Use mmap MAP_NOSYNC option to limit shared memory writes

bmomjian · bmomjian · commit 34afbba84e8f · 2015-03-21T22:06:19.000-04:00
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
@@ -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
@@ -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. */
