From 34afbba84e8fd12bbf905d68d1112f51ed584df9 Mon Sep 17 00:00:00 2001
From: Bruce Momjian <bruce@momjian.us>
Date: Sat, 21 Mar 2015 22:06:19 -0400
Subject: [PATCH] Use mmap MAP_NOSYNC option to limit shared memory writes
mmap() is rarely used for shared memory, but when it is, this option is
useful, particularly on the BSDs.
Patch by Sean Chittenden
---
src/backend/storage/ipc/dsm_impl.c | 4 ++--
src/include/portability/mem.h | 8 ++++++++
2 files changed, 10 insertions(+), 2 deletions(-)
diff --git a/src/backend/storage/ipc/dsm_impl.c b/src/backend/storage/ipc/dsm_impl.c
index cf7938cb44d..0859fbfc930 100644
--- a/src/backend/storage/ipc/dsm_impl.c
+++ b/src/backend/storage/ipc/dsm_impl.c
@@ -368,7 +368,7 @@ dsm_impl_posix(dsm_op op, dsm_handle handle, Size request_size,
/* Map it. */
address = mmap(NULL, request_size, PROT_READ | PROT_WRITE,
- MAP_SHARED | MAP_HASSEMAPHORE, fd, 0);
+ MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0);
if (address == MAP_FAILED)
{
int save_errno;
@@ -960,7 +960,7 @@ dsm_impl_mmap(dsm_op op, dsm_handle handle, Size request_size,
/* Map it. */
address = mmap(NULL, request_size, PROT_READ | PROT_WRITE,
- MAP_SHARED | MAP_HASSEMAPHORE, fd, 0);
+ MAP_SHARED | MAP_HASSEMAPHORE | MAP_NOSYNC, fd, 0);
if (address == MAP_FAILED)
{
int save_errno;
diff --git a/src/include/portability/mem.h b/src/include/portability/mem.h
index 7c77dbd1284..a4b01fecaed 100644
--- a/src/include/portability/mem.h
+++ b/src/include/portability/mem.h
@@ -30,6 +30,14 @@
#define MAP_HASSEMAPHORE 0
#endif
+/*
+ * BSD-derived systems use the MAP_NOSYNC flag to prevent dirty mmap(2)
+ * pages from being gratuitously flushed to disk.
+ */
+#ifndef MAP_NOSYNC
+#define MAP_NOSYNC 0
+#endif
+
#define PG_MMAP_FLAGS (MAP_SHARED|MAP_ANONYMOUS|MAP_HASSEMAPHORE)
/* Some really old systems don't define MAP_FAILED. */
--
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