From 69ab44651422c49a6256d1b6cca6c20b5060ad92 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=C3=81lvaro=20Herrera?= <alvherre@alvh.no-ip.org>
Date: Thu, 9 Jan 2025 07:33:52 +0100
Subject: [PATCH] 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
---
src/backend/access/transam/slru.c | 6 +++---
src/include/access/slru.h | 9 +++------
2 files changed, 6 insertions(+), 9 deletions(-)
diff --git a/src/backend/access/transam/slru.c b/src/backend/access/transam/slru.c
index 7eeaafe2cb3..9ce628e62a5 100644
--- 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;
@@ -1180,7 +1180,7 @@ 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
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);
}