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Commit5b9a058

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Tweak LWLock algorithms so that an awakened waiter for a lock is not
granted the lock when awakened; the signal now only means that the lockis potentially available. The waiting process must retry its attemptto get the lock when it gets to run. This allows the lock releasingprocess to re-acquire the lock later in its timeslice. Since LWLocksare usually held for short periods, it is possible for a process toacquire and release the same lock many times in a timeslice. The oldspinlock-based implementation of these locks allowed for that; but theoriginal coding of LWLock would force a process swap for each acquisitionif there was any contention. Although this approach reopens the door toprocess starvation (a waiter might repeatedly fail to get the lock),the odds of that being a big problem seem low, and the performance costof the previous approach is considerable.
1 parent5445283 commit5b9a058

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1 file changed

+81
-62
lines changed

1 file changed

+81
-62
lines changed

‎src/backend/storage/lmgr/lwlock.c‎

Lines changed: 81 additions & 62 deletions
Original file line numberDiff line numberDiff line change
@@ -15,7 +15,7 @@
1515
* Portions Copyright (c) 1994, Regents of the University of California
1616
*
1717
* IDENTIFICATION
18-
* $Header: /cvsroot/pgsql/src/backend/storage/lmgr/lwlock.c,v 1.7 2001/12/29 21:30:32 momjian Exp $
18+
* $Header: /cvsroot/pgsql/src/backend/storage/lmgr/lwlock.c,v 1.8 2002/01/07 16:33:00 tgl Exp $
1919
*
2020
*-------------------------------------------------------------------------
2121
*/
@@ -30,6 +30,7 @@
3030
typedefstructLWLock
3131
{
3232
slock_tmutex;/* Protects LWLock and queue of PROCs */
33+
boolreleaseOK;/* T if ok to release waiters */
3334
charexclusive;/* # of exclusive holders (0 or 1) */
3435
intshared;/* # of shared holders (0..MaxBackends) */
3536
PROC*head;/* head of list of waiting PROCs */
@@ -67,9 +68,10 @@ inline static void
6768
PRINT_LWDEBUG(constchar*where,LWLockIdlockid,constvolatileLWLock*lock)
6869
{
6970
if (Trace_lwlocks)
70-
elog(DEBUG,"%s(%d): excl %d shared %d head %p",
71+
elog(DEBUG,"%s(%d): excl %d shared %d head %p rOK %d",
7172
where, (int)lockid,
72-
(int)lock->exclusive,lock->shared,lock->head);
73+
(int)lock->exclusive,lock->shared,lock->head,
74+
(int)lock->releaseOK);
7375
}
7476

7577
inlinestaticvoid
@@ -153,6 +155,7 @@ CreateLWLocks(void)
153155
for (id=0,lock=LWLockArray;id<numLocks;id++,lock++)
154156
{
155157
SpinLockInit(&lock->mutex);
158+
lock->releaseOK= true;
156159
lock->exclusive=0;
157160
lock->shared=0;
158161
lock->head=NULL;
@@ -195,7 +198,9 @@ void
195198
LWLockAcquire(LWLockIdlockid,LWLockModemode)
196199
{
197200
volatileLWLock*lock=LWLockArray+lockid;
198-
boolmustwait;
201+
PROC*proc=MyProc;
202+
boolretry= false;
203+
intextraWaits=0;
199204

200205
PRINT_LWDEBUG("LWLockAcquire",lockid,lock);
201206

@@ -206,43 +211,62 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
206211
*/
207212
HOLD_INTERRUPTS();
208213

209-
/* Acquire mutex. Time spent holding mutex should be short! */
210-
SpinLockAcquire_NoHoldoff(&lock->mutex);
211-
212-
/* If I can get the lock, do so quickly. */
213-
if (mode==LW_EXCLUSIVE)
214+
/*
215+
* Loop here to try to acquire lock after each time we are signaled
216+
* by LWLockRelease.
217+
*
218+
* NOTE: it might seem better to have LWLockRelease actually grant us
219+
* the lock, rather than retrying and possibly having to go back to
220+
* sleep. But in practice that is no good because it means a process
221+
* swap for every lock acquisition when two or more processes are
222+
* contending for the same lock. Since LWLocks are normally used to
223+
* protect not-very-long sections of computation, a process needs to
224+
* be able to acquire and release the same lock many times during a
225+
* single CPU time slice, even in the presence of contention. The
226+
* efficiency of being able to do that outweighs the inefficiency of
227+
* sometimes wasting a process dispatch cycle because the lock is not
228+
* free when a released waiter finally gets to run. See pgsql-hackers
229+
* archives for 29-Dec-01.
230+
*/
231+
for (;;)
214232
{
215-
if (lock->exclusive==0&&lock->shared==0)
233+
boolmustwait;
234+
235+
/* Acquire mutex. Time spent holding mutex should be short! */
236+
SpinLockAcquire_NoHoldoff(&lock->mutex);
237+
238+
/* If retrying, allow LWLockRelease to release waiters again */
239+
if (retry)
240+
lock->releaseOK= true;
241+
242+
/* If I can get the lock, do so quickly. */
243+
if (mode==LW_EXCLUSIVE)
216244
{
217-
lock->exclusive++;
218-
mustwait= false;
245+
if (lock->exclusive==0&&lock->shared==0)
246+
{
247+
lock->exclusive++;
248+
mustwait= false;
249+
}
250+
else
251+
mustwait= true;
219252
}
220253
else
221-
mustwait= true;
222-
}
223-
else
224-
{
225-
/*
226-
* If there is someone waiting (presumably for exclusive access),
227-
* queue up behind him even though I could get the lock. This
228-
* prevents a stream of read locks from starving a writer.
229-
*/
230-
if (lock->exclusive==0&&lock->head==NULL)
231254
{
232-
lock->shared++;
233-
mustwait= false;
255+
if (lock->exclusive==0)
256+
{
257+
lock->shared++;
258+
mustwait= false;
259+
}
260+
else
261+
mustwait= true;
234262
}
235-
else
236-
mustwait= true;
237-
}
238263

239-
if (mustwait)
240-
{
241-
/* Add myself to wait queue */
242-
PROC*proc=MyProc;
243-
intextraWaits=0;
264+
if (!mustwait)
265+
break;/* got the lock */
244266

245267
/*
268+
* Add myself to wait queue.
269+
*
246270
* If we don't have a PROC structure, there's no way to wait. This
247271
* should never occur, since MyProc should only be null during
248272
* shared memory initialization.
@@ -267,9 +291,9 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
267291
*
268292
* Since we share the process wait semaphore with the regular lock
269293
* manager and ProcWaitForSignal, and we may need to acquire an
270-
* LWLock while one of those is pending, it is possible that we
271-
*getawakened for a reason other than beinggranted the LWLock.
272-
* If so, loop back and wait again. Once we've gotten thelock,
294+
* LWLock while one of those is pending, it is possible that we get
295+
* awakened for a reason other than beingsignaled by LWLockRelease.
296+
* If so, loop back and wait again. Once we've gotten theLWLock,
273297
* re-increment the sema by the number of additional signals
274298
* received, so that the lock manager or signal manager will see
275299
* the received signal when it next waits.
@@ -287,23 +311,22 @@ LWLockAcquire(LWLockId lockid, LWLockMode mode)
287311

288312
LOG_LWDEBUG("LWLockAcquire",lockid,"awakened");
289313

290-
/*
291-
* The awakener already updated the lock struct's state, so we
292-
* don't need to do anything more to it. Just need to fix the
293-
* semaphore count.
294-
*/
295-
while (extraWaits-->0)
296-
IpcSemaphoreUnlock(proc->sem.semId,proc->sem.semNum);
297-
}
298-
else
299-
{
300-
/* Got the lock without waiting */
301-
SpinLockRelease_NoHoldoff(&lock->mutex);
314+
/* Now loop back and try to acquire lock again. */
315+
retry= true;
302316
}
303317

318+
/* We are done updating shared state of the lock itself. */
319+
SpinLockRelease_NoHoldoff(&lock->mutex);
320+
304321
/* Add lock to list of locks held by this backend */
305322
Assert(num_held_lwlocks<MAX_SIMUL_LWLOCKS);
306323
held_lwlocks[num_held_lwlocks++]=lockid;
324+
325+
/*
326+
* Fix the process wait semaphore's count for any absorbed wakeups.
327+
*/
328+
while (extraWaits-->0)
329+
IpcSemaphoreUnlock(proc->sem.semId,proc->sem.semNum);
307330
}
308331

309332
/*
@@ -344,12 +367,7 @@ LWLockConditionalAcquire(LWLockId lockid, LWLockMode mode)
344367
}
345368
else
346369
{
347-
/*
348-
* If there is someone waiting (presumably for exclusive access),
349-
* queue up behind him even though I could get the lock. This
350-
* prevents a stream of read locks from starving a writer.
351-
*/
352-
if (lock->exclusive==0&&lock->head==NULL)
370+
if (lock->exclusive==0)
353371
{
354372
lock->shared++;
355373
mustwait= false;
@@ -419,33 +437,34 @@ LWLockRelease(LWLockId lockid)
419437

420438
/*
421439
* See if I need to awaken any waiters. If I released a non-last
422-
* shared hold, there cannot be anything to do.
440+
* shared hold, there cannot be anything to do. Also, do not awaken
441+
* any waiters if someone has already awakened waiters that haven't
442+
* yet acquired the lock.
423443
*/
424444
head=lock->head;
425445
if (head!=NULL)
426446
{
427-
if (lock->exclusive==0&&lock->shared==0)
447+
if (lock->exclusive==0&&lock->shared==0&&lock->releaseOK)
428448
{
429449
/*
430-
* Remove the to-be-awakened PROCs from the queue, and update
431-
* the lock state to show them as holding the lock.
450+
* Remove the to-be-awakened PROCs from the queue. If the
451+
* front waiter wants exclusive lock, awaken him only.
452+
* Otherwise awaken as many waiters as want shared access.
432453
*/
433454
proc=head;
434-
if (proc->lwExclusive)
435-
lock->exclusive++;
436-
else
455+
if (!proc->lwExclusive)
437456
{
438-
lock->shared++;
439457
while (proc->lwWaitLink!=NULL&&
440458
!proc->lwWaitLink->lwExclusive)
441459
{
442460
proc=proc->lwWaitLink;
443-
lock->shared++;
444461
}
445462
}
446463
/* proc is now the last PROC to be released */
447464
lock->head=proc->lwWaitLink;
448465
proc->lwWaitLink=NULL;
466+
/* prevent additional wakeups until retryer gets to run */
467+
lock->releaseOK= false;
449468
}
450469
else
451470
{

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