From bfd42993d2bf9ba88ffd26815565a321eec12440 Mon Sep 17 00:00:00 2001 From: Amit Kapila Date: Thu, 12 Mar 2020 17:00:05 +0530 Subject: [PATCH] Allow relation extension and page locks to conflict among parallel group members. --- src/backend/storage/lmgr/README | 60 ++++++++++++++++++++----------------- src/backend/storage/lmgr/deadlock.c | 9 ++++++ src/backend/storage/lmgr/lock.c | 12 ++++++++ src/backend/storage/lmgr/proc.c | 8 ++++- src/include/storage/lock.h | 1 + 5 files changed, 62 insertions(+), 28 deletions(-) diff --git a/src/backend/storage/lmgr/README b/src/backend/storage/lmgr/README index 56b0a12..13eb1cc 100644 --- a/src/backend/storage/lmgr/README +++ b/src/backend/storage/lmgr/README @@ -597,21 +597,22 @@ deadlock detection algorithm very much, but it makes the bookkeeping more complicated. We choose to regard locks held by processes in the same parallel group as -non-conflicting. This means that two processes in a parallel group can hold a -self-exclusive lock on the same relation at the same time, or one process can -acquire an AccessShareLock while the other already holds AccessExclusiveLock. -This might seem dangerous and could be in some cases (more on that below), but -if we didn't do this then parallel query would be extremely prone to -self-deadlock. For example, a parallel query against a relation on which the -leader already had AccessExclusiveLock would hang, because the workers would -try to lock the same relation and be blocked by the leader; yet the leader -can't finish until it receives completion indications from all workers. An -undetected deadlock results. This is far from the only scenario where such a -problem happens. The same thing will occur if the leader holds only -AccessShareLock, the worker seeks AccessShareLock, but between the time the -leader attempts to acquire the lock and the time the worker attempts to -acquire it, some other process queues up waiting for an AccessExclusiveLock. -In this case, too, an indefinite hang results. +non-conflicting with the exception of relation extension and page locks. This +means that two processes in a parallel group can hold a self-exclusive lock on +the same relation at the same time, or one process can acquire an AccessShareLock +while the other already holds AccessExclusiveLock. This might seem dangerous and +could be in some cases (more on that below), but if we didn't do this then +parallel query would be extremely prone to self-deadlock. For example, a +parallel query against a relation on which the leader already had +AccessExclusiveLock would hang, because the workers would try to lock the same +relation and be blocked by the leader; yet the leader can't finish until it +receives completion indications from all workers. An undetected deadlock +results. This is far from the only scenario where such a problem happens. The +same thing will occur if the leader holds only AccessShareLock, the worker +seeks AccessShareLock, but between the time the leader attempts to acquire the +lock and the time the worker attempts to acquire it, some other process queues +up waiting for an AccessExclusiveLock. In this case, too, an indefinite hang +results. It might seem that we could predict which locks the workers will attempt to acquire and ensure before going parallel that those locks would be acquired @@ -637,18 +638,23 @@ the other is safe enough. Problems would occur if the leader initiated parallelism from a point in the code at which it had some backend-private state that made table access from another process unsafe, for example after calling SetReindexProcessing and before calling ResetReindexProcessing, -catastrophe could ensue, because the worker won't have that state. Similarly, -problems could occur with certain kinds of non-relation locks, such as -relation extension locks. It's no safer for two related processes to extend -the same relation at the time than for unrelated processes to do the same. -However, since parallel mode is strictly read-only at present, neither this -nor most of the similar cases can arise at present. To allow parallel writes, -we'll either need to (1) further enhance the deadlock detector to handle those -types of locks in a different way than other types; or (2) have parallel -workers use some other mutual exclusion method for such cases; or (3) revise -those cases so that they no longer use heavyweight locking in the first place -(which is not a crazy idea, given that such lock acquisitions are not expected -to deadlock and that heavyweight lock acquisition is fairly slow anyway). +catastrophe could ensue, because the worker won't have that state. + +To allow parallel inserts and parallel copy, we have ensured that relation +extension and page locks don't participate in group locking which means such +locks can conflict among the same group members. This is required as it is no +safer for two related processes to extend the same relation or perform clean up +in gin indexes at a time than for unrelated processes to do the same. We don't +acquire a heavyweight lock on any other object after relation extension lock +which means such a lock can never participate in the deadlock cycle. After +acquiring page locks, we can acquire relation extension lock but reverse never +happens, so those will also not participate in deadlock. To allow for other +parallel writes like parallel update or parallel delete, we'll either need to +(1) further enhance the deadlock detector to handle those tuple locks in a +different way than other types; or (2) have parallel workers use some other +mutual exclusion method for such cases. Currently, the parallel mode is +strictly read-only, but now we have the infrastructure to allow parallel +inserts and parallel copy. Group locking adds three new members to each PGPROC: lockGroupLeader, lockGroupMembers, and lockGroupLink. A PGPROC's lockGroupLeader is NULL for diff --git a/src/backend/storage/lmgr/deadlock.c b/src/backend/storage/lmgr/deadlock.c index f8c5df0..80ec88b 100644 --- a/src/backend/storage/lmgr/deadlock.c +++ b/src/backend/storage/lmgr/deadlock.c @@ -555,6 +555,15 @@ FindLockCycleRecurseMember(PGPROC *checkProc, int numLockModes, lm; + /* + * The relation extension or page lock can never participate in actual + * deadlock cycle. See Asserts in LockAcquireExtended. So, there is + * no advantage in checking wait edges from it. + */ + if ((LOCK_LOCKTAG(*lock) == LOCKTAG_RELATION_EXTEND) || + (LOCK_LOCKTAG(*lock) == LOCKTAG_PAGE)) + return false; + lockMethodTable = GetLocksMethodTable(lock); numLockModes = lockMethodTable->numLockModes; conflictMask = lockMethodTable->conflictTab[checkProc->waitLockMode]; diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c index 56dba09..6fdfeba 100644 --- a/src/backend/storage/lmgr/lock.c +++ b/src/backend/storage/lmgr/lock.c @@ -1404,6 +1404,18 @@ LockCheckConflicts(LockMethod lockMethodTable, } /* + * The relation extension or page lock conflict even between the group + * members. + */ + if ((LOCK_LOCKTAG(*lock) == LOCKTAG_RELATION_EXTEND) || + (LOCK_LOCKTAG(*lock) == LOCKTAG_PAGE)) + { + PROCLOCK_PRINT("LockCheckConflicts: conflicting (group)", + proclock); + return true; + } + + /* * Locks held in conflicting modes by members of our own lock group are * not real conflicts; we can subtract those out and see if we still have * a conflict. This is O(N) in the number of processes holding or diff --git a/src/backend/storage/lmgr/proc.c b/src/backend/storage/lmgr/proc.c index eb321f7..b18f61b 100644 --- a/src/backend/storage/lmgr/proc.c +++ b/src/backend/storage/lmgr/proc.c @@ -1077,7 +1077,13 @@ ProcSleep(LOCALLOCK *locallock, LockMethod lockMethodTable) /* * If group locking is in use, locks held by members of my locking group - * need to be included in myHeldLocks. + * need to be included in myHeldLocks. This is not required for + * relation extension or page locks which conflict among group members. + * However, including them in myHeldLocks will give group members the + * priority to get those locks as compared to other backends which are + * also trying to acquire those locks. OTOH, we can avoid giving + * priority to group members for that kind of locks, but there + * doesn't appear to be a clear advantage of the same. */ if (leader != NULL) { diff --git a/src/include/storage/lock.h b/src/include/storage/lock.h index bb8e4e6..fac979d 100644 --- a/src/include/storage/lock.h +++ b/src/include/storage/lock.h @@ -301,6 +301,7 @@ typedef struct LOCK } LOCK; #define LOCK_LOCKMETHOD(lock) ((LOCKMETHODID) (lock).tag.locktag_lockmethodid) +#define LOCK_LOCKTAG(lock) ((LockTagType) (lock).tag.locktag_type) /* -- 1.8.3.1