From 7517a498cb1cbb23f60189019b3a46b27db35f72 Mon Sep 17 00:00:00 2001 From: Melanie Plageman Date: Mon, 20 Apr 2026 11:14:31 -0400 Subject: [PATCH] Make local buffers pin limit more conservative MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit GetLocalPinLimit() and GetAdditionalLocalPinLimit(), currently in use only by the read stream, previously allowed a backend to pin all num_temp_buffers local buffers. This meant that the read stream could use every available local buffer for read-ahead, leaving none for other concurrent pin-holders like other read streams and related buffers like the visibility map buffer needed during on-access pruning. Cap the local pin limit to num_temp_buffers / 4, providing some headroom. This doesn't guarantee that all needed pins will be available — for example, a backend can still open more cursors than there are buffers — but it makes it less likely that read-ahead will exhaust the pool. Note that these functions are not limited by definition to use in the read stream; however, this cap should be appropriate in other contexts. Author: Melanie Plageman Reported-by: Alexander Lakhin Reviewed-by: Andres Freund Discussion: https://postgr.es/m/97529f5a-ec10-46b1-ab50-4653126c6889%40gmail.com --- src/backend/storage/buffer/localbuf.c | 15 ++++++++++++--- 1 file changed, 12 insertions(+), 3 deletions(-) diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c index 396da84b25c..24ef95ecb10 100644 --- a/src/backend/storage/buffer/localbuf.c +++ b/src/backend/storage/buffer/localbuf.c @@ -307,16 +307,25 @@ GetLocalVictimBuffer(void) uint32 GetLocalPinLimit(void) { - /* Every backend has its own temporary buffers, and can pin them all. */ - return num_temp_buffers; + /* + * Every backend has its own temporary buffers, but we leave headroom to + * avoid running out of pins for discretionary demand, such as for + * read-ahead. + */ + return num_temp_buffers / 4; } /* see GetAdditionalPinLimit() */ uint32 GetAdditionalLocalPinLimit(void) { + uint32 total = GetLocalPinLimit(); + Assert(NLocalPinnedBuffers <= num_temp_buffers); - return num_temp_buffers - NLocalPinnedBuffers; + + if (NLocalPinnedBuffers >= total) + return 0; + return total - NLocalPinnedBuffers; } /* see LimitAdditionalPins() */ -- 2.43.0