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Commit7714c63

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Remove extra copies of LogwrtResult.
This simplifies the code a little bit. The new rule is that to updateXLogCtl->LogwrtResult, you must hold both WALWriteLock and info_lck, whereasbefore we had two copies, one that was protected by WALWriteLock and anotherprotected by info_lck. The code that updates them was already holding bothlocks, so merging the two is trivial.The third copy, XLogCtl->Insert.LogwrtResult, was not totally redundant, itwas used in AdvanceXLInsertBuffer to update the backend-local copy, beforeacquiring the info_lck to read the up-to-date value. But the value of thatseems dubious; at best it's saving one spinlock acquisition per completedWAL page, which is not significant compared to all the other work involved.And in practice, it's probably not saving even that much.
1 parent3b682df commit7714c63

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1 file changed

+22
-52
lines changed
  • src/backend/access/transam

1 file changed

+22
-52
lines changed

‎src/backend/access/transam/xlog.c‎

Lines changed: 22 additions & 52 deletions
Original file line numberDiff line numberDiff line change
@@ -289,35 +289,16 @@ static XLogRecPtr RedoStartLSN = {0, 0};
289289
* These structs are identical but are declared separately to indicate their
290290
* slightly different functions.
291291
*
292-
* We do a lot of pushups to minimize the amount of access to lockable
293-
* shared memory values. There are actually three shared-memory copies of
294-
* LogwrtResult, plus one unshared copy in each backend. Here's how it works:
295-
*XLogCtl->LogwrtResult is protected by info_lck
296-
*XLogCtl->Write.LogwrtResult is protected by WALWriteLock
297-
*XLogCtl->Insert.LogwrtResult is protected by WALInsertLock
298-
* One must hold the associated lock to read or write any of these, but
299-
* of course no lock is needed to read/write the unshared LogwrtResult.
300-
*
301-
* XLogCtl->LogwrtResult and XLogCtl->Write.LogwrtResult are both "always
302-
* right", since both are updated by a write or flush operation before
303-
* it releases WALWriteLock. The point of keeping XLogCtl->Write.LogwrtResult
304-
* is that it can be examined/modified by code that already holds WALWriteLock
305-
* without needing to grab info_lck as well.
306-
*
307-
* XLogCtl->Insert.LogwrtResult may lag behind the reality of the other two,
308-
* but is updated when convenient.Again, it exists for the convenience of
309-
* code that is already holding WALInsertLock but not the other locks.
310-
*
311-
* The unshared LogwrtResult may lag behind any or all of these, and again
312-
* is updated when convenient.
292+
* To read XLogCtl->LogwrtResult, you must hold either info_lck or
293+
* WALWriteLock. To update it, you need to hold both locks. The point of
294+
* this arrangement is that the value can be examined by code that already
295+
* holds WALWriteLock without needing to grab info_lck as well. In addition
296+
* to the shared variable, each backend has a private copy of LogwrtResult,
297+
* which is updated when convenient.
313298
*
314299
* The request bookkeeping is simpler: there is a shared XLogCtl->LogwrtRqst
315300
* (protected by info_lck), but we don't need to cache any copies of it.
316301
*
317-
* Note that this all works because the request and result positions can only
318-
* advance forward, never back up, and so we can easily determine which of two
319-
* values is "more up to date".
320-
*
321302
* info_lck is only held long enough to read/update the protected variables,
322303
* so it's a plain spinlock. The other locks are held longer (potentially
323304
* over I/O operations), so we use LWLocks for them. These locks are:
@@ -354,7 +335,6 @@ typedef struct XLogwrtResult
354335
*/
355336
typedefstructXLogCtlInsert
356337
{
357-
XLogwrtResultLogwrtResult;/* a recent value of LogwrtResult */
358338
XLogRecPtrPrevRecord;/* start of previously-inserted record */
359339
intcurridx;/* current block index in cache */
360340
XLogPageHeadercurrpage;/* points to header of block in cache */
@@ -388,7 +368,6 @@ typedef struct XLogCtlInsert
388368
*/
389369
typedefstructXLogCtlWrite
390370
{
391-
XLogwrtResultLogwrtResult;/* current value of LogwrtResult */
392371
intcurridx;/* cache index of next block to write */
393372
pg_time_tlastSegSwitchTime;/* time of last xlog segment switch */
394373
}XLogCtlWrite;
@@ -403,7 +382,6 @@ typedef struct XLogCtlData
403382

404383
/* Protected by info_lck: */
405384
XLogwrtRqstLogwrtRqst;
406-
XLogwrtResultLogwrtResult;
407385
uint32ckptXidEpoch;/* nextXID & epoch of latest checkpoint */
408386
TransactionIdckptXid;
409387
XLogRecPtrasyncXactLSN;/* LSN of newest async commit/abort */
@@ -413,6 +391,12 @@ typedef struct XLogCtlData
413391
/* Protected by WALWriteLock: */
414392
XLogCtlWriteWrite;
415393

394+
/*
395+
* Protected by info_lck and WALWriteLock (you must hold either lock to
396+
* read it, but both to update)
397+
*/
398+
XLogwrtResultLogwrtResult;
399+
416400
/*
417401
* These values do not change after startup, although the pointed-to pages
418402
* and xlblocks values certainly do. Permission to read/write the pages
@@ -1015,7 +999,7 @@ begin:;
1015999
}
10161000

10171001
LWLockAcquire(WALWriteLock,LW_EXCLUSIVE);
1018-
LogwrtResult=XLogCtl->Write.LogwrtResult;
1002+
LogwrtResult=XLogCtl->LogwrtResult;
10191003
if (!XLByteLE(RecPtr,LogwrtResult.Flush))
10201004
{
10211005
XLogwrtRqstFlushRqst;
@@ -1188,8 +1172,6 @@ begin:;
11881172
SpinLockRelease(&xlogctl->info_lck);
11891173
}
11901174

1191-
Write->LogwrtResult=LogwrtResult;
1192-
11931175
LWLockRelease(WALWriteLock);
11941176

11951177
updrqst= false;/* done already */
@@ -1477,18 +1459,13 @@ static bool
14771459
AdvanceXLInsertBuffer(boolnew_segment)
14781460
{
14791461
XLogCtlInsert*Insert=&XLogCtl->Insert;
1480-
XLogCtlWrite*Write=&XLogCtl->Write;
14811462
intnextidx=NextBufIdx(Insert->curridx);
14821463
boolupdate_needed= true;
14831464
XLogRecPtrOldPageRqstPtr;
14841465
XLogwrtRqstWriteRqst;
14851466
XLogRecPtrNewPageEndPtr;
14861467
XLogPageHeaderNewPage;
14871468

1488-
/* Use Insert->LogwrtResult copy if it's more fresh */
1489-
if (XLByteLT(LogwrtResult.Write,Insert->LogwrtResult.Write))
1490-
LogwrtResult=Insert->LogwrtResult;
1491-
14921469
/*
14931470
* Get ending-offset of the buffer page we need to replace (this may be
14941471
* zero if the buffer hasn't been used yet). Fall through if it's already
@@ -1516,21 +1493,19 @@ AdvanceXLInsertBuffer(bool new_segment)
15161493

15171494
update_needed= false;/* Did the shared-request update */
15181495

1519-
if (XLByteLE(OldPageRqstPtr,LogwrtResult.Write))
1520-
{
1521-
/* OK, someone wrote it already */
1522-
Insert->LogwrtResult=LogwrtResult;
1523-
}
1524-
else
1496+
/*
1497+
* Now that we have an up-to-date LogwrtResult value, see if we still
1498+
* need to write it or if someone else already did.
1499+
*/
1500+
if (!XLByteLE(OldPageRqstPtr,LogwrtResult.Write))
15251501
{
15261502
/* Must acquire write lock */
15271503
LWLockAcquire(WALWriteLock,LW_EXCLUSIVE);
1528-
LogwrtResult=Write->LogwrtResult;
1504+
LogwrtResult=XLogCtl->LogwrtResult;
15291505
if (XLByteLE(OldPageRqstPtr,LogwrtResult.Write))
15301506
{
15311507
/* OK, someone wrote it already */
15321508
LWLockRelease(WALWriteLock);
1533-
Insert->LogwrtResult=LogwrtResult;
15341509
}
15351510
else
15361511
{
@@ -1544,7 +1519,6 @@ AdvanceXLInsertBuffer(bool new_segment)
15441519
WriteRqst.Flush.xrecoff=0;
15451520
XLogWrite(WriteRqst, false, false);
15461521
LWLockRelease(WALWriteLock);
1547-
Insert->LogwrtResult=LogwrtResult;
15481522
TRACE_POSTGRESQL_WAL_BUFFER_WRITE_DIRTY_DONE();
15491523
}
15501524
}
@@ -1697,7 +1671,7 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
16971671
/*
16981672
* Update local LogwrtResult (caller probably did this already, but...)
16991673
*/
1700-
LogwrtResult=Write->LogwrtResult;
1674+
LogwrtResult=XLogCtl->LogwrtResult;
17011675

17021676
/*
17031677
* Since successive pages in the xlog cache are consecutively allocated,
@@ -1931,8 +1905,6 @@ XLogWrite(XLogwrtRqst WriteRqst, bool flexible, bool xlog_switch)
19311905
xlogctl->LogwrtRqst.Flush=LogwrtResult.Flush;
19321906
SpinLockRelease(&xlogctl->info_lck);
19331907
}
1934-
1935-
Write->LogwrtResult=LogwrtResult;
19361908
}
19371909

19381910
/*
@@ -2126,7 +2098,7 @@ XLogFlush(XLogRecPtr record)
21262098
continue;
21272099
}
21282100
/* Got the lock */
2129-
LogwrtResult=XLogCtl->Write.LogwrtResult;
2101+
LogwrtResult=XLogCtl->LogwrtResult;
21302102
if (!XLByteLE(record,LogwrtResult.Flush))
21312103
{
21322104
/* try to write/flush later additions to XLOG as well */
@@ -2268,7 +2240,7 @@ XLogBackgroundFlush(void)
22682240

22692241
/* now wait for the write lock */
22702242
LWLockAcquire(WALWriteLock,LW_EXCLUSIVE);
2271-
LogwrtResult=XLogCtl->Write.LogwrtResult;
2243+
LogwrtResult=XLogCtl->LogwrtResult;
22722244
if (!XLByteLE(WriteRqstPtr,LogwrtResult.Flush))
22732245
{
22742246
XLogwrtRqstWriteRqst;
@@ -6831,8 +6803,6 @@ StartupXLOG(void)
68316803

68326804
LogwrtResult.Write=LogwrtResult.Flush=EndOfLog;
68336805

6834-
XLogCtl->Write.LogwrtResult=LogwrtResult;
6835-
Insert->LogwrtResult=LogwrtResult;
68366806
XLogCtl->LogwrtResult=LogwrtResult;
68376807

68386808
XLogCtl->LogwrtRqst.Write=EndOfLog;

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