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Delete empty pages in each pass during GIST VACUUM. 

Earlier, we use to postpone deleting empty pages till the second stage of
vacuum to amortize the cost of scanning internal pages.  However, that can
sometimes (say vacuum is canceled or errored between first and second
stage) delay the pages to be recycled.

Another thing is that to facilitate deleting empty pages in the second
stage, we need to share the information about internal and empty pages
between different stages of vacuum.  It will be quite tricky to share this
information via DSM which is required for the upcoming parallel vacuum
patch.

Also, it will bring the logic to reclaim deleted pages closer to nbtree
where we delete empty pages in each pass.

Overall, the advantages of deleting empty pages in each pass outweigh the
advantages of postponing the same.

Author: Dilip Kumar, with changes by Amit Kapila
Reviewed-by: Sawada Masahiko and Amit Kapila
Discussion: https://postgr.es/m/CAA4eK1LGr+MN0xHZpJ2dfS8QNQ1a_aROKowZB+MPNep8FVtwAA@mail.gmail.com
This commit is contained in:
Amit Kapila 2020-01-13 07:59:44 +05:30
parent eae056c19e
commit 4e514c6180
2 changed files with 78 additions and 105 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

21
src/backend/access/gist/README
View File

@ -429,18 +429,17 @@ splits during searches, we don't need a "vacuum cycle ID" concept for that
like B-tree does.
While we scan all the pages, we also make note of any completely empty leaf
pages. We will try to unlink them from the tree in the second stage. We also
record the block numbers of all internal pages; they are needed in the second
stage, to locate parents of the empty pages.
pages. We will try to unlink them from the tree after the scan. We also record
the block numbers of all internal pages; they are needed to locate parents of
the empty pages while unlinking them.
In the second stage, we try to unlink any empty leaf pages from the tree, so
that their space can be reused. In order to delete an empty page, its
downlink must be removed from the parent. We scan all the internal pages,
whose block numbers we memorized in the first stage, and look for downlinks
to pages that we have memorized as being empty. Whenever we find one, we
acquire a lock on the parent and child page, re-check that the child page is
still empty. Then, we remove the downlink and mark the child as deleted, and
release the locks.
We try to unlink any empty leaf pages from the tree, so that their space can
be reused. In order to delete an empty page, its downlink must be removed from
the parent. We scan all the internal pages, whose block numbers we memorized
in the first stage, and look for downlinks to pages that we have memorized as
being empty. Whenever we find one, we acquire a lock on the parent and child
page, re-check that the child page is still empty. Then, we remove the
downlink and mark the child as deleted, and release the locks.
The insertion algorithm would get confused, if an internal page was completely
empty. So we never delete the last child of an internal page, even if it's

162
src/backend/access/gist/gistvacuum.c
View File

@ -24,58 +24,34 @@
#include "storage/lmgr.h"
#include "utils/memutils.h"
/*
* State kept across vacuum stages.
*/
typedef struct
{
IndexBulkDeleteResult stats; /* must be first */
/*
* These are used to memorize all internal and empty leaf pages in the 1st
* vacuum stage. They are used in the 2nd stage, to delete all the empty
* pages.
*/
IntegerSet *internal_page_set;
IntegerSet *empty_leaf_set;
MemoryContext page_set_context;
} GistBulkDeleteResult;
/* Working state needed by gistbulkdelete */
typedef struct
{
IndexVacuumInfo *info;
GistBulkDeleteResult *stats;
IndexBulkDeleteResult *stats;
IndexBulkDeleteCallback callback;
void *callback_state;
GistNSN startNSN;
/*
* These are used to memorize all internal and empty leaf pages. They are
* used for deleting all the empty pages.
*/
IntegerSet *internal_page_set;
IntegerSet *empty_leaf_set;
MemoryContext page_set_context;
} GistVacState;
static void gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
static void gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
IndexBulkDeleteCallback callback, void *callback_state);
static void gistvacuumpage(GistVacState *vstate, BlockNumber blkno,
BlockNumber orig_blkno);
static void gistvacuum_delete_empty_pages(IndexVacuumInfo *info,
GistBulkDeleteResult *stats);
static bool gistdeletepage(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
GistVacState *vstate);
static bool gistdeletepage(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
Buffer buffer, OffsetNumber downlink,
Buffer leafBuffer);
/* allocate the 'stats' struct that's kept over vacuum stages */
static GistBulkDeleteResult *
create_GistBulkDeleteResult(void)
{
GistBulkDeleteResult *gist_stats;
gist_stats = (GistBulkDeleteResult *) palloc0(sizeof(GistBulkDeleteResult));
gist_stats->page_set_context =
GenerationContextCreate(CurrentMemoryContext,
"GiST VACUUM page set context",
16 * 1024);
return gist_stats;
}
/*
* VACUUM bulkdelete stage: remove index entries.
*/
@ -83,15 +59,13 @@ IndexBulkDeleteResult *
gistbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
IndexBulkDeleteCallback callback, void *callback_state)
{
GistBulkDeleteResult *gist_stats = (GistBulkDeleteResult *) stats;
/* allocate stats if first time through, else re-use existing struct */
if (gist_stats == NULL)
gist_stats = create_GistBulkDeleteResult();
if (stats == NULL)
stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
gistvacuumscan(info, gist_stats, callback, callback_state);
gistvacuumscan(info, stats, callback, callback_state);
return (IndexBulkDeleteResult *) gist_stats;
return stats;
}
/*
@ -100,8 +74,6 @@ gistbulkdelete(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
IndexBulkDeleteResult *
gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
{
GistBulkDeleteResult *gist_stats = (GistBulkDeleteResult *) stats;
/* No-op in ANALYZE ONLY mode */
if (info->analyze_only)
return stats;
@ -111,24 +83,12 @@ gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
* stats from the latest gistbulkdelete call. If it wasn't called, we
* still need to do a pass over the index, to obtain index statistics.
*/
if (gist_stats == NULL)
if (stats == NULL)
{
gist_stats = create_GistBulkDeleteResult();
gistvacuumscan(info, gist_stats, NULL, NULL);
stats = (IndexBulkDeleteResult *) palloc0(sizeof(IndexBulkDeleteResult));
gistvacuumscan(info, stats, NULL, NULL);
}
/*
* If we saw any empty pages, try to unlink them from the tree so that
* they can be reused.
*/
gistvacuum_delete_empty_pages(info, gist_stats);
/* we don't need the internal and empty page sets anymore */
MemoryContextDelete(gist_stats->page_set_context);
gist_stats->page_set_context = NULL;
gist_stats->internal_page_set = NULL;
gist_stats->empty_leaf_set = NULL;
/*
* It's quite possible for us to be fooled by concurrent page splits into
* double-counting some index tuples, so disbelieve any total that exceeds
@ -137,11 +97,11 @@ gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
*/
if (!info->estimated_count)
{
if (gist_stats->stats.num_index_tuples > info->num_heap_tuples)
gist_stats->stats.num_index_tuples = info->num_heap_tuples;
if (stats->num_index_tuples > info->num_heap_tuples)
stats->num_index_tuples = info->num_heap_tuples;
}
return (IndexBulkDeleteResult *) gist_stats;
return stats;
}
/*
@ -153,15 +113,16 @@ gistvacuumcleanup(IndexVacuumInfo *info, IndexBulkDeleteResult *stats)
* occurred).
*
* This also makes note of any empty leaf pages, as well as all internal
* pages. The second stage, gistvacuum_delete_empty_pages(), needs that
* information. Any deleted pages are added directly to the free space map.
* (They should've been added there when they were originally deleted, already,
* but it's possible that the FSM was lost at a crash, for example.)
* pages while looping over all index pages. After scanning all the pages, we
* remove the empty pages so that they can be reused. Any deleted pages are
* added directly to the free space map. (They should've been added there
* when they were originally deleted, already, but it's possible that the FSM
* was lost at a crash, for example.)
*
* The caller is responsible for initially allocating/zeroing a stats struct.
*/
static void
gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
gistvacuumscan(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
IndexBulkDeleteCallback callback, void *callback_state)
{
Relation rel = info->index;
@ -175,11 +136,10 @@ gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
* Reset counts that will be incremented during the scan; needed in case
* of multiple scans during a single VACUUM command.
*/
stats->stats.estimated_count = false;
stats->stats.num_index_tuples = 0;
stats->stats.pages_deleted = 0;
stats->stats.pages_free = 0;
MemoryContextReset(stats->page_set_context);
stats->estimated_count = false;
stats->num_index_tuples = 0;
stats->pages_deleted = 0;
stats->pages_free = 0;
/*
* Create the integer sets to remember all the internal and the empty leaf
@ -187,9 +147,12 @@ gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
* this context so that the subsequent allocations for these integer sets
* will be done from the same context.
*/
oldctx = MemoryContextSwitchTo(stats->page_set_context);
stats->internal_page_set = intset_create();
stats->empty_leaf_set = intset_create();
vstate.page_set_context = GenerationContextCreate(CurrentMemoryContext,
"GiST VACUUM page set context",
16 * 1024);
oldctx = MemoryContextSwitchTo(vstate.page_set_context);
vstate.internal_page_set = intset_create();
vstate.empty_leaf_set = intset_create();
MemoryContextSwitchTo(oldctx);
/* Set up info to pass down to gistvacuumpage */
@ -257,11 +220,23 @@ gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
* Note that if no recyclable pages exist, we don't bother vacuuming the
* FSM at all.
*/
if (stats->stats.pages_free > 0)
if (stats->pages_free > 0)
IndexFreeSpaceMapVacuum(rel);
/* update statistics */
stats->stats.num_pages = num_pages;
stats->num_pages = num_pages;
/*
* If we saw any empty pages, try to unlink them from the tree so that
* they can be reused.
*/
gistvacuum_delete_empty_pages(info, &vstate);
/* we don't need the internal and empty page sets anymore */
MemoryContextDelete(vstate.page_set_context);
vstate.page_set_context = NULL;
vstate.internal_page_set = NULL;
vstate.empty_leaf_set = NULL;
}
/*
@ -278,7 +253,6 @@ gistvacuumscan(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
static void
gistvacuumpage(GistVacState *vstate, BlockNumber blkno, BlockNumber orig_blkno)
{
GistBulkDeleteResult *stats = vstate->stats;
IndexVacuumInfo *info = vstate->info;
IndexBulkDeleteCallback callback = vstate->callback;
void *callback_state = vstate->callback_state;
@ -307,13 +281,13 @@ restart:
{
/* Okay to recycle this page */
RecordFreeIndexPage(rel, blkno);
stats->stats.pages_free++;
stats->stats.pages_deleted++;
vstate->stats->pages_free++;
vstate->stats->pages_deleted++;
}
else if (GistPageIsDeleted(page))
{
/* Already deleted, but can't recycle yet */
stats->stats.pages_deleted++;
vstate->stats->pages_deleted++;
}
else if (GistPageIsLeaf(page))
{
@ -388,7 +362,7 @@ restart:
END_CRIT_SECTION();
stats->stats.tuples_removed += ntodelete;
vstate->stats->tuples_removed += ntodelete;
/* must recompute maxoff */
maxoff = PageGetMaxOffsetNumber(page);
}
@ -405,10 +379,10 @@ restart:
* it up.
*/
if (blkno == orig_blkno)
intset_add_member(stats->empty_leaf_set, blkno);
intset_add_member(vstate->empty_leaf_set, blkno);
}
else
stats->stats.num_index_tuples += nremain;
vstate->stats->num_index_tuples += nremain;
}
else
{
@ -443,7 +417,7 @@ restart:
* parents of empty leaf pages.
*/
if (blkno == orig_blkno)
intset_add_member(stats->internal_page_set, blkno);
intset_add_member(vstate->internal_page_set, blkno);
}
UnlockReleaseBuffer(buffer);
@ -466,7 +440,7 @@ restart:
* Scan all internal pages, and try to delete their empty child pages.
*/
static void
gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats)
gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistVacState *vstate)
{
Relation rel = info->index;
BlockNumber empty_pages_remaining;
@ -475,10 +449,10 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
/*
* Rescan all inner pages to find those that have empty child pages.
*/
empty_pages_remaining = intset_num_entries(stats->empty_leaf_set);
intset_begin_iterate(stats->internal_page_set);
empty_pages_remaining = intset_num_entries(vstate->empty_leaf_set);
intset_begin_iterate(vstate->internal_page_set);
while (empty_pages_remaining > 0 &&
intset_iterate_next(stats->internal_page_set, &blkno))
intset_iterate_next(vstate->internal_page_set, &blkno))
{
Buffer buffer;
Page page;
@ -521,7 +495,7 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
BlockNumber leafblk;
leafblk = ItemPointerGetBlockNumber(&(idxtuple->t_tid));
if (intset_is_member(stats->empty_leaf_set, leafblk))
if (intset_is_member(vstate->empty_leaf_set, leafblk))
{
leafs_to_delete[ntodelete] = leafblk;
todelete[ntodelete++] = off;
@ -561,7 +535,7 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
gistcheckpage(rel, leafbuf);
LockBuffer(buffer, GIST_EXCLUSIVE);
if (gistdeletepage(info, stats,
if (gistdeletepage(info, vstate->stats,
buffer, todelete[i] - deleted,
leafbuf))
deleted++;
@ -573,7 +547,7 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
ReleaseBuffer(buffer);
/* update stats */
stats->stats.pages_removed += deleted;
vstate->stats->pages_removed += deleted;
/*
* We can stop the scan as soon as we have seen the downlinks, even if
@ -596,7 +570,7 @@ gistvacuum_delete_empty_pages(IndexVacuumInfo *info, GistBulkDeleteResult *stats
* prevented it.
*/
static bool
gistdeletepage(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
gistdeletepage(IndexVacuumInfo *info, IndexBulkDeleteResult *stats,
Buffer parentBuffer, OffsetNumber downlink,
Buffer leafBuffer)
{
@ -665,7 +639,7 @@ gistdeletepage(IndexVacuumInfo *info, GistBulkDeleteResult *stats,
/* mark the page as deleted */
MarkBufferDirty(leafBuffer);
GistPageSetDeleted(leafPage, txid);
stats->stats.pages_deleted++;
stats->pages_deleted++;
/* remove the downlink from the parent */
MarkBufferDirty(parentBuffer);