[PATCH] Tweak VM dirty thresholds

The below patch on 2.5.12 takes the 'dbench 32' throughput on a
1024 megabyte machine from 35 megs/sec up to 125 megs/sec.
That's on the second run.  The first run is much slower because
the ext2 bitmaps aren't in cache.

Presumably, s/60/80/ will take it even higher.

fs/fs-writeback.c

@@ -219,7 +219,7 @@ static void __sync_list(struct list_head *head, int sync_mode,
 	struct list_head * tmp;
 	LIST_HEAD(hold);	/* Unready inodes go here */
 
-	while ((tmp = head->next) != head) {
+	while ((tmp = head->prev) != head) {
 		struct inode *inode = list_entry(tmp, struct inode, i_list);
 		struct address_space *mapping = inode->i_mapping;
 		int really_sync;

mm/page-writeback.c

@@ -29,12 +29,12 @@
 /*
  * Start background writeback (via pdflush) at this level
  */
-static int dirty_background_ratio = 30;
+static int dirty_background_ratio = 20;
 
 /*
  * The generator of dirty data starts async writeback at this level
  */
-static int dirty_async_ratio = 45;
+static int dirty_async_ratio = 60;
 
 /*
  * The generator of dirty data performs sync writeout at this level