From 464a996b85c333ffc781086263c2e491758b248f Mon Sep 17 00:00:00 2001 From: Alvaro Herrera Date: Wed, 31 Jan 2024 12:27:51 +0100 Subject: [PATCH 1/2] Use atomics for SlruSharedData->latest_page_number --- src/backend/access/transam/clog.c | 7 ++---- src/backend/access/transam/commit_ts.c | 7 +++--- src/backend/access/transam/multixact.c | 30 ++++++++++++++++---------- src/backend/access/transam/slru.c | 19 ++++++++++------ src/include/access/slru.h | 5 ++++- 5 files changed, 41 insertions(+), 27 deletions(-) diff --git a/src/backend/access/transam/clog.c b/src/backend/access/transam/clog.c index f6e7da7ffc..245fd21c8d 100644 --- a/src/backend/access/transam/clog.c +++ b/src/backend/access/transam/clog.c @@ -766,14 +766,11 @@ StartupCLOG(void) TransactionId xid = XidFromFullTransactionId(TransamVariables->nextXid); int64 pageno = TransactionIdToPage(xid); - LWLockAcquire(XactSLRULock, LW_EXCLUSIVE); - /* * Initialize our idea of the latest page number. */ - XactCtl->shared->latest_page_number = pageno; - - LWLockRelease(XactSLRULock); + pg_atomic_init_u64(&XactCtl->shared->latest_page_number, pageno); + pg_write_barrier(); } /* diff --git a/src/backend/access/transam/commit_ts.c b/src/backend/access/transam/commit_ts.c index 61b82385f3..f68705989d 100644 --- a/src/backend/access/transam/commit_ts.c +++ b/src/backend/access/transam/commit_ts.c @@ -689,9 +689,7 @@ ActivateCommitTs(void) /* * Re-Initialize our idea of the latest page number. */ - LWLockAcquire(CommitTsSLRULock, LW_EXCLUSIVE); - CommitTsCtl->shared->latest_page_number = pageno; - LWLockRelease(CommitTsSLRULock); + pg_atomic_init_u64(&CommitTsCtl->shared->latest_page_number, pageno); /* * If CommitTs is enabled, but it wasn't in the previous server run, we @@ -1006,7 +1004,8 @@ commit_ts_redo(XLogReaderState *record) * During XLOG replay, latest_page_number isn't set up yet; insert a * suitable value to bypass the sanity test in SimpleLruTruncate. */ - CommitTsCtl->shared->latest_page_number = trunc->pageno; + pg_atomic_write_u64(&CommitTsCtl->shared->latest_page_number, + trunc->pageno); SimpleLruTruncate(CommitTsCtl, trunc->pageno); } diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c index 59523be901..a886c29892 100644 --- a/src/backend/access/transam/multixact.c +++ b/src/backend/access/transam/multixact.c @@ -2017,13 +2017,17 @@ StartupMultiXact(void) * Initialize offset's idea of the latest page number. */ pageno = MultiXactIdToOffsetPage(multi); - MultiXactOffsetCtl->shared->latest_page_number = pageno; + pg_atomic_init_u64(&MultiXactOffsetCtl->shared->latest_page_number, + pageno); /* * Initialize member's idea of the latest page number. */ pageno = MXOffsetToMemberPage(offset); - MultiXactMemberCtl->shared->latest_page_number = pageno; + pg_atomic_init_u64(&MultiXactMemberCtl->shared->latest_page_number, + pageno); + + pg_write_barrier(); } /* @@ -2047,14 +2051,15 @@ TrimMultiXact(void) oldestMXactDB = MultiXactState->oldestMultiXactDB; LWLockRelease(MultiXactGenLock); - /* Clean up offsets state */ - LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE); - /* * (Re-)Initialize our idea of the latest page number for offsets. */ pageno = MultiXactIdToOffsetPage(nextMXact); - MultiXactOffsetCtl->shared->latest_page_number = pageno; + pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number, + pageno); + + /* Clean up offsets state */ + LWLockAcquire(MultiXactOffsetSLRULock, LW_EXCLUSIVE); /* * Zero out the remainder of the current offsets page. See notes in @@ -2081,14 +2086,16 @@ TrimMultiXact(void) LWLockRelease(MultiXactOffsetSLRULock); - /* And the same for members */ - LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE); - /* + * And the same for members. + * * (Re-)Initialize our idea of the latest page number for members. */ pageno = MXOffsetToMemberPage(offset); - MultiXactMemberCtl->shared->latest_page_number = pageno; + pg_atomic_write_u64(&MultiXactMemberCtl->shared->latest_page_number, + pageno); + + LWLockAcquire(MultiXactMemberSLRULock, LW_EXCLUSIVE); /* * Zero out the remainder of the current members page. See notes in @@ -3333,7 +3340,8 @@ multixact_redo(XLogReaderState *record) * SimpleLruTruncate. */ pageno = MultiXactIdToOffsetPage(xlrec.endTruncOff); - MultiXactOffsetCtl->shared->latest_page_number = pageno; + pg_atomic_write_u64(&MultiXactOffsetCtl->shared->latest_page_number, + pageno); PerformOffsetsTruncation(xlrec.startTruncOff, xlrec.endTruncOff); LWLockRelease(MultiXactTruncationLock); diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c index 9ac4790f16..57949fbab4 100644 --- a/src/backend/access/transam/slru.c +++ b/src/backend/access/transam/slru.c @@ -17,7 +17,8 @@ * per-buffer LWLocks that synchronize I/O for each buffer. The control lock * must be held to examine or modify any shared state. A process that is * reading in or writing out a page buffer does not hold the control lock, - * only the per-buffer lock for the buffer it is working on. + * only the per-buffer lock for the buffer it is working on. One exception + * is latest_page_number, which is read and written using atomic ops. * * "Holding the control lock" means exclusive lock in all cases except for * SimpleLruReadPage_ReadOnly(); see comments for SlruRecentlyUsed() for @@ -330,7 +331,8 @@ SimpleLruZeroPage(SlruCtl ctl, int64 pageno) SimpleLruZeroLSNs(ctl, slotno); /* Assume this page is now the latest active page */ - shared->latest_page_number = pageno; + pg_atomic_write_u64(&shared->latest_page_number, pageno); + pg_write_barrier(); /* update the stats counter of zeroed pages */ pgstat_count_slru_page_zeroed(shared->slru_stats_idx); @@ -1113,8 +1115,11 @@ SlruSelectLRUPage(SlruCtl ctl, int64 pageno) shared->page_lru_count[slotno] = cur_count; this_delta = 0; } + this_page_number = shared->page_number[slotno]; - if (this_page_number == shared->latest_page_number) + pg_read_barrier(); + if (this_page_number == + pg_atomic_read_u64(&shared->latest_page_number)) continue; if (shared->page_status[slotno] == SLRU_PAGE_VALID) { @@ -1270,10 +1275,12 @@ SimpleLruTruncate(SlruCtl ctl, int64 cutoffPage) restart: /* - * While we are holding the lock, make an important safety check: the - * current endpoint page must not be eligible for removal. + * An important safety check: the current endpoint page must not be + * eligible for removal. */ - if (ctl->PagePrecedes(shared->latest_page_number, cutoffPage)) + pg_read_barrier(); + if (ctl->PagePrecedes(pg_atomic_read_u64(&shared->latest_page_number), + cutoffPage)) { LWLockRelease(shared->ControlLock); ereport(LOG, diff --git a/src/include/access/slru.h b/src/include/access/slru.h index b05f6bc71d..2109488654 100644 --- a/src/include/access/slru.h +++ b/src/include/access/slru.h @@ -49,6 +49,9 @@ typedef enum /* * Shared-memory state + * + * ControlLock is used to protect access to the other fields, except + * latest_page_number, which uses atomics; see comment in slru.c. */ typedef struct SlruSharedData { @@ -95,7 +98,7 @@ typedef struct SlruSharedData * this is not critical data, since we use it only to avoid swapping out * the latest page. */ - int64 latest_page_number; + pg_atomic_uint64 latest_page_number; /* SLRU's index for statistics purposes (might not be unique) */ int slru_stats_idx; -- 2.39.2