On 12/6/2012 12:45 PM, Robert Haas wrote:
On Wed, Dec 5, 2012 at 10:16 PM, Jan Wieck <janwi...@yahoo.com> wrote:
That sort of "dynamic" approach would indeed be interesting. But I fear that
it is going to be complex at best. The amount of time spent in scanning
heavily depends on the visibility map. The initial vacuum scan of a table
can take hours or more, but it does update the visibility map even if the
vacuum itself is aborted later. The next vacuum may scan that table in
almost no time at all, because it skips all blocks that are marked "all
visible".
Well, if that's true, then there's little reason to worry about giving
up quickly, because the next autovacuum a minute later won't consume
many resources.
"Almost no time" is of course "relative" to what an actual scan and dead
tuple removal cost. Looking at a table with 3 GB of dead tuples at the
end, the initial vacuum scan takes hours. When vacuum comes back to this
table, cleaning up a couple megabytes of newly deceased tuples and then
skipping over the all visible pages may take a minute.
Based on the discussion and what I feel is a consensus I have created an
updated patch that has no GUC at all. The hard coded parameters in
include/postmaster/autovacuum.h are
AUTOVACUUM_TRUNCATE_LOCK_CHECK_INTERVAL 20 /* ms */
AUTOVACUUM_TRUNCATE_LOCK_WAIT_INTERVAL 50 /* ms */
AUTOVACUUM_TRUNCATE_LOCK_TIMEOUT 5000 /* ms */
I gave that the worst workload I can think of. A pgbench (style)
application that throws about 10 transactions per second at it, so that
there is constantly the need to give up the lock due to conflicting lock
requests and then reacquiring it again. A "cleanup" process is
periodically moving old tuples from the history table to an archive
table, making history a rolling window table. And a third job that 2-3
times per minute produces a 10 second lasting transaction, forcing
autovacuum to give up on the lock reacquisition.
Even with that workload autovacuum slow but steady is chopping away at
the table.
Jan
--
Anyone who trades liberty for security deserves neither
liberty nor security. -- Benjamin Franklin
diff --git a/src/backend/commands/vacuumlazy.c b/src/backend/commands/vacuumlazy.c
index c9253a9..fd3e91e 100644
*** a/src/backend/commands/vacuumlazy.c
--- b/src/backend/commands/vacuumlazy.c
***************
*** 52,62 ****
--- 52,64 ----
#include "storage/bufmgr.h"
#include "storage/freespace.h"
#include "storage/lmgr.h"
+ #include "storage/proc.h"
#include "utils/lsyscache.h"
#include "utils/memutils.h"
#include "utils/pg_rusage.h"
#include "utils/timestamp.h"
#include "utils/tqual.h"
+ #include "portability/instr_time.h"
/*
*************** typedef struct LVRelStats
*** 103,108 ****
--- 105,111 ----
ItemPointer dead_tuples; /* array of ItemPointerData */
int num_index_scans;
TransactionId latestRemovedXid;
+ bool lock_waiter_detected;
} LVRelStats;
*************** lazy_vacuum_rel(Relation onerel, VacuumS
*** 193,198 ****
--- 196,203 ----
vacrelstats->old_rel_pages = onerel->rd_rel->relpages;
vacrelstats->old_rel_tuples = onerel->rd_rel->reltuples;
vacrelstats->num_index_scans = 0;
+ vacrelstats->pages_removed = 0;
+ vacrelstats->lock_waiter_detected = false;
/* Open all indexes of the relation */
vac_open_indexes(onerel, RowExclusiveLock, &nindexes, &Irel);
*************** lazy_vacuum_rel(Relation onerel, VacuumS
*** 259,268 ****
vacrelstats->hasindex,
new_frozen_xid);
! /* report results to the stats collector, too */
! pgstat_report_vacuum(RelationGetRelid(onerel),
onerel->rd_rel->relisshared,
new_rel_tuples);
/* and log the action if appropriate */
if (IsAutoVacuumWorkerProcess() && Log_autovacuum_min_duration >= 0)
--- 264,281 ----
vacrelstats->hasindex,
new_frozen_xid);
! /*
! * report results to the stats collector, too.
! * An early terminated lazy_truncate_heap attempt
! * suppresses the message and also cancels the
! * execution of ANALYZE, if that was ordered.
! */
! if (!vacrelstats->lock_waiter_detected)
! pgstat_report_vacuum(RelationGetRelid(onerel),
onerel->rd_rel->relisshared,
new_rel_tuples);
+ else
+ vacstmt->options &= ~VACOPT_ANALYZE;
/* and log the action if appropriate */
if (IsAutoVacuumWorkerProcess() && Log_autovacuum_min_duration >= 0)
*************** lazy_truncate_heap(Relation onerel, LVRe
*** 1255,1334 ****
BlockNumber old_rel_pages = vacrelstats->rel_pages;
BlockNumber new_rel_pages;
PGRUsage ru0;
pg_rusage_init(&ru0);
/*
! * We need full exclusive lock on the relation in order to do truncation.
! * If we can't get it, give up rather than waiting --- we don't want to
! * block other backends, and we don't want to deadlock (which is quite
! * possible considering we already hold a lower-grade lock).
! */
! if (!ConditionalLockRelation(onerel, AccessExclusiveLock))
! return;
!
! /*
! * Now that we have exclusive lock, look to see if the rel has grown
! * whilst we were vacuuming with non-exclusive lock. If so, give up; the
! * newly added pages presumably contain non-deletable tuples.
*/
! new_rel_pages = RelationGetNumberOfBlocks(onerel);
! if (new_rel_pages != old_rel_pages)
{
/*
! * Note: we intentionally don't update vacrelstats->rel_pages with the
! * new rel size here. If we did, it would amount to assuming that the
! * new pages are empty, which is unlikely. Leaving the numbers alone
! * amounts to assuming that the new pages have the same tuple density
! * as existing ones, which is less unlikely.
*/
! UnlockRelation(onerel, AccessExclusiveLock);
! return;
! }
! /*
! * Scan backwards from the end to verify that the end pages actually
! * contain no tuples. This is *necessary*, not optional, because other
! * backends could have added tuples to these pages whilst we were
! * vacuuming.
! */
! new_rel_pages = count_nondeletable_pages(onerel, vacrelstats);
! if (new_rel_pages >= old_rel_pages)
! {
! /* can't do anything after all */
! UnlockRelation(onerel, AccessExclusiveLock);
! return;
! }
! /*
! * Okay to truncate.
! */
! RelationTruncate(onerel, new_rel_pages);
! /*
! * We can release the exclusive lock as soon as we have truncated. Other
! * backends can't safely access the relation until they have processed the
! * smgr invalidation that smgrtruncate sent out ... but that should happen
! * as part of standard invalidation processing once they acquire lock on
! * the relation.
! */
! UnlockRelation(onerel, AccessExclusiveLock);
! /*
! * Update statistics. Here, it *is* correct to adjust rel_pages without
! * also touching reltuples, since the tuple count wasn't changed by the
! * truncation.
! */
! vacrelstats->rel_pages = new_rel_pages;
! vacrelstats->pages_removed = old_rel_pages - new_rel_pages;
! ereport(elevel,
! (errmsg("\"%s\": truncated %u to %u pages",
! RelationGetRelationName(onerel),
! old_rel_pages, new_rel_pages),
! errdetail("%s.",
! pg_rusage_show(&ru0))));
}
/*
--- 1268,1393 ----
BlockNumber old_rel_pages = vacrelstats->rel_pages;
BlockNumber new_rel_pages;
PGRUsage ru0;
+ int lock_retry;
pg_rusage_init(&ru0);
/*
! * Loop until no more truncating can be done.
*/
! do
{
/*
! * We need full exclusive lock on the relation in order to do
! * truncation.
! * If we can't get it, give up rather than waiting --- we don't want to
! * block other backends, and we don't want to deadlock (which is quite
! * possible considering we already hold a lower-grade lock).
*/
! vacrelstats->lock_waiter_detected = false;
! lock_retry = 0;
! while (true)
! {
! if (ConditionalLockRelation(onerel, AccessExclusiveLock))
! break;
! /*
! * Check for interrupts while trying to (re-)acquire
! * the exclusive lock.
! */
! CHECK_FOR_INTERRUPTS();
! if (++lock_retry > (AUTOVACUUM_TRUNCATE_LOCK_TIMEOUT /
! AUTOVACUUM_TRUNCATE_LOCK_WAIT_INTERVAL))
! {
! /*
! * We failed to establish the lock in the specified
! * number of retries. This means we give up truncating.
! * Suppress the ANALYZE step. Doing an ANALYZE at
! * this point will reset the dead_tuple_count in the
! * stats collector, so we will not get called by the
! * autovacuum launcher again to do the truncate.
! */
! vacrelstats->lock_waiter_detected = true;
! ereport(LOG,
! (errmsg("automatic vacuum of table \"%s.%s.%s\": "
! "cannot (re)acquire exclusive "
! "lock for truncate scan",
! get_database_name(MyDatabaseId),
! get_namespace_name(RelationGetNamespace(onerel)),
! RelationGetRelationName(onerel))));
! return;
! }
! pg_usleep(AUTOVACUUM_TRUNCATE_LOCK_WAIT_INTERVAL);
! }
! /*
! * Now that we have exclusive lock, look to see if the rel has grown
! * whilst we were vacuuming with non-exclusive lock. If so, give up;
! * the newly added pages presumably contain non-deletable tuples.
! */
! new_rel_pages = RelationGetNumberOfBlocks(onerel);
! if (new_rel_pages != old_rel_pages)
! {
! /*
! * Note: we intentionally don't update vacrelstats->rel_pages
! * with the new rel size here. If we did, it would amount to
! * assuming that the new pages are empty, which is unlikely.
! * Leaving the numbers alone amounts to assuming that the new
! * pages have the same tuple density as existing ones, which
! * is less unlikely.
! */
! UnlockRelation(onerel, AccessExclusiveLock);
! return;
! }
! /*
! * Scan backwards from the end to verify that the end pages actually
! * contain no tuples. This is *necessary*, not optional, because other
! * backends could have added tuples to these pages whilst we were
! * vacuuming.
! */
! new_rel_pages = count_nondeletable_pages(onerel, vacrelstats);
! if (new_rel_pages >= old_rel_pages)
! {
! /* can't do anything after all */
! UnlockRelation(onerel, AccessExclusiveLock);
! return;
! }
!
! /*
! * Okay to truncate.
! */
! RelationTruncate(onerel, new_rel_pages);
!
! /*
! * We can release the exclusive lock as soon as we have truncated.
! * Other backends can't safely access the relation until they have
! * processed the smgr invalidation that smgrtruncate sent out ...
! * but that should happen as part of standard invalidation
! * processing once they acquire lock on the relation.
! */
! UnlockRelation(onerel, AccessExclusiveLock);
!
! /*
! * Update statistics. Here, it *is* correct to adjust rel_pages without
! * also touching reltuples, since the tuple count wasn't changed by the
! * truncation.
! */
! vacrelstats->pages_removed += old_rel_pages - new_rel_pages;
! vacrelstats->rel_pages = new_rel_pages;
!
! ereport(elevel,
! (errmsg("\"%s\": truncated %u to %u pages",
! RelationGetRelationName(onerel),
! old_rel_pages, new_rel_pages),
! errdetail("%s.",
! pg_rusage_show(&ru0))));
! old_rel_pages = new_rel_pages;
! } while (new_rel_pages > vacrelstats->nonempty_pages &&
! vacrelstats->lock_waiter_detected);
}
/*
*************** static BlockNumber
*** 1340,1345 ****
--- 1399,1410 ----
count_nondeletable_pages(Relation onerel, LVRelStats *vacrelstats)
{
BlockNumber blkno;
+ instr_time starttime;
+ instr_time currenttime;
+ instr_time elapsed;
+
+ /* Initialize the starttime if we check for conflicting lock requests */
+ INSTR_TIME_SET_CURRENT(starttime);
/* Strange coding of loop control is needed because blkno is unsigned */
blkno = vacrelstats->rel_pages;
*************** count_nondeletable_pages(Relation onerel
*** 1352,1357 ****
--- 1417,1453 ----
bool hastup;
/*
+ * Check if another process requests a lock on our relation.
+ * We are holding an AccessExclusiveLock here, so they will
+ * be waiting. We only do this in autovacuum_truncate_lock_check
+ * millisecond intervals, and we only check if that interval
+ * has elapsed once every 32 blocks to keep the number of
+ * system calls and actual shared lock table lookups to a
+ * minimum.
+ */
+ if ((blkno % 32) == 0)
+ {
+ INSTR_TIME_SET_CURRENT(currenttime);
+ elapsed = currenttime;
+ INSTR_TIME_SUBTRACT(elapsed, starttime);
+ if ((INSTR_TIME_GET_MICROSEC(elapsed) / 1000)
+ >= AUTOVACUUM_TRUNCATE_LOCK_CHECK_INTERVAL)
+ {
+ if (LockHasWaitersRelation(onerel, AccessExclusiveLock))
+ {
+ ereport(elevel,
+ (errmsg("\"%s\": suspending truncate "
+ "due to conflicting lock request",
+ RelationGetRelationName(onerel))));
+
+ vacrelstats->lock_waiter_detected = true;
+ return blkno;
+ }
+ starttime = currenttime;
+ }
+ }
+
+ /*
* We don't insert a vacuum delay point here, because we have an
* exclusive lock on the table which we want to hold for as short a
* time as possible. We still need to check for interrupts however.
diff --git a/src/backend/storage/lmgr/lmgr.c b/src/backend/storage/lmgr/lmgr.c
index a7786d0..e1fa74f 100644
*** a/src/backend/storage/lmgr/lmgr.c
--- b/src/backend/storage/lmgr/lmgr.c
*************** UnlockRelation(Relation relation, LOCKMO
*** 233,238 ****
--- 233,256 ----
}
/*
+ * LockHasWaitersRelation
+ *
+ * This is a functiion to check if someone else is waiting on a
+ * lock, we are currently holding.
+ */
+ bool
+ LockHasWaitersRelation(Relation relation, LOCKMODE lockmode)
+ {
+ LOCKTAG tag;
+
+ SET_LOCKTAG_RELATION(tag,
+ relation->rd_lockInfo.lockRelId.dbId,
+ relation->rd_lockInfo.lockRelId.relId);
+
+ return LockHasWaiters(&tag, lockmode, false);
+ }
+
+ /*
* LockRelationIdForSession
*
* This routine grabs a session-level lock on the target relation. The
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index 0183443..ec4da20 100644
*** a/src/backend/storage/lmgr/lock.c
--- b/src/backend/storage/lmgr/lock.c
*************** ProcLockHashCode(const PROCLOCKTAG *proc
*** 538,543 ****
--- 538,635 ----
return lockhash;
}
+ /*
+ * LockHasWaiters -- look up 'locktag' and check if releasing this
+ * lock would wake up other processes waiting for it.
+ */
+ bool
+ LockHasWaiters(const LOCKTAG *locktag, LOCKMODE lockmode, bool sessionLock)
+ {
+ LOCKMETHODID lockmethodid = locktag->locktag_lockmethodid;
+ LockMethod lockMethodTable;
+ LOCALLOCKTAG localtag;
+ LOCALLOCK *locallock;
+ LOCK *lock;
+ PROCLOCK *proclock;
+ LWLockId partitionLock;
+ bool hasWaiters = false;
+
+ if (lockmethodid <= 0 || lockmethodid >= lengthof(LockMethods))
+ elog(ERROR, "unrecognized lock method: %d", lockmethodid);
+ lockMethodTable = LockMethods[lockmethodid];
+ if (lockmode <= 0 || lockmode > lockMethodTable->numLockModes)
+ elog(ERROR, "unrecognized lock mode: %d", lockmode);
+
+ #ifdef LOCK_DEBUG
+ if (LOCK_DEBUG_ENABLED(locktag))
+ elog(LOG, "LockHasWaiters: lock [%u,%u] %s",
+ locktag->locktag_field1, locktag->locktag_field2,
+ lockMethodTable->lockModeNames[lockmode]);
+ #endif
+
+ /*
+ * Find the LOCALLOCK entry for this lock and lockmode
+ */
+ MemSet(&localtag, 0, sizeof(localtag)); /* must clear padding */
+ localtag.lock = *locktag;
+ localtag.mode = lockmode;
+
+ locallock = (LOCALLOCK *) hash_search(LockMethodLocalHash,
+ (void *) &localtag,
+ HASH_FIND, NULL);
+
+ /*
+ * let the caller print its own error message, too. Do not ereport(ERROR).
+ */
+ if (!locallock || locallock->nLocks <= 0)
+ {
+ elog(WARNING, "you don't own a lock of type %s",
+ lockMethodTable->lockModeNames[lockmode]);
+ return false;
+ }
+
+ /*
+ * Check the shared lock table.
+ */
+ partitionLock = LockHashPartitionLock(locallock->hashcode);
+
+ LWLockAcquire(partitionLock, LW_SHARED);
+
+ /*
+ * We don't need to re-find the lock or proclock, since we kept their
+ * addresses in the locallock table, and they couldn't have been removed
+ * while we were holding a lock on them.
+ */
+ lock = locallock->lock;
+ LOCK_PRINT("LockHasWaiters: found", lock, lockmode);
+ proclock = locallock->proclock;
+ PROCLOCK_PRINT("LockHasWaiters: found", proclock);
+
+ /*
+ * Double-check that we are actually holding a lock of the type we want to
+ * release.
+ */
+ if (!(proclock->holdMask & LOCKBIT_ON(lockmode)))
+ {
+ PROCLOCK_PRINT("LockHasWaiters: WRONGTYPE", proclock);
+ LWLockRelease(partitionLock);
+ elog(WARNING, "you don't own a lock of type %s",
+ lockMethodTable->lockModeNames[lockmode]);
+ RemoveLocalLock(locallock);
+ return false;
+ }
+
+ /*
+ * Do the checking.
+ */
+ if ((lockMethodTable->conflictTab[lockmode] & lock->waitMask) != 0)
+ hasWaiters = true;
+
+ LWLockRelease(partitionLock);
+
+ return hasWaiters;
+ }
+
/*
* LockAcquire -- Check for lock conflicts, sleep if conflict found,
diff --git a/src/include/postmaster/autovacuum.h b/src/include/postmaster/autovacuum.h
index a851758..7c14d92 100644
*** a/src/include/postmaster/autovacuum.h
--- b/src/include/postmaster/autovacuum.h
***************
*** 15,20 ****
--- 15,27 ----
#define AUTOVACUUM_H
+ /* Truncate exclusive lock configuration */
+ #define AUTOVACUUM_TRUNCATE_LOCK_CHECK_INTERVAL 20 /* ms */
+ #define AUTOVACUUM_TRUNCATE_LOCK_WAIT_INTERVAL 50 /* ms */
+ #define AUTOVACUUM_TRUNCATE_LOCK_TIMEOUT 5000 /* ms */
+
+
+
/* GUC variables */
extern bool autovacuum_start_daemon;
extern int autovacuum_max_workers;
diff --git a/src/include/storage/lmgr.h b/src/include/storage/lmgr.h
index de340c4..aa79eda 100644
*** a/src/include/storage/lmgr.h
--- b/src/include/storage/lmgr.h
*************** extern void UnlockRelationOid(Oid relid,
*** 31,36 ****
--- 31,37 ----
extern void LockRelation(Relation relation, LOCKMODE lockmode);
extern bool ConditionalLockRelation(Relation relation, LOCKMODE lockmode);
extern void UnlockRelation(Relation relation, LOCKMODE lockmode);
+ extern bool LockHasWaitersRelation(Relation relation, LOCKMODE lockmode);
extern void LockRelationIdForSession(LockRelId *relid, LOCKMODE lockmode);
extern void UnlockRelationIdForSession(LockRelId *relid, LOCKMODE lockmode);
diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h
index e01a5c5..680a132 100644
*** a/src/include/storage/lock.h
--- b/src/include/storage/lock.h
*************** extern void LockReleaseAll(LOCKMETHODID
*** 494,499 ****
--- 494,501 ----
extern void LockReleaseSession(LOCKMETHODID lockmethodid);
extern void LockReleaseCurrentOwner(LOCALLOCK **locallocks, int nlocks);
extern void LockReassignCurrentOwner(LOCALLOCK **locallocks, int nlocks);
+ extern bool LockHasWaiters(const LOCKTAG *locktag,
+ LOCKMODE lockmode, bool sessionLock);
extern VirtualTransactionId *GetLockConflicts(const LOCKTAG *locktag,
LOCKMODE lockmode);
extern void AtPrepare_Locks(void);
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