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Commit3cda10f

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Use atomic ops to hand out pages to scan in parallel scan.
With a lot of CPUs, the spinlock that protects the current scan locationin a parallel scan can become a bottleneck. Use an atomic fetch-and-addinstruction instead.David RowleyDiscussion:https://www.postgresql.org/message-id/CAKJS1f9tgsPhqBcoPjv9_KUPZvTLCZ4jy%3DB%3DbhqgaKn7cYzm-w@mail.gmail.com
1 parent0c504a8 commit3cda10f

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2 files changed

+62
-45
lines changed

2 files changed

+62
-45
lines changed

‎src/backend/access/heap/heapam.c‎

Lines changed: 59 additions & 43 deletions
Original file line numberDiff line numberDiff line change
@@ -58,6 +58,7 @@
5858
#include"catalog/namespace.h"
5959
#include"miscadmin.h"
6060
#include"pgstat.h"
61+
#include"port/atomics.h"
6162
#include"storage/bufmgr.h"
6263
#include"storage/freespace.h"
6364
#include"storage/lmgr.h"
@@ -89,6 +90,7 @@ static HeapScanDesc heap_beginscan_internal(Relation relation,
8990
boolis_bitmapscan,
9091
boolis_samplescan,
9192
booltemp_snap);
93+
staticvoidheap_parallelscan_startblock_init(HeapScanDescscan);
9294
staticBlockNumberheap_parallelscan_nextpage(HeapScanDescscan);
9395
staticHeapTupleheap_prepare_insert(Relationrelation,HeapTupletup,
9496
TransactionIdxid,CommandIdcid,intoptions);
@@ -510,6 +512,8 @@ heapgettup(HeapScanDesc scan,
510512
}
511513
if (scan->rs_parallel!=NULL)
512514
{
515+
heap_parallelscan_startblock_init(scan);
516+
513517
page=heap_parallelscan_nextpage(scan);
514518

515519
/* Other processes might have already finished the scan. */
@@ -812,6 +816,8 @@ heapgettup_pagemode(HeapScanDesc scan,
812816
}
813817
if (scan->rs_parallel!=NULL)
814818
{
819+
heap_parallelscan_startblock_init(scan);
820+
815821
page=heap_parallelscan_nextpage(scan);
816822

817823
/* Other processes might have already finished the scan. */
@@ -1535,14 +1541,10 @@ heap_rescan(HeapScanDesc scan,
15351541

15361542
/*
15371543
* Caller is responsible for making sure that all workers have
1538-
* finished the scan before calling this, so it really shouldn't be
1539-
* necessary to acquire the mutex at all. We acquire it anyway, just
1540-
* to be tidy.
1544+
* finished the scan before calling this.
15411545
*/
15421546
parallel_scan=scan->rs_parallel;
1543-
SpinLockAcquire(&parallel_scan->phs_mutex);
1544-
parallel_scan->phs_cblock=parallel_scan->phs_startblock;
1545-
SpinLockRelease(&parallel_scan->phs_mutex);
1547+
pg_atomic_write_u64(&parallel_scan->phs_nallocated,0);
15461548
}
15471549
}
15481550

@@ -1635,8 +1637,8 @@ heap_parallelscan_initialize(ParallelHeapScanDesc target, Relation relation,
16351637
!RelationUsesLocalBuffers(relation)&&
16361638
target->phs_nblocks>NBuffers /4;
16371639
SpinLockInit(&target->phs_mutex);
1638-
target->phs_cblock=InvalidBlockNumber;
16391640
target->phs_startblock=InvalidBlockNumber;
1641+
pg_atomic_write_u64(&target->phs_nallocated,0);
16401642
SerializeSnapshot(snapshot,target->phs_snapshot_data);
16411643
}
16421644

@@ -1660,20 +1662,17 @@ heap_beginscan_parallel(Relation relation, ParallelHeapScanDesc parallel_scan)
16601662
}
16611663

16621664
/* ----------------
1663-
*heap_parallelscan_nextpage -get the next page toscan
1665+
*heap_parallelscan_startblock_init -find and set thescan's startblock
16641666
*
1665-
*Get the next page to scan. Even if there are no pages left to scan,
1666-
*another backend could have grabbed a page to scan and not yet finished
1667-
*looking at it, so it doesn't follow that the scan is done when the
1668-
*first backend gets an InvalidBlockNumber return.
1667+
*Determine where the parallel seq scan should start. This function may
1668+
*be called many times, once by each parallel worker. We must be careful
1669+
*only to set the startblock once.
16691670
* ----------------
16701671
*/
1671-
staticBlockNumber
1672-
heap_parallelscan_nextpage(HeapScanDescscan)
1672+
staticvoid
1673+
heap_parallelscan_startblock_init(HeapScanDescscan)
16731674
{
1674-
BlockNumberpage=InvalidBlockNumber;
16751675
BlockNumbersync_startpage=InvalidBlockNumber;
1676-
BlockNumberreport_page=InvalidBlockNumber;
16771676
ParallelHeapScanDescparallel_scan;
16781677

16791678
Assert(scan->rs_parallel);
@@ -1705,46 +1704,63 @@ heap_parallelscan_nextpage(HeapScanDesc scan)
17051704
sync_startpage=ss_get_location(scan->rs_rd,scan->rs_nblocks);
17061705
gotoretry;
17071706
}
1708-
parallel_scan->phs_cblock=parallel_scan->phs_startblock;
17091707
}
1708+
SpinLockRelease(&parallel_scan->phs_mutex);
1709+
}
1710+
1711+
/* ----------------
1712+
*heap_parallelscan_nextpage - get the next page to scan
1713+
*
1714+
*Get the next page to scan. Even if there are no pages left to scan,
1715+
*another backend could have grabbed a page to scan and not yet finished
1716+
*looking at it, so it doesn't follow that the scan is done when the
1717+
*first backend gets an InvalidBlockNumber return.
1718+
* ----------------
1719+
*/
1720+
staticBlockNumber
1721+
heap_parallelscan_nextpage(HeapScanDescscan)
1722+
{
1723+
BlockNumberpage;
1724+
ParallelHeapScanDescparallel_scan;
1725+
uint64nallocated;
1726+
1727+
Assert(scan->rs_parallel);
1728+
parallel_scan=scan->rs_parallel;
17101729

17111730
/*
1712-
* The current block number is the next one that needs to be scanned,
1713-
* unless it's InvalidBlockNumber already, in which case there are no more
1714-
* blocks to scan. After remembering the current value, we must advance
1715-
* it so that the next call to this function returns the next block to be
1716-
* scanned.
1731+
* phs_nallocated tracks how many pages have been allocated to workers
1732+
* already. When phs_nallocated >= rs_nblocks, all blocks have been
1733+
* allocated.
1734+
*
1735+
* Because we use an atomic fetch-and-add to fetch the current value, the
1736+
* phs_nallocated counter will exceed rs_nblocks, because workers will
1737+
* still increment the value, when they try to allocate the next block but
1738+
* all blocks have been allocated already. The counter must be 64 bits
1739+
* wide because of that, to avoid wrapping around when rs_nblocks is close
1740+
* to 2^32.
1741+
*
1742+
* The actual page to return is calculated by adding the counter to the
1743+
* starting block number, modulo nblocks.
17171744
*/
1718-
page=parallel_scan->phs_cblock;
1719-
if (page!=InvalidBlockNumber)
1720-
{
1721-
parallel_scan->phs_cblock++;
1722-
if (parallel_scan->phs_cblock >=scan->rs_nblocks)
1723-
parallel_scan->phs_cblock=0;
1724-
if (parallel_scan->phs_cblock==parallel_scan->phs_startblock)
1725-
{
1726-
parallel_scan->phs_cblock=InvalidBlockNumber;
1727-
report_page=parallel_scan->phs_startblock;
1728-
}
1729-
}
1730-
1731-
/* Release the lock. */
1732-
SpinLockRelease(&parallel_scan->phs_mutex);
1745+
nallocated=pg_atomic_fetch_add_u64(&parallel_scan->phs_nallocated,1);
1746+
if (nallocated >=scan->rs_nblocks)
1747+
page=InvalidBlockNumber;/* all blocks have been allocated */
1748+
else
1749+
page= (nallocated+parallel_scan->phs_startblock) %scan->rs_nblocks;
17331750

17341751
/*
17351752
* Report scan location. Normally, we report the current page number.
17361753
* When we reach the end of the scan, though, we report the starting page,
17371754
* not the ending page, just so the starting positions for later scans
17381755
* doesn't slew backwards. We only report the position at the end of the
1739-
* scan once, though: subsequent callers will have report nothing, since
1740-
* they will have page == InvalidBlockNumber.
1756+
* scan once, though: subsequent callers will report nothing.
17411757
*/
17421758
if (scan->rs_syncscan)
17431759
{
1744-
if (report_page==InvalidBlockNumber)
1745-
report_page=page;
1746-
if (report_page!=InvalidBlockNumber)
1747-
ss_report_location(scan->rs_rd,report_page);
1760+
if (page!=InvalidBlockNumber)
1761+
ss_report_location(scan->rs_rd,page);
1762+
elseif (nallocated==scan->rs_nblocks)
1763+
ss_report_location(scan->rs_rd,parallel_scan->phs_startblock);
17481764
}
17491765

17501766
returnpage;

‎src/include/access/relscan.h‎

Lines changed: 3 additions & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -35,9 +35,10 @@ typedef struct ParallelHeapScanDescData
3535
Oidphs_relid;/* OID of relation to scan */
3636
boolphs_syncscan;/* report location to syncscan logic? */
3737
BlockNumberphs_nblocks;/* # blocks in relation at start of scan */
38-
slock_tphs_mutex;/* mutual exclusion forblock number fields */
38+
slock_tphs_mutex;/* mutual exclusion forsetting startblock */
3939
BlockNumberphs_startblock;/* starting block number */
40-
BlockNumberphs_cblock;/* current block number */
40+
pg_atomic_uint64phs_nallocated;/* number of blocks allocated to
41+
* workers so far. */
4142
charphs_snapshot_data[FLEXIBLE_ARRAY_MEMBER];
4243
}ParallelHeapScanDescData;
4344

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