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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 location
in a parallel scan can become a bottleneck. Use an atomic fetch-and-add
instruction instead.

David Rowley

Discussion: https://www.postgresql.org/message-id/CAKJS1f9tgsPhqBcoPjv9_KUPZvTLCZ4jy%3DB%3DbhqgaKn7cYzm-w@mail.gmail.com
This commit is contained in:
Heikki Linnakangas 2017-08-16 16:18:41 +03:00
parent 0c504a80cf
commit 3cda10f41b
2 changed files with 62 additions and 45 deletions
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102
src/backend/access/heap/heapam.c
View File

@ -58,6 +58,7 @@
#include "catalog/namespace.h"
#include "miscadmin.h"
#include "pgstat.h"
#include "port/atomics.h"
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/lmgr.h"
@ -89,6 +90,7 @@ static HeapScanDesc heap_beginscan_internal(Relation relation,
bool is_bitmapscan,
bool is_samplescan,
bool temp_snap);
static void heap_parallelscan_startblock_init(HeapScanDesc scan);
static BlockNumber heap_parallelscan_nextpage(HeapScanDesc scan);
static HeapTuple heap_prepare_insert(Relation relation, HeapTuple tup,
TransactionId xid, CommandId cid, int options);
@ -510,6 +512,8 @@ heapgettup(HeapScanDesc scan,
}
if (scan->rs_parallel != NULL)
{
heap_parallelscan_startblock_init(scan);
page = heap_parallelscan_nextpage(scan);
/* Other processes might have already finished the scan. */
@ -812,6 +816,8 @@ heapgettup_pagemode(HeapScanDesc scan,
}
if (scan->rs_parallel != NULL)
{
heap_parallelscan_startblock_init(scan);
page = heap_parallelscan_nextpage(scan);
/* Other processes might have already finished the scan. */
@ -1535,14 +1541,10 @@ heap_rescan(HeapScanDesc scan,
/*
* Caller is responsible for making sure that all workers have
* finished the scan before calling this, so it really shouldn't be
* necessary to acquire the mutex at all. We acquire it anyway, just
* to be tidy.
* finished the scan before calling this.
*/
parallel_scan = scan->rs_parallel;
SpinLockAcquire(&parallel_scan->phs_mutex);
parallel_scan->phs_cblock = parallel_scan->phs_startblock;
SpinLockRelease(&parallel_scan->phs_mutex);
pg_atomic_write_u64(&parallel_scan->phs_nallocated, 0);
}
}
@ -1635,8 +1637,8 @@ heap_parallelscan_initialize(ParallelHeapScanDesc target, Relation relation,
!RelationUsesLocalBuffers(relation) &&
target->phs_nblocks > NBuffers / 4;
SpinLockInit(&target->phs_mutex);
target->phs_cblock = InvalidBlockNumber;
target->phs_startblock = InvalidBlockNumber;
pg_atomic_write_u64(&target->phs_nallocated, 0);
SerializeSnapshot(snapshot, target->phs_snapshot_data);
}
@ -1660,20 +1662,17 @@ heap_beginscan_parallel(Relation relation, ParallelHeapScanDesc parallel_scan)
}
/* ----------------
* heap_parallelscan_nextpage - get the next page to scan
* heap_parallelscan_startblock_init - find and set the scan's startblock
*
* Get the next page to scan. Even if there are no pages left to scan,
* another backend could have grabbed a page to scan and not yet finished
* looking at it, so it doesn't follow that the scan is done when the
* first backend gets an InvalidBlockNumber return.
* Determine where the parallel seq scan should start. This function may
* be called many times, once by each parallel worker. We must be careful
* only to set the startblock once.
* ----------------
*/
static BlockNumber
heap_parallelscan_nextpage(HeapScanDesc scan)
static void
heap_parallelscan_startblock_init(HeapScanDesc scan)
{
BlockNumber page = InvalidBlockNumber;
BlockNumber sync_startpage = InvalidBlockNumber;
BlockNumber report_page = InvalidBlockNumber;
ParallelHeapScanDesc parallel_scan;
Assert(scan->rs_parallel);
@ -1705,46 +1704,63 @@ retry:
sync_startpage = ss_get_location(scan->rs_rd, scan->rs_nblocks);
goto retry;
}
parallel_scan->phs_cblock = parallel_scan->phs_startblock;
}
SpinLockRelease(&parallel_scan->phs_mutex);
}
/* ----------------
* heap_parallelscan_nextpage - get the next page to scan
*
* Get the next page to scan. Even if there are no pages left to scan,
* another backend could have grabbed a page to scan and not yet finished
* looking at it, so it doesn't follow that the scan is done when the
* first backend gets an InvalidBlockNumber return.
* ----------------
*/
static BlockNumber
heap_parallelscan_nextpage(HeapScanDesc scan)
{
BlockNumber page;
ParallelHeapScanDesc parallel_scan;
uint64 nallocated;
Assert(scan->rs_parallel);
parallel_scan = scan->rs_parallel;
/*
* The current block number is the next one that needs to be scanned,
* unless it's InvalidBlockNumber already, in which case there are no more
* blocks to scan. After remembering the current value, we must advance
* it so that the next call to this function returns the next block to be
* scanned.
* phs_nallocated tracks how many pages have been allocated to workers
* already. When phs_nallocated >= rs_nblocks, all blocks have been
* allocated.
*
* Because we use an atomic fetch-and-add to fetch the current value, the
* phs_nallocated counter will exceed rs_nblocks, because workers will
* still increment the value, when they try to allocate the next block but
* all blocks have been allocated already. The counter must be 64 bits
* wide because of that, to avoid wrapping around when rs_nblocks is close
* to 2^32.
*
* The actual page to return is calculated by adding the counter to the
* starting block number, modulo nblocks.
*/
page = parallel_scan->phs_cblock;
if (page != InvalidBlockNumber)
{
parallel_scan->phs_cblock++;
if (parallel_scan->phs_cblock >= scan->rs_nblocks)
parallel_scan->phs_cblock = 0;
if (parallel_scan->phs_cblock == parallel_scan->phs_startblock)
{
parallel_scan->phs_cblock = InvalidBlockNumber;
report_page = parallel_scan->phs_startblock;
}
}
/* Release the lock. */
SpinLockRelease(&parallel_scan->phs_mutex);
nallocated = pg_atomic_fetch_add_u64(&parallel_scan->phs_nallocated, 1);
if (nallocated >= scan->rs_nblocks)
page = InvalidBlockNumber; /* all blocks have been allocated */
else
page = (nallocated + parallel_scan->phs_startblock) % scan->rs_nblocks;
/*
* Report scan location. Normally, we report the current page number.
* When we reach the end of the scan, though, we report the starting page,
* not the ending page, just so the starting positions for later scans
* doesn't slew backwards. We only report the position at the end of the
* scan once, though: subsequent callers will have report nothing, since
* they will have page == InvalidBlockNumber.
* scan once, though: subsequent callers will report nothing.
*/
if (scan->rs_syncscan)
{
if (report_page == InvalidBlockNumber)
report_page = page;
if (report_page != InvalidBlockNumber)
ss_report_location(scan->rs_rd, report_page);
if (page != InvalidBlockNumber)
ss_report_location(scan->rs_rd, page);
else if (nallocated == scan->rs_nblocks)
ss_report_location(scan->rs_rd, parallel_scan->phs_startblock);
}
return page;

5
src/include/access/relscan.h
View File

@ -35,9 +35,10 @@ typedef struct ParallelHeapScanDescData
Oid phs_relid; /* OID of relation to scan */
bool phs_syncscan; /* report location to syncscan logic? */
BlockNumber phs_nblocks; /* # blocks in relation at start of scan */
slock_t phs_mutex; /* mutual exclusion for block number fields */
slock_t phs_mutex; /* mutual exclusion for setting startblock */
BlockNumber phs_startblock; /* starting block number */
BlockNumber phs_cblock; /* current block number */
pg_atomic_uint64 phs_nallocated; /* number of blocks allocated to
* workers so far. */
char phs_snapshot_data[FLEXIBLE_ARRAY_MEMBER];
} ParallelHeapScanDescData;