The context method will lead to noticable overhead on sparse random reads.
Having the readahead window to start slowly makes it much better.

Signed-off-by: Wu Fengguang <wfg@mail.ustc.edu.cn>
---


--- linux-2.6.17-mm2.orig/mm/readahead.c
+++ linux-2.6.17-mm2/mm/readahead.c
@@ -1548,6 +1548,11 @@ try_context_based_readahead(struct addre
 			return -1;
 	} else if (prev_page || probe_page(mapping, index - 1)) {
 		ra_index = index;
+		/*
+		 * Slow start of readahead window.
+		 * It helps avoid most readahead miss on sparse random reads.
+		 */
+		ra_min = readahead_hit_rate;
 	} else if (readahead_hit_rate > 1) {
 		ra_index = find_segtail_backward(mapping, index,
 						readahead_hit_rate + ra_min);

--