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 cf7938cb44d2a3a7fe24661abe342eaf395818c0..0859fbfc930b70fd4d02da01636d0245332d26dc 100644 (file)
--- 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 7c77dbd1284d0bbe8045b1246326b1c10a1e7409..a4b01fecaed3a4d66f9ce44c7cc3f8d0d49ab9c8 100644 (file)
--- 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. */