set MAP_NOCACHE if we can

Set the MAP_NOCACHE flags in mmap() if we can. On OS X this stops the
mad super-aggressive disc caching and produces a huge improveemnt in
performance.

ChangeLog

@@ -28,6 +28,8 @@
 - move im_stretch3() to deprecated
 - move im_clamp() to deprecated
 - gtk-doc for video ... all operators done! amazing argh
+- set MAP_NOCACHE on OS X, otherwise performance dives off a cliff with 
+  files larger than memory
 
 30/11/10 started 7.24.0
 - bump for new stable

TODO

@@ -1,7 +1,8 @@
 
-- does OS X have any mmap flags which could stop their crazy caching?
+- use g_mmap() or whatever it's called ... or perhaps they don't have all
+  these options
+
 
-- use g_mmap() or whatever it's called 
 
 
 - use |_O_TEMPORARY to open()'s mode on Windows for tmpfiles to increase the

libvips/iofuncs/im_mapfile.c

@@ -163,12 +163,23 @@ im__mmap( int fd, int writeable, size_t length, gint64 offset )
 #else /*!OS_WIN32*/
 {
 	int prot;
+	int flags;
 
 	if( writeable ) 
 		prot = PROT_WRITE;
 	else 
 		prot = PROT_READ;
 
+	flags = MAP_SHARED;
+
+	/* OS X caches mmapped files very aggressively if this flags is not
+	 * set. Scanning a large file without this flag will cause every other
+	 * process to get swapped out and kill performance.
+	 */
+#ifdef MAP_NOCACHE
+	flags |= MAP_NOCACHE;
+#endif /*MAP_NOCACHE*/
+
 	/* Casting gint64 to off_t should be safe, even on *nixes without
 	 * LARGEFILE.
 	 */