From: Bruce Momjian <bruce@momjian.us>
Date: Sun, 22 Mar 2015 02:06:19 +0000 (-0400)
Subject: Use mmap MAP_NOSYNC option to limit shared memory writes
X-Git-Tag: REL9_5_ALPHA1~580
X-Git-Url: https://granicus.if.org/sourcecode?a=commitdiff_plain;h=34afbba84e8fd12bbf905d68d1112f51ed584df9;p=postgresql

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
---

diff --git a/src/backend/storage/ipc/dsm_impl.c b/src/backend/storage/ipc/dsm_impl.c
index cf7938cb44..0859fbfc93 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 7c77dbd128..a4b01fecae 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. */