Here is a new version of the patch that uses the new atomic read/write functions with full barriers that were added in commit bd5132d. Thoughts?
-- Nathan Bossart Amazon Web Services: https://aws.amazon.com
>From c40594c62ccfbf75cb0d3787cb9367d15ae37de8 Mon Sep 17 00:00:00 2001 From: Nathan Bossart <nat...@postgresql.org> Date: Thu, 29 Feb 2024 10:25:46 -0600 Subject: [PATCH v6 1/1] Convert unloggedLSN to an atomic variable. Currently, this variable is an XLogRecPtr protected by a spinlock. By converting it to an atomic variable, we can remove the spinlock, which saves a small amount of shared memory space. Since this code is not performance-critical, we use atomic operations with full barrier semantics to make it easy to reason about correctness. Author: John Morris Reviewed-by: Michael Paquier, Robert Haas, Andres Freund, Stephen Frost, Bharath Rupireddy Discussion: https://postgr.es/m/BYAPR13MB26772534335255E50318C574A0409%40BYAPR13MB2677.namprd13.prod.outlook.com Discussion: https://postgr.es/m/MN2PR13MB2688FD8B757316CB5C54C8A2A0DDA%40MN2PR13MB2688.namprd13.prod.outlook.com --- src/backend/access/transam/xlog.c | 26 +++++++++----------------- 1 file changed, 9 insertions(+), 17 deletions(-) diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c index c1162d55bf..eb02e3b6a6 100644 --- a/src/backend/access/transam/xlog.c +++ b/src/backend/access/transam/xlog.c @@ -470,9 +470,8 @@ typedef struct XLogCtlData XLogSegNo lastRemovedSegNo; /* latest removed/recycled XLOG segment */ - /* Fake LSN counter, for unlogged relations. Protected by ulsn_lck. */ - XLogRecPtr unloggedLSN; - slock_t ulsn_lck; + /* Fake LSN counter, for unlogged relations. */ + pg_atomic_uint64 unloggedLSN; /* Time and LSN of last xlog segment switch. Protected by WALWriteLock. */ pg_time_t lastSegSwitchTime; @@ -4498,14 +4497,7 @@ DataChecksumsEnabled(void) XLogRecPtr GetFakeLSNForUnloggedRel(void) { - XLogRecPtr nextUnloggedLSN; - - /* increment the unloggedLSN counter, need SpinLock */ - SpinLockAcquire(&XLogCtl->ulsn_lck); - nextUnloggedLSN = XLogCtl->unloggedLSN++; - SpinLockRelease(&XLogCtl->ulsn_lck); - - return nextUnloggedLSN; + return pg_atomic_fetch_add_u64(&XLogCtl->unloggedLSN, 1); } /* @@ -4921,7 +4913,7 @@ XLOGShmemInit(void) SpinLockInit(&XLogCtl->Insert.insertpos_lck); SpinLockInit(&XLogCtl->info_lck); - SpinLockInit(&XLogCtl->ulsn_lck); + pg_atomic_init_u64(&XLogCtl->unloggedLSN, InvalidXLogRecPtr); } /* @@ -5526,9 +5518,11 @@ StartupXLOG(void) * the unlogged LSN counter can be reset too. */ if (ControlFile->state == DB_SHUTDOWNED) - XLogCtl->unloggedLSN = ControlFile->unloggedLSN; + pg_atomic_write_membarrier_u64(&XLogCtl->unloggedLSN, + ControlFile->unloggedLSN); else - XLogCtl->unloggedLSN = FirstNormalUnloggedLSN; + pg_atomic_write_membarrier_u64(&XLogCtl->unloggedLSN, + FirstNormalUnloggedLSN); /* * Copy any missing timeline history files between 'now' and the recovery @@ -7110,9 +7104,7 @@ CreateCheckPoint(int flags) * unused on non-shutdown checkpoints, but seems useful to store it always * for debugging purposes. */ - SpinLockAcquire(&XLogCtl->ulsn_lck); - ControlFile->unloggedLSN = XLogCtl->unloggedLSN; - SpinLockRelease(&XLogCtl->ulsn_lck); + ControlFile->unloggedLSN = pg_atomic_read_membarrier_u64(&XLogCtl->unloggedLSN); UpdateControlFile(); LWLockRelease(ControlFileLock); -- 2.25.1
