Make-local-buffers-pin-limit-more-conservative.patch
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Filename: Make-local-buffers-pin-limit-more-conservative.patch
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Subject: Make local buffers pin limit more conservative
| File | + | − |
|---|---|---|
| src/backend/storage/buffer/localbuf.c | 12 | 3 |
From 7517a498cb1cbb23f60189019b3a46b27db35f72 Mon Sep 17 00:00:00 2001
From: Melanie Plageman <melanieplageman@gmail.com>
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
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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 <melanieplageman@gmail.com>
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
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