From 375d8526f2900d0c377f44532f6d09ee06531f67 Mon Sep 17 00:00:00 2001
From: Heikki Linnakangas <heikki.linnakangas@iki.fi>
Date: Wed, 4 Sep 2013 23:14:33 +0300
Subject: [PATCH] Keep heavily-contended fields in XLogCtlInsert on different
cache lines.
Performance testing shows that if the insertpos_lck spinlock and the fields
that it protects are on the same cache line with other variables that are
frequently accessed, the false sharing can hurt performance a lot. Keep
them apart by adding some padding.
---
src/backend/access/transam/xlog.c | 15 ++++++++++++---
src/include/pg_config_manual.h | 11 +++++++++++
2 files changed, 23 insertions(+), 3 deletions(-)
diff --git a/src/backend/access/transam/xlog.c b/src/backend/access/transam/xlog.c
index 39c58d00fe6..386811389d7 100644
--- a/src/backend/access/transam/xlog.c
+++ b/src/backend/access/transam/xlog.c
@@ -408,7 +408,7 @@ typedef struct
typedef union XLogInsertSlotPadded
{
XLogInsertSlot slot;
- char pad[64];
+ char pad[CACHE_LINE_SIZE];
} XLogInsertSlotPadded;
/*
@@ -428,8 +428,14 @@ typedef struct XLogCtlInsert
uint64 CurrBytePos;
uint64 PrevBytePos;
- /* insertion slots, see above for details */
- XLogInsertSlotPadded *insertSlots;
+ /*
+ * Make sure the above heavily-contended spinlock and byte positions are
+ * on their own cache line. In particular, the RedoRecPtr and full page
+ * write variables below should be on a different cache line. They are
+ * read on every WAL insertion, but updated rarely, and we don't want
+ * those reads to steal the cache line containing Curr/PrevBytePos.
+ */
+ char pad[CACHE_LINE_SIZE];
/*
* fullPageWrites is the master copy used by all backends to determine
@@ -455,6 +461,9 @@ typedef struct XLogCtlInsert
bool exclusiveBackup;
int nonExclusiveBackups;
XLogRecPtr lastBackupStart;
+
+ /* insertion slots, see XLogInsertSlot struct above for details */
+ XLogInsertSlotPadded *insertSlots;
} XLogCtlInsert;
/*
diff --git a/src/include/pg_config_manual.h b/src/include/pg_config_manual.h
index 1d60be2c477..2e6aad1ca56 100644
--- a/src/include/pg_config_manual.h
+++ b/src/include/pg_config_manual.h
@@ -199,6 +199,17 @@
#define USE_PPC_LWSYNC
#endif
+/*
+ * Assumed cache line size. This doesn't affect correctness, but can be
+ * used for low-level optimizations. Currently, this is only used to pad
+ * some data structures in xlog.c, to ensure that highly-contended fields
+ * are on different cache lines. Too small a value can hurt performance due
+ * to false sharing, while the only downside of too large a value is a few
+ * bytes of wasted memory. The default is 128, which should be large enough
+ * for all supported platforms.
+ */
+#define CACHE_LINE_SIZE 128
+
/*
*------------------------------------------------------------------------
* The following symbols are for enabling debugging code, not for
--
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