Fix SLRU bank selection code

alvherre · alvherre · commit ffd9b8134658 · 2025-01-09T07:44:30.000+01:00
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 53c2a97, 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
diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
@@ -343,7 +343,7 @@ SimpleLruInit(SlruCtl ctl, const char *name, int nslots, int nlsns,
 	ctl->shared = shared;
 	ctl->sync_handler = sync_handler;
 	ctl->long_segment_names = long_segment_names;
-	ctl->bank_mask = (nslots / SLRU_BANK_SIZE) - 1;
+	ctl->nbanks = nbanks;
 	strlcpy(ctl->Dir, subdir, sizeof(ctl->Dir));
 }
 
@@ -606,7 +606,7 @@ SimpleLruReadPage_ReadOnly(SlruCtl ctl, int64 pageno, TransactionId xid)
 {
 	SlruShared	shared = ctl->shared;
 	LWLock	   *banklock = SimpleLruGetBankLock(ctl, pageno);
-	int			bankno = pageno & ctl->bank_mask;
+	int			bankno = pageno % ctl->nbanks;
 	int			bankstart = bankno * SLRU_BANK_SIZE;
 	int			bankend = bankstart + SLRU_BANK_SIZE;
 
@@ -1177,7 +1177,7 @@ SlruSelectLRUPage(SlruCtl ctl, int64 pageno)
 		int			bestinvalidslot = 0;	/* keep compiler quiet */
 		int			best_invalid_delta = -1;
 		int64		best_invalid_page_number = 0;	/* keep compiler quiet */
-		int			bankno = pageno & ctl->bank_mask;
+		int			bankno = pageno % ctl->nbanks;
 		int			bankstart = bankno * SLRU_BANK_SIZE;
 		int			bankend = bankstart + SLRU_BANK_SIZE;
 
diff --git 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);
 }
 
