hey ho, line cache goes back too

TODO

@@ -2,14 +2,6 @@
 
 - need tests for reducel3, test every kernel plues every numeric type
 
-- removed the cache from resize, can we get out-of-order reads?
-
-  yup, try
-
-    for i in 200 300 400 500 600; do time vipsthumbnail Chicago.png -s $i; done
-
-  bench on ws as well
-
 - try orc version of reducev?
 
 - still see rounding problems on resize, try ghibli3

libvips/resample/resize.c

@@ -143,6 +143,56 @@ vips_resize_build( VipsObject *object )
 	else
 		vresidual = hresidual;
 
+	/* We want to make sure we read the image sequentially.
+	 * However, the convolution we may be doing later will force us 
+	 * into SMALLTILE or maybe FATSTRIP mode and that will break
+	 * sequentiality.
+	 *
+	 * So ... read into a cache where tiles are scanlines, and make sure
+	 * we keep enough scanlines.
+	 *
+	 * We use a threaded tilecache to avoid a deadlock: suppose thread1,
+	 * evaluating the top block of the output, is delayed, and thread2, 
+	 * evaluating the second block, gets here first (this can happen on 
+	 * a heavily-loaded system). 
+	 *
+	 * With an unthreaded tilecache, thread2 will get
+	 * the cache lock and start evaling the second block of the shrink. 
+	 * When it reaches the png reader it will stall until the first block 
+	 * has been used ... but it never will, since thread1 will block on 
+	 * this cache lock. 
+	 *
+	 * Cache sizing: we double-buffer writes, so threads can be up to one 
+	 * line of tiles behind. For example, one thread could be allocated
+	 * tile (0,0) and then stall, the whole write system won't stall until
+	 * it tries to allocate tile (0, 2).
+	 *
+	 * We reduce down after this, which can be a scale of up to @residual, 
+	 * perhaps 0.5 or down as low as 0.3 depending on the interpolator. So
+	 * the number of scanlines we need to keep for the worst case is
+	 * 2 * @tile_height / @residual, plus a little extra.
+	 */
+	if( int_vshrink > 1 ) { 
+		int tile_width;
+		int tile_height;
+		int n_lines;
+
+		int need_lines;
+
+		vips_get_tile_size( in, 
+			&tile_width, &tile_height, &n_lines );
+		need_lines = 1.2 * n_lines / vresidual;
+		if( vips_tilecache( in, &t[6], 
+			"tile_width", in->Xsize,
+			"tile_height", 10,
+			"max_tiles", 1 + need_lines / 10,
+			"access", VIPS_ACCESS_SEQUENTIAL,
+			"threaded", TRUE, 
+			NULL ) )
+			return( -1 );
+		in = t[6];
+	}
+
 	/* If the final affine will be doing a large downsample, we can get 
 	 * nasty aliasing on hard edges. Blur before affine to smooth this out.
 	 *