oldest_buffer_index],
611+
next_blocknum,
612+
stream->advice_enabled ?
613+
READ_BUFFERS_ISSUE_ADVICE :0)))
614+
{
615+
/* Fast return. */
616+
returnbuffer;
617+
}
628618

629-
if (need_wait)
630-
{
631619
/* Next call must wait for I/O for the newly pinned buffer. */
632620
stream->oldest_io_index=0;
633621
stream->next_io_index=stream->max_ios>1 ?1 :0;
634622
stream->ios_in_progress=1;
635623
stream->ios[0].buffer_index=oldest_buffer_index;
636624
stream->seq_blocknum=next_blocknum+1;
637625
}
638-
if (next_blocknum==InvalidBlockNumber)
639-
{
640-
/* Next call hits end of stream and can't pin anything more. */
641-
stream->distance=0;
642-
stream->pending_read_nblocks=0;
643-
}
644626
else
645627
{
646-
/* Set up the pending read. */
647-
stream->pending_read_blocknum=next_blocknum;
628+
/* No more blocks, end of stream. */
629+
stream->distance=0;
630+
stream->oldest_buffer_index=stream->next_buffer_index;
631+
stream->pinned_buffers=0;
648632
}
633+
634+
stream->fast_path= false;
649635
returnbuffer;
650636
}
651637
#endif
@@ -762,15 +748,11 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
762748
if (stream->ios_in_progress==0&&
763749
stream->pinned_buffers==1&&
764750
stream->distance==1&&
765-
stream->pending_read_nblocks==1&&
751+
stream->pending_read_nblocks==0&&
766752
stream->per_buffer_data_size==0)
767753
{
768754
stream->fast_path= true;
769755
}
770-
else
771-
{
772-
stream->fast_path= false;
773-
}
774756
#endif
775757

776758
returnbuffer;
@@ -790,6 +772,11 @@ read_stream_reset(ReadStream *stream)
790772
/* Stop looking ahead. */
791773
stream->distance=0;
792774

775+
/* Forget buffered block numbers and fast path state. */
776+
stream->blocknums_next=0;
777+
stream->blocknums_count=0;
778+
stream->fast_path= false;
779+
793780
/* Unpin anything that wasn't consumed. */
794781
while ((buffer=read_stream_next_buffer(stream,NULL))!=InvalidBuffer)
795782
ReleaseBuffer(buffer);

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