From 202b12774d092baf7e5d3309aa3cb0cd1e8a606b Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Thu, 20 Mar 2025 19:58:22 -0400 Subject: [PATCH] bufmgr: Improve stats when a buffer is read in concurrently Previously we would have the following inaccuracies when a backend tried to read in a buffer, but that buffer was read in concurrently by another backend: - the read IO was double-counted in the global buffer access stats (pgBufferUsage) - the buffer hit was not accounted for in: - global buffer access statistics - pg_stat_io - relation level IO stats - vacuum cost balancing While trying to read in a buffer that is concurrently read in by another backend is not a common occurrence, it's also not that rare, e.g. due to concurrent sequential scans on the same relation. This scenario has become more likely in PG 17, due to the introducing of read streams, which can pin multiple buffers before calling StartBufferIO() for all the buffers. This behaviour has historically grown, but there doesn't seem to be any reason to continue with the wrong accounting. Reviewed-by: Melanie Plageman Discussion: https://postgr.es/m/CAAKRu_Zk-B08AzPsO-6680LUHLOCGaNJYofaxTFseLa=OepV1g@mail.gmail.com --- src/backend/storage/buffer/bufmgr.c | 37 +++++++++++++++++------------ 1 file changed, 22 insertions(+), 15 deletions(-) diff --git a/src/backend/storage/buffer/bufmgr.c b/src/backend/storage/buffer/bufmgr.c index d04afa5ab9c..14fc1bd1248 100644 --- a/src/backend/storage/buffer/bufmgr.c +++ b/src/backend/storage/buffer/bufmgr.c @@ -1436,19 +1436,6 @@ WaitReadBuffers(ReadBuffersOperation *operation) io_object = IOOBJECT_RELATION; } - /* - * We count all these blocks as read by this backend. This is traditional - * behavior, but might turn out to be not true if we find that someone - * else has beaten us and completed the read of some of these blocks. In - * that case the system globally double-counts, but we traditionally don't - * count this as a "hit", and we don't have a separate counter for "miss, - * but another backend completed the read". - */ - if (persistence == RELPERSISTENCE_TEMP) - pgBufferUsage.local_blks_read += nblocks; - else - pgBufferUsage.shared_blks_read += nblocks; - for (int i = 0; i < nblocks; ++i) { int io_buffers_len; @@ -1466,8 +1453,9 @@ WaitReadBuffers(ReadBuffersOperation *operation) if (!WaitReadBuffersCanStartIO(buffers[i], false)) { /* - * Report this as a 'hit' for this backend, even though it must - * have started out as a miss in PinBufferForBlock(). + * Report and track this as a 'hit' for this backend, even though + * it must have started out as a miss in PinBufferForBlock(). The + * other backend will track this as a 'read'. */ TRACE_POSTGRESQL_BUFFER_READ_DONE(forknum, blocknum + i, operation->smgr->smgr_rlocator.locator.spcOid, @@ -1475,6 +1463,20 @@ WaitReadBuffers(ReadBuffersOperation *operation) operation->smgr->smgr_rlocator.locator.relNumber, operation->smgr->smgr_rlocator.backend, true); + + if (persistence == RELPERSISTENCE_TEMP) + pgBufferUsage.local_blks_hit += 1; + else + pgBufferUsage.shared_blks_hit += 1; + + if (operation->rel) + pgstat_count_buffer_hit(operation->rel); + + pgstat_count_io_op(io_object, io_context, IOOP_HIT, 1, 0); + + if (VacuumCostActive) + VacuumCostBalance += VacuumCostPageHit; + continue; } @@ -1560,6 +1562,11 @@ WaitReadBuffers(ReadBuffersOperation *operation) false); } + if (persistence == RELPERSISTENCE_TEMP) + pgBufferUsage.local_blks_read += io_buffers_len; + else + pgBufferUsage.shared_blks_read += io_buffers_len; + if (VacuumCostActive) VacuumCostBalance += VacuumCostPageMiss * io_buffers_len; } -- 2.30.2