Make local buffers pin limit more conservative

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.

This became more noticeable since b46e1e54d07, which allows on-access
pruning to set the visibility map, which meant that some scans also
needed to pin a page of the VM. It caused a test in
src/test/regress/sql/temp.sql to fail in some cases.

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.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: Melanie Plageman <melanieplageman@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/97529f5a-ec10-46b1-ab50-4653126c6889%40gmail.com

diff --git a/src/backend/storage/buffer/localbuf.c b/src/backend/storage/buffer/localbuf.c
index 396da84b25c55cdfc361278f6ad58840c95c3673..4870c8e13d0105accecdbaccdfaf9f1303075ee8 100644 (file)
--- a/src/backend/storage/buffer/localbuf.c
+++ b/src/backend/storage/buffer/localbuf.c
@@ -307,16 +307,24 @@ 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 for
+        * concurrent pin-holders -- like multiple scans in the same query.
+        */
+       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() */