[PATCH 3/8] readahead: state based method: prevent tiny size

[Fengguang Wu <wfg@mail.ustc.edu.cn>]

- cancel look-ahead if readahead size is small compared to the look-ahead size,
  to avoid too small I/O size, and not to add more memory/io pressure on the
  possibly loaded system.
- prevent issuing tiny readahead size, by applying a minimal limit

Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
---
 mm/readahead.c |   21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

--- linux-2.6.20-rc4-mm1.orig/mm/readahead.c
+++ linux-2.6.20-rc4-mm1/mm/readahead.c
@@ -932,17 +932,21 @@ static int adjust_rala(unsigned long ra_
 			unsigned long *ra_size, unsigned long *la_size)
 {
 	/*
-	 * Substract the old look-ahead to get real safe size for the next
-	 * read-ahead request.
+	 * Cancel asynchrous read-ahead,
+	 * if there is a major upsurge of load, or fall of this stream's speed.
 	 */
-	if (*ra_size > *la_size)
-		*ra_size -= *la_size;
-	else {
+	if (*ra_size <= *la_size * 2) {
 		ra_account(NULL, RA_EVENT_READAHEAD_SHRINK, *ra_size);
 		return 0;
 	}
 
 	/*
+	 * Substract the old look-ahead to get real safe size for the next
+	 * read-ahead request.
+	 */
+	*ra_size -= *la_size;
+
+	/*
 	 * Set new la_size according to the (still large) ra_size.
 	 */
 	*la_size = *ra_size / LOOKAHEAD_RATIO;
@@ -956,6 +960,13 @@ static void limit_rala(unsigned long ra_
 	unsigned long stream_shift;
 
 	/*
+	 * Protect against too small I/O sizes,
+	 * by mapping [0, 4*min] to [min, 4*min].
+	 */
+	if (*ra_size < 4 * MIN_RA_PAGES)
+		*ra_size = MIN_RA_PAGES + *ra_size * 3 / 4;
+
+	/*
 	 * Apply basic upper limits.
 	 */
 	if (*ra_size > ra_max)

--
-
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