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Reduce page locking in GIN vacuum

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GIN vacuum during cleaning posting tree can lock this whole tree for a long
time with by holding LockBufferForCleanup() on root. Patch changes it with
two ways: first, cleanup lock will be taken only if there is an empty page
(which should be deleted) and, second, it tries to lock only subtree, not the
whole posting tree.

Author: Andrey Borodin with minor editorization by me
Reviewed-by: Jeff Davis, me

https://commitfest.postgresql.org/13/896/
This commit is contained in:
Teodor Sigaev
2017-03-23 19:38:47 +03:00
parent 73561013e5
commit 218f51584d
4 changed files with 152 additions and 117 deletions
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250
src/backend/access/gin/ginvacuum.c
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@@ -109,75 +109,17 @@ xlogVacuumPage(Relation index, Buffer buffer)
PageSetLSN(page, recptr);
}
static bool
ginVacuumPostingTreeLeaves(GinVacuumState *gvs, BlockNumber blkno, bool isRoot, Buffer *rootBuffer)
typedef struct DataPageDeleteStack
{
Buffer buffer;
Page page;
bool hasVoidPage = FALSE;
MemoryContext oldCxt;
struct DataPageDeleteStack *child;
struct DataPageDeleteStack *parent;
buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
RBM_NORMAL, gvs->strategy);
page = BufferGetPage(buffer);
BlockNumber blkno; /* current block number */
BlockNumber leftBlkno; /* rightest non-deleted page on left */
bool isRoot;
} DataPageDeleteStack;
/*
* We should be sure that we don't concurrent with inserts, insert process
* never release root page until end (but it can unlock it and lock
* again). New scan can't start but previously started ones work
* concurrently.
*/
if (isRoot)
LockBufferForCleanup(buffer);
else
LockBuffer(buffer, GIN_EXCLUSIVE);
Assert(GinPageIsData(page));
if (GinPageIsLeaf(page))
{
oldCxt = MemoryContextSwitchTo(gvs->tmpCxt);
ginVacuumPostingTreeLeaf(gvs->index, buffer, gvs);
MemoryContextSwitchTo(oldCxt);
MemoryContextReset(gvs->tmpCxt);
/* if root is a leaf page, we don't desire further processing */
if (!isRoot && !hasVoidPage && GinDataLeafPageIsEmpty(page))
hasVoidPage = TRUE;
}
else
{
OffsetNumber i;
bool isChildHasVoid = FALSE;
for (i = FirstOffsetNumber; i <= GinPageGetOpaque(page)->maxoff; i++)
{
PostingItem *pitem = GinDataPageGetPostingItem(page, i);
if (ginVacuumPostingTreeLeaves(gvs, PostingItemGetBlockNumber(pitem), FALSE, NULL))
isChildHasVoid = TRUE;
}
if (isChildHasVoid)
hasVoidPage = TRUE;
}
/*
* if we have root and there are empty pages in tree, then we don't
* release lock to go further processing and guarantee that tree is unused
*/
if (!(isRoot && hasVoidPage))
{
UnlockReleaseBuffer(buffer);
}
else
{
Assert(rootBuffer);
*rootBuffer = buffer;
}
return hasVoidPage;
}
/*
* Delete a posting tree page.
@@ -194,8 +136,13 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
BlockNumber rightlink;
/*
* Lock the pages in the same order as an insertion would, to avoid
* deadlocks: left, then right, then parent.
* This function MUST be called only if someone of parent pages hold
* exclusive cleanup lock. This guarantees that no insertions currently
* happen in this subtree. Caller also acquire Exclusive lock on deletable
* page and is acquiring and releasing exclusive lock on left page before.
* Left page was locked and released. Then parent and this page are locked.
* We acquire left page lock here only to mark page dirty after changing
* right pointer.
*/
lBuffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, leftBlkno,
RBM_NORMAL, gvs->strategy);
@@ -205,10 +152,6 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
RBM_NORMAL, gvs->strategy);
LockBuffer(lBuffer, GIN_EXCLUSIVE);
LockBuffer(dBuffer, GIN_EXCLUSIVE);
if (!isParentRoot) /* parent is already locked by
* LockBufferForCleanup() */
LockBuffer(pBuffer, GIN_EXCLUSIVE);
START_CRIT_SECTION();
@@ -272,26 +215,15 @@ ginDeletePage(GinVacuumState *gvs, BlockNumber deleteBlkno, BlockNumber leftBlkn
PageSetLSN(BufferGetPage(lBuffer), recptr);
}
if (!isParentRoot)
LockBuffer(pBuffer, GIN_UNLOCK);
ReleaseBuffer(pBuffer);
UnlockReleaseBuffer(lBuffer);
UnlockReleaseBuffer(dBuffer);
ReleaseBuffer(dBuffer);
END_CRIT_SECTION();
gvs->result->pages_deleted++;
}
typedef struct DataPageDeleteStack
{
struct DataPageDeleteStack *child;
struct DataPageDeleteStack *parent;
BlockNumber blkno; /* current block number */
BlockNumber leftBlkno; /* rightest non-deleted page on left */
bool isRoot;
} DataPageDeleteStack;
/*
* scans posting tree and deletes empty pages
@@ -325,6 +257,10 @@ ginScanToDelete(GinVacuumState *gvs, BlockNumber blkno, bool isRoot,
buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
RBM_NORMAL, gvs->strategy);
if(!isRoot)
LockBuffer(buffer, GIN_EXCLUSIVE);
page = BufferGetPage(buffer);
Assert(GinPageIsData(page));
@@ -359,6 +295,9 @@ ginScanToDelete(GinVacuumState *gvs, BlockNumber blkno, bool isRoot,
}
}
if(!isRoot)
LockBuffer(buffer, GIN_UNLOCK);
ReleaseBuffer(buffer);
if (!meDelete)
@@ -367,37 +306,124 @@ ginScanToDelete(GinVacuumState *gvs, BlockNumber blkno, bool isRoot,
return meDelete;
}
/*
* Scan through posting tree, delete empty tuples from leaf pages.
* Also, this function collects empty subtrees (with all empty leafs).
* For parents of these subtrees CleanUp lock is taken, then we call
* ScanToDelete. This is done for every inner page, which points to
* empty subtree.
*/
static bool
ginVacuumPostingTreeLeaves(GinVacuumState *gvs, BlockNumber blkno, bool isRoot)
{
Buffer buffer;
Page page;
bool hasVoidPage = FALSE;
MemoryContext oldCxt;
buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
RBM_NORMAL, gvs->strategy);
page = BufferGetPage(buffer);
ginTraverseLock(buffer,false);
Assert(GinPageIsData(page));
if (GinPageIsLeaf(page))
{
oldCxt = MemoryContextSwitchTo(gvs->tmpCxt);
ginVacuumPostingTreeLeaf(gvs->index, buffer, gvs);
MemoryContextSwitchTo(oldCxt);
MemoryContextReset(gvs->tmpCxt);
/* if root is a leaf page, we don't desire further processing */
if (GinDataLeafPageIsEmpty(page))
hasVoidPage = TRUE;
UnlockReleaseBuffer(buffer);
return hasVoidPage;
}
else
{
OffsetNumber i;
bool hasEmptyChild = FALSE;
bool hasNonEmptyChild = FALSE;
OffsetNumber maxoff = GinPageGetOpaque(page)->maxoff;
BlockNumber* children = palloc(sizeof(BlockNumber) * (maxoff + 1));
/*
* Read all children BlockNumbers.
* Not sure it is safe if there are many concurrent vacuums.
*/
for (i = FirstOffsetNumber; i <= maxoff; i++)
{
PostingItem *pitem = GinDataPageGetPostingItem(page, i);
children[i] = PostingItemGetBlockNumber(pitem);
}
UnlockReleaseBuffer(buffer);
for (i = FirstOffsetNumber; i <= maxoff; i++)
{
if (ginVacuumPostingTreeLeaves(gvs, children[i], FALSE))
hasEmptyChild = TRUE;
else
hasNonEmptyChild = TRUE;
}
pfree(children);
vacuum_delay_point();
/*
* All subtree is empty - just return TRUE to indicate that parent must
* do a cleanup. Unless we are ROOT an there is way to go upper.
*/
if(hasEmptyChild && !hasNonEmptyChild && !isRoot)
return TRUE;
if(hasEmptyChild)
{
DataPageDeleteStack root,
*ptr,
*tmp;
buffer = ReadBufferExtended(gvs->index, MAIN_FORKNUM, blkno,
RBM_NORMAL, gvs->strategy);
LockBufferForCleanup(buffer);
memset(&root, 0, sizeof(DataPageDeleteStack));
root.leftBlkno = InvalidBlockNumber;
root.isRoot = TRUE;
ginScanToDelete(gvs, blkno, TRUE, &root, InvalidOffsetNumber);
ptr = root.child;
while (ptr)
{
tmp = ptr->child;
pfree(ptr);
ptr = tmp;
}
UnlockReleaseBuffer(buffer);
}
/* Here we have deleted all empty subtrees */
return FALSE;
}
}
static void
ginVacuumPostingTree(GinVacuumState *gvs, BlockNumber rootBlkno)
{
Buffer rootBuffer = InvalidBuffer;
DataPageDeleteStack root,
*ptr,
*tmp;
if (ginVacuumPostingTreeLeaves(gvs, rootBlkno, TRUE, &rootBuffer) == FALSE)
{
Assert(rootBuffer == InvalidBuffer);
return;
}
memset(&root, 0, sizeof(DataPageDeleteStack));
root.leftBlkno = InvalidBlockNumber;
root.isRoot = TRUE;
vacuum_delay_point();
ginScanToDelete(gvs, rootBlkno, TRUE, &root, InvalidOffsetNumber);
ptr = root.child;
while (ptr)
{
tmp = ptr->child;
pfree(ptr);
ptr = tmp;
}
UnlockReleaseBuffer(rootBuffer);
ginVacuumPostingTreeLeaves(gvs, rootBlkno, TRUE);
}
/*