Fix SLRU bank selection code

The originally submitted code (using bit masking) was correct when the
number of slots was restricted to be a power of two -- but that
limitation was removed during development that led to commit
53c2a97a9266, which made the bank selection code incorrect.  This led to
always using a smaller number of banks than available.  Change said code
to use integer modulo instead, which works correctly with an arbitrary
number of banks.

It's likely that we could improve on this to avoid runtime use of
integer division.  But with this change we're, at least, not wasting
memory on unused banks, and more banks mean less contention, which is
likely to have a much higher performance impact than a single
instruction's latency.

Author: Yura Sokolov <y.sokolov@postgrespro.ru>
Reviewed-by: Andrey Borodin <x4mmm@yandex-team.ru>
Discussion: https://postgr.es/m/9444dc46-ca47-43ed-9058-89c456316306@postgrespro.ru

1 files changed, 3 insertions, 6 deletions

diff --git a/src/include/access/slru.h b/src/include/access/slru.h
index ae871b640f8..e142800aab2 100644
--- a/src/include/access/slru.h
+++ b/src/include/access/slru.h
@@ -128,10 +128,8 @@ typedef struct SlruCtlData
 {
 	SlruShared	shared;
 
-	/*
-	 * Bitmask to determine bank number from page number.
-	 */
-	bits16		bank_mask;
+	/* Number of banks in this SLRU. */
+	uint16		nbanks;
 
 	/*
 	 * If true, use long segment file names.  Otherwise, use short file names.
@@ -163,7 +161,6 @@ typedef struct SlruCtlData
 	 * it's always the same, it doesn't need to be in shared memory.
 	 */
 	char		Dir[64];
-
 } SlruCtlData;
 
 typedef SlruCtlData *SlruCtl;
@@ -179,7 +176,7 @@ SimpleLruGetBankLock(SlruCtl ctl, int64 pageno)
 {
 	int			bankno;
 
-	bankno = pageno & ctl->bank_mask;
+	bankno = pageno % ctl->nbanks;
 	return &(ctl->shared->bank_locks[bankno].lock);
 }