From 99047de70afa74c508b55f23ff47b2510117d550 Mon Sep 17 00:00:00 2001 From: Tomas Vondra Date: Sun, 25 Oct 2026 19:11:35 +0100 Subject: [PATCH v1 9/9] Skip FPI lookups when no FPIs are pending XLogPrefetcherHasFPI() is called for every main fork block reference that gets past the other checks in XLogPrefetcherNextBlock(), i.e. for nearly every block that ends up being prefetched or found in shared buffers. It has to hash the block, and compare it with the entries in the group it maps to. But the entries only matter until the record with the FPI is replayed, and quite often there are no outstanding FPIs at all, e.g. later in a checkpoint cycle, when most of the modified blocks were already modified since the checkpoint. Track the highest LSN of all FPIs in the history, and return right away if the record with that LSN has been replayed already. This does not change which blocks get prefetched, it only skips searching the history when it can't find a pending FPI. --- src/backend/access/transam/xlogprefetcher.c | 26 +++++++++++++++++---- 1 file changed, 22 insertions(+), 4 deletions(-) diff --git a/src/backend/access/transam/xlogprefetcher.c b/src/backend/access/transam/xlogprefetcher.c index ba9e4e40a7e..1eddaac558e 100644 --- a/src/backend/access/transam/xlogprefetcher.c +++ b/src/backend/access/transam/xlogprefetcher.c @@ -177,6 +177,7 @@ struct XLogPrefetcher /* Book-keeping to avoid prefetching blocks restored from FPIs. */ XLogPrefetcherFPI *recent_fpi; uint32 recent_fpi_groups; /* always a power of two */ + XLogRecPtr recent_fpi_max_lsn; /* highest LSN in recent_fpi */ /* Book-keeping to disable prefetching temporarily. */ XLogRecPtr no_readahead_until; @@ -770,8 +771,6 @@ XLogPrefetcherNextBlock(uintptr_t pgsr_private, XLogRecPtr *lsn) * restored from an FPI in a record that has not been replayed * yet. The block will be in shared buffers by the time this * record gets replayed. - * - * XXX Maybe we could skip this with full-page writes disabled? */ if (XLogPrefetcherHasFPI(prefetcher, block->rlocator, block->blkno, replaying_lsn)) @@ -1104,6 +1103,13 @@ XLogPrefetcherAddFPI(XLogPrefetcher *prefetcher, RelFileLocator rlocator, slot->rlocator = rlocator; slot->blkno = blkno; slot->lsn = lsn; + + /* + * FPIs are normally added in LSN order, but use Max() so that the quick + * exit in XLogPrefetcherHasFPI() can't give a different answer than + * searching the history, no matter what. + */ + prefetcher->recent_fpi_max_lsn = Max(prefetcher->recent_fpi_max_lsn, lsn); } /* @@ -1121,8 +1127,19 @@ static inline bool XLogPrefetcherHasFPI(XLogPrefetcher *prefetcher, RelFileLocator rlocator, BlockNumber blkno, XLogRecPtr replaying_lsn) { - XLogPrefetcherFPI *slots = XLogPrefetcherFPIGroup(prefetcher, rlocator, - blkno); + XLogPrefetcherFPI *slots; + + /* + * Quick exit if all FPIs in the history have been replayed already. This + * is the common case when there are no FPIs within the look-ahead + * distance, e.g. with full_page_writes=off, or later in a checkpoint + * cycle, when most of the modified blocks were modified since the + * checkpoint already. + */ + if (prefetcher->recent_fpi_max_lsn <= replaying_lsn) + return false; + + slots = XLogPrefetcherFPIGroup(prefetcher, rlocator, blkno); for (int i = 0; i < XLOGPREFETCHER_FPI_PER_GROUP; i++) { @@ -1216,6 +1233,7 @@ XLogPrefetcherReadRecord(XLogPrefetcher *prefetcher, char **errmsg) palloc0_array(XLogPrefetcherFPI, prefetcher->recent_fpi_groups * XLOGPREFETCHER_FPI_PER_GROUP); + prefetcher->recent_fpi_max_lsn = InvalidXLogRecPtr; prefetcher->reconfigure_count = XLogPrefetchReconfigureCount; } -- 2.55.0