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Commit4effd08

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Fix unfairness in all-cached parallel seq scan.
Commitb5a9b18 introduced block streaming infrastructure with a specialfast path for all-cached scans, and commitb7b0f3f connected theinfrastructure up to sequential scans. One of the fast pathmicro-optimizations had an unintended consequence: it interfered withparallel sequential scan's block range allocator (from commit56788d2),which has its own ramp-up and ramp-down algorithm when handing outgroups of pages to workers. A scan of an all-cached table could giveextra blocks to one worker, when others had finished. In some plans(probably already very bad plans, such as the one reported byAlexander), the unfairness could be magnified.An internal buffer of 16 block numbers is removed, keeping just a singleblock buffer for technical reasons.Back-patch to 17.Reported-by: Alexander Lakhin <exclusion@gmail.com>Discussion:https://postgr.es/m/63a63690-dd92-c809-0b47-af05459e95d1%40gmail.com
1 parentecd5645 commit4effd08

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1 file changed

+23
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‎src/backend/storage/aio/read_stream.c

Lines changed: 23 additions & 58 deletions
Original file line numberDiff line numberDiff line change
@@ -117,13 +117,10 @@ struct ReadStream
117117
booladvice_enabled;
118118

119119
/*
120-
* Small buffer of block numbers, useful for 'ungetting' to resolve flow
121-
* control problems when I/Os are split. Also useful for batch-loading
122-
* block numbers in the fast path.
120+
* One-block buffer to support 'ungetting' a block number, to resolve flow
121+
* control problems when I/Os are split.
123122
*/
124-
BlockNumberblocknums[16];
125-
int16blocknums_count;
126-
int16blocknums_next;
123+
BlockNumberbuffered_blocknum;
127124

128125
/*
129126
* The callback that will tell us which block numbers to read, and an
@@ -167,23 +164,23 @@ get_per_buffer_data(ReadStream *stream, int16 buffer_index)
167164
}
168165

169166
/*
170-
* Ask the callback which block it would like us to read next, with a small
171-
* buffer in front to allow read_stream_unget_block() to work and to allow the
172-
* fast path to skip this function and work directly from the array.
167+
* Ask the callback which block it would like us to read next, with a one block
168+
* buffer in front to allow read_stream_unget_block() to work.
173169
*/
174170
staticinlineBlockNumber
175171
read_stream_get_block(ReadStream*stream,void*per_buffer_data)
176172
{
177-
if (stream->blocknums_next<stream->blocknums_count)
178-
returnstream->blocknums[stream->blocknums_next++];
173+
BlockNumberblocknum;
179174

180-
/*
181-
* We only bother to fetch one at a time here (but see the fast path which
182-
* uses more).
183-
*/
184-
returnstream->callback(stream,
185-
stream->callback_private_data,
186-
per_buffer_data);
175+
blocknum=stream->buffered_blocknum;
176+
if (blocknum!=InvalidBlockNumber)
177+
stream->buffered_blocknum=InvalidBlockNumber;
178+
else
179+
blocknum=stream->callback(stream,
180+
stream->callback_private_data,
181+
per_buffer_data);
182+
183+
returnblocknum;
187184
}
188185

189186
/*
@@ -193,42 +190,12 @@ read_stream_get_block(ReadStream *stream, void *per_buffer_data)
193190
staticinlinevoid
194191
read_stream_unget_block(ReadStream*stream,BlockNumberblocknum)
195192
{
196-
if (stream->blocknums_next==stream->blocknums_count)
197-
{
198-
/* Never initialized or entirely consumed. Re-initialize. */
199-
stream->blocknums[0]=blocknum;
200-
stream->blocknums_count=1;
201-
stream->blocknums_next=0;
202-
}
203-
else
204-
{
205-
/* Must be the last value return from blocknums array. */
206-
Assert(stream->blocknums_next>0);
207-
stream->blocknums_next--;
208-
Assert(stream->blocknums[stream->blocknums_next]==blocknum);
209-
}
193+
/* We shouldn't ever unget more than one block. */
194+
Assert(stream->buffered_blocknum==InvalidBlockNumber);
195+
Assert(blocknum!=InvalidBlockNumber);
196+
stream->buffered_blocknum=blocknum;
210197
}
211198

212-
#ifndefREAD_STREAM_DISABLE_FAST_PATH
213-
staticvoid
214-
read_stream_fill_blocknums(ReadStream*stream)
215-
{
216-
BlockNumberblocknum;
217-
inti=0;
218-
219-
do
220-
{
221-
blocknum=stream->callback(stream,
222-
stream->callback_private_data,
223-
NULL);
224-
stream->blocknums[i++]=blocknum;
225-
}while (i<lengthof(stream->blocknums)&&
226-
blocknum!=InvalidBlockNumber);
227-
stream->blocknums_count=i;
228-
stream->blocknums_next=0;
229-
}
230-
#endif
231-
232199
staticvoid
233200
read_stream_start_pending_read(ReadStream*stream,boolsuppress_advice)
234201
{
@@ -530,6 +497,7 @@ read_stream_begin_impl(int flags,
530497
stream->queue_size=queue_size;
531498
stream->callback=callback;
532499
stream->callback_private_data=callback_private_data;
500+
stream->buffered_blocknum=InvalidBlockNumber;
533501

534502
/*
535503
* Skip the initial ramp-up phase if the caller says we're going to be
@@ -649,9 +617,7 @@ read_stream_next_buffer(ReadStream *stream, void **per_buffer_data)
649617
Assert(buffer!=InvalidBuffer);
650618

651619
/* Choose the next block to pin. */
652-
if (unlikely(stream->blocknums_next==stream->blocknums_count))
653-
read_stream_fill_blocknums(stream);
654-
next_blocknum=stream->blocknums[stream->blocknums_next++];
620+
next_blocknum=read_stream_get_block(stream,NULL);
655621

656622
if (likely(next_blocknum!=InvalidBlockNumber))
657623
{
@@ -827,9 +793,8 @@ read_stream_reset(ReadStream *stream)
827793
/* Stop looking ahead. */
828794
stream->distance=0;
829795

830-
/* Forget buffered block numbers and fast path state. */
831-
stream->blocknums_next=0;
832-
stream->blocknums_count=0;
796+
/* Forget buffered block number and fast path state. */
797+
stream->buffered_blocknum=InvalidBlockNumber;
833798
stream->fast_path= false;
834799

835800
/* Unpin anything that wasn't consumed. */

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