minimum bytes for mmap windows

ChangeLog

@@ -4,6 +4,8 @@
 - argh, im_measure() was not looping over bands correctly (thanks Peter)
 - removed mmap_limit, we now always use windows ... reduces VM use hugely,
   because mmap windows are freed when their regions are freed
+- have a min bytes for mmap windows as well, so we don't make too many tiny
+  windows
 
 26/11/09 started 7.20.3
 - updated en_GB.po translation 

TODO

@@ -1,6 +1,3 @@
-WONTFIX for 7.20
-================
-
 - some of deprecated.h repeats stuff we do better in image.h, eg. addr()
   should just call IM_REGION_ADDR()
 
@@ -63,10 +60,6 @@ Actions:
   
   how can we move them to im_affinei ?
 
-- make a "deprecated" package too
-
-- update the Portfiles on the mac and on the website from the macports ones
-
 - radiance read/write needs docs
 
   maybe have an im_format(3) page with all the built-in formats?

libvips/include/vips/private.h

@@ -41,10 +41,15 @@ extern "C" {
 
 #define IM_SPARE (8)
 
-/* Private to iofuncs: the image size above which we switch from
- * mmap()-whole-image behaviour to mmap()-window, plus window margins.
+/* Private to iofuncs: the minimum number of scanlines we add above and below 
+ * the window as a margin for slop.
  */
-#define IM__WINDOW_MARGIN (128)
+#define IM__WINDOW_MARGIN_PIXELS (128)
+
+/* Private to iofuncs: add at least this many bytes above and below the window. 
+ * There's no point mapping just a few KB of a small image.
+ */
+#define IM__WINDOW_MARGIN_BYTES (1024 * 1024 * 10)
 
 /* sizeof() a VIPS header on disc.
  */

libvips/iofuncs/window.c

@@ -33,7 +33,6 @@
  */
 
 /*
-#define DEBUG_ENVIRONMENT
 #define DEBUG_TOTAL
 #define DEBUG
  */
@@ -74,7 +73,12 @@ int im__read_test;
 
 /* Add this many lines above and below the mmap() window.
  */
-int im__window_margin = IM__WINDOW_MARGIN;
+int im__window_margin_pixels = IM__WINDOW_MARGIN_PIXELS;
+
+/* Always map at least this many bytes. There's no point making tiny windows
+ * on small files.
+ */
+int im__window_margin_bytes = IM__WINDOW_MARGIN_BYTES;
 
 /* Track global mmap usage.
  */
@@ -219,7 +223,7 @@ im_window_set( im_window_t *window, int top, int height )
 	gint64 start, end, pagestart;
 	size_t length, pagelength;
 
-	/* Calculate start and length for our window.
+	/* Calculate start and length for our window. 
 	 */
 	start = window->im->sizeof_header + 
 		(gint64) IM_IMAGE_SIZEOF_LINE( window->im ) * top;
@@ -347,31 +351,20 @@ im_window_t *
 im_window_ref( IMAGE *im, int top, int height )
 {
 	im_window_t *window;
-#ifdef DEBUG_ENVIRONMENT
-	static const char *str = NULL;
-#endif /*DEBUG_ENVIRONMENT*/
-
-#ifdef DEBUG_ENVIRONMENT
-	if( !str ) {
-		if( (str = g_getenv( "IM_WINDOW_MARGIN" )) ) {
-			printf( "iofuncs/region.c: "
-				"compiled with DEBUG_ENVIRONMENT enabled\n" );
-			im__window_margin = atoi( str );
-			printf( "im_region_create: setting window margin to %d "
-				"from environment\n", im__window_margin );
-		}
-		else
-			str = "done";
-	}
-#endif /*DEBUG_ENVIRONMENT*/
 
 	g_mutex_lock( im->sslock );
 
 	if( !(window = im_window_find( im, top, height )) ) {
-		/* No existing window ... make a new one.
+		/* No existing window ... make a new one. Ask for a larger
+		 * window than we strictly need. There's no point making tiny
+		 * windows.
 		 */
-		top -= im__window_margin;
-		height += im__window_margin * 2;
+		int margin = IM_MIN( im__window_margin_pixels,
+			im__window_margin_bytes / IM_IMAGE_SIZEOF_LINE( im ) );
+
+		top -= margin;
+		height += margin * 2;
+
 		top = IM_CLIP( 0, top, im->Ysize - 1 );
 		height = IM_CLIP( 0, height, im->Ysize - top );