On Tue, Mar 22, 2016 at 1:12 PM, Petr Jelinek <[email protected]> wrote:
> I also think the code simplicity makes this worth it.
Agreed. I went over this patch and did a cleanup pass today. I
discovered that the LockWaiterCount() function was broken if you try
to tried to use it on a lock that you didn't hold or a lock that you
held in any mode other than exclusive, so I tried to fix that. I
rewrote a lot of the comments and tightened some other things up. The
result is attached.
I'm baffled by the same code Amit asked about upthread, even though
there's now a comment:
+ /*
+ * Here we are calling
RecordAndGetPageWithFreeSpace
+ * instead of GetPageWithFreeSpace,
because other backend
+ * who have got the lock might have
added extra blocks in
+ * the FSM and its possible that FSM
tree might not have
+ * been updated so far and we will not
get the page by
+ * searching from top using
GetPageWithFreeSpace, so we
+ * need to search the leaf page
directly using our last
+ * valid target block.
+ *
+ * XXX. I don't understand what is
happening here. -RMH
+ */
I've read this over several times and looked at
RecordAndGetPageWithFreeSpace() and I'm still confused. First of all,
if the lock was acquired by some other backend which did
RelationAddExtraBlocks(), it *will* have updated the FSM - that's the
whole point. Second, if the other backend extended the relation in
some other manner and did not extend the FSM, how does calling
RecordAndGetPageWithFreeSpace help? As far as I can see,
GetPageWithFreeSpace and RecordAndGetPageWithFreeSpace are both just
searching the FSM, so if one is stymied the other will be too. What
am I missing?
--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company
diff --git a/src/backend/access/heap/hio.c b/src/backend/access/heap/hio.c
index 8140418..3ab911f 100644
--- a/src/backend/access/heap/hio.c
+++ b/src/backend/access/heap/hio.c
@@ -169,6 +169,55 @@ GetVisibilityMapPins(Relation relation, Buffer buffer1, Buffer buffer2,
}
/*
+ * Extend a relation by multiple blocks to avoid future contention on the
+ * relation extension lock. Our goal is to pre-extend the relation by an
+ * amount which ramps up as the degree of contention ramps up, but limiting
+ * the result to some sane overall value.
+ */
+static void
+RelationAddExtraBlocks(Relation relation, BulkInsertState bistate)
+{
+ Page page;
+ Size freespace;
+ BlockNumber blockNum;
+ int extraBlocks = 0;
+ int lockWaiters = 0;
+ Buffer buffer;
+
+ /*
+ * We use the length of the lock wait queue to judge how much to extend.
+ * It might seem like multiplying the number of lock waiters by as much
+ * as 20 is too aggressive, but benchmarking revealed that smaller numbers
+ * were insufficient. 512 is just an arbitrary cap to prevent pathological
+ * results (and excessive wasted disk space).
+ */
+ lockWaiters = RelationExtensionLockWaiterCount(relation);
+ extraBlocks = Min(512, lockWaiters * 20);
+
+ while (extraBlocks-- >= 0)
+ {
+ /* Ouch - an unnecessary lseek() each time through the loop! */
+ buffer = ReadBufferBI(relation, P_NEW, bistate);
+
+ /* Extend by one page. */
+ LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
+ page = BufferGetPage(buffer);
+ PageInit(page, BufferGetPageSize(buffer), 0);
+ freespace = PageGetHeapFreeSpace(page);
+ MarkBufferDirty(buffer);
+ blockNum = BufferGetBlockNumber(buffer);
+ UnlockReleaseBuffer(buffer);
+
+ /*
+ * Put the page in the freespace map so other backends can find it.
+ * This is what will keep those other backends from also queueing up
+ * on the relation extension lock.
+ */
+ RecordPageWithFreeSpace(relation, blockNum, freespace);
+ }
+}
+
+/*
* RelationGetBufferForTuple
*
* Returns pinned and exclusive-locked buffer of a page in given relation
@@ -233,10 +282,11 @@ RelationGetBufferForTuple(Relation relation, Size len,
bool use_fsm = !(options & HEAP_INSERT_SKIP_FSM);
Buffer buffer = InvalidBuffer;
Page page;
- Size pageFreeSpace,
- saveFreeSpace;
+ Size pageFreeSpace = 0,
+ saveFreeSpace = 0;
BlockNumber targetBlock,
- otherBlock;
+ otherBlock,
+ lastValidBlock = InvalidBlockNumber;
bool needLock;
len = MAXALIGN(len); /* be conservative */
@@ -308,6 +358,7 @@ RelationGetBufferForTuple(Relation relation, Size len,
}
}
+loop:
while (targetBlock != InvalidBlockNumber)
{
/*
@@ -388,6 +439,8 @@ RelationGetBufferForTuple(Relation relation, Size len,
otherBlock, targetBlock, vmbuffer_other,
vmbuffer);
+ lastValidBlock = targetBlock;
+
/*
* Now we can check to see if there's enough free space here. If so,
* we're done.
@@ -440,10 +493,60 @@ RelationGetBufferForTuple(Relation relation, Size len,
*/
needLock = !RELATION_IS_LOCAL(relation);
+ /*
+ * If we need the lock but are not able to acquire it immediately, we'll
+ * consider extending the relation by multiple blocks at a time to manage
+ * contention on the relation extension lock. However, this only makes
+ * sense if we're using the FSM; otherwise, there's no point.
+ */
if (needLock)
- LockRelationForExtension(relation, ExclusiveLock);
+ {
+ if (use_fsm)
+ LockRelationForExtension(relation, ExclusiveLock);
+ else if (!ConditionLockRelationForExtension(relation, ExclusiveLock))
+ {
+ /* Couldn't get the lock immmediately; wait for it. */
+ LockRelationForExtension(relation, ExclusiveLock);
+
+ if (lastValidBlock != InvalidBlockNumber)
+ {
+ /*
+ * Here we are calling RecordAndGetPageWithFreeSpace
+ * instead of GetPageWithFreeSpace, because other backend
+ * who have got the lock might have added extra blocks in
+ * the FSM and its possible that FSM tree might not have
+ * been updated so far and we will not get the page by
+ * searching from top using GetPageWithFreeSpace, so we
+ * need to search the leaf page directly using our last
+ * valid target block.
+ *
+ * XXX. I don't understand what is happening here. -RMH
+ */
+ targetBlock = RecordAndGetPageWithFreeSpace(relation,
+ lastValidBlock,
+ pageFreeSpace,
+ len + saveFreeSpace);
+ }
+
+ /*
+ * If some other waiter has already extended the relation, we
+ * don't need to do so; just use the existing freespace.
+ */
+ if (targetBlock != InvalidBlockNumber)
+ {
+ UnlockRelationForExtension(relation, ExclusiveLock);
+ goto loop;
+ }
+
+ /* Time to bulk-extend. */
+ RelationAddExtraBlocks(relation, bistate);
+ }
+ }
/*
+ * In addition to whatever extension we performed above, we always add
+ * at least one block to satisfy our own request.
+ *
* XXX This does an lseek - rather expensive - but at the moment it is the
* only way to accurately determine how many blocks are in a relation. Is
* it worth keeping an accurate file length in shared memory someplace,
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index 0632fc0..7e04137 100644
--- a/src/backend/storage/lmgr/lmgr.c
+++ b/src/backend/storage/lmgr/lmgr.c
@@ -341,6 +341,41 @@ LockRelationForExtension(Relation relation, LOCKMODE lockmode)
}
/*
+ * ConditionLockRelationForExtension
+ *
+ * As above, but only lock if we can get the lock without blocking.
+ * Returns TRUE iff the lock was acquired.
+ */
+bool
+ConditionLockRelationForExtension(Relation relation, LOCKMODE lockmode)
+{
+ LOCKTAG tag;
+
+ SET_LOCKTAG_RELATION_EXTEND(tag,
+ relation->rd_lockInfo.lockRelId.dbId,
+ relation->rd_lockInfo.lockRelId.relId);
+
+ return (LockAcquire(&tag, lockmode, false, true) != LOCKACQUIRE_NOT_AVAIL);
+}
+
+/*
+ * RelationExtensionLockWaiterCount
+ *
+ * Count the lock requester for the RelationExtension lock.
+ */
+int
+RelationExtensionLockWaiterCount(Relation relation)
+{
+ LOCKTAG tag;
+
+ SET_LOCKTAG_RELATION_EXTEND(tag,
+ relation->rd_lockInfo.lockRelId.dbId,
+ relation->rd_lockInfo.lockRelId.relId);
+
+ return LockWaiterCount(&tag);
+}
+
+/*
* UnlockRelationForExtension
*/
void
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index b30b7b1..353f705 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -4380,3 +4380,40 @@ VirtualXactLock(VirtualTransactionId vxid, bool wait)
LockRelease(&tag, ShareLock, false);
return true;
}
+
+/*
+ * LockWaiterCount
+ *
+ * Find the number of lock requester on this locktag
+ */
+int
+LockWaiterCount(const LOCKTAG *locktag)
+{
+ LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
+ LOCK *lock;
+ bool found;
+ uint32 hashcode;
+ LWLock *partitionLock;
+ int waiters = 0;
+
+ if (lockmethodid <= 0 || lockmethodid >= lengthof(LockMethods))
+ elog(ERROR, "unrecognized lock method: %d", lockmethodid);
+
+ hashcode = LockTagHashCode(locktag);
+ partitionLock = LockHashPartitionLock(hashcode);
+ LWLockAcquire(partitionLock, LW_EXCLUSIVE);
+
+ lock = (LOCK *) hash_search_with_hash_value(LockMethodLockHash,
+ (const void *) locktag,
+ hashcode,
+ HASH_FIND,
+ &found);
+ if (found)
+ {
+ Assert(lock != NULL);
+ waiters = lock->nRequested;
+ }
+ LWLockRelease(partitionLock);
+
+ return waiters;
+}
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index 975b6f8..4460756 100644
--- a/src/include/storage/lmgr.h
+++ b/src/include/storage/lmgr.h
@@ -53,6 +53,9 @@ extern void UnlockRelationIdForSession(LockRelId *relid, LOCKMODE lockmode);
/* Lock a relation for extension */
extern void LockRelationForExtension(Relation relation, LOCKMODE lockmode);
extern void UnlockRelationForExtension(Relation relation, LOCKMODE lockmode);
+extern bool ConditionLockRelationForExtension(Relation relation,
+ LOCKMODE lockmode);
+extern int RelationExtensionLockWaiterCount(Relation relation);
/* Lock a page (currently only used within indexes) */
extern void LockPage(Relation relation, BlockNumber blkno, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index b26427d..9c08679 100644
--- a/src/include/storage/lock.h
+++ b/src/include/storage/lock.h
@@ -574,6 +574,8 @@ extern void RememberSimpleDeadLock(PGPROC *proc1,
PGPROC *proc2);
extern void InitDeadLockChecking(void);
+extern int LockWaiterCount(const LOCKTAG *locktag);
+
#ifdef LOCK_DEBUG
extern void DumpLocks(PGPROC *proc);
extern void DumpAllLocks(void);
--
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