[PATCH 17/28] readahead: initial method - thrashing guard size

[Wu Fengguang]

backing_dev_info.ra_thrash_bytes is dynamicly updated to be a little above
the thrashing safe read-ahead size. It is used in the initial method where
the thrashing threshold for the particular reader is still unknown.

Signed-off-by: Wu Fengguang <wfg@xxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxx>
--- linux-2.6.19-rc5-mm2.orig/mm/readahead.c
+++ linux-2.6.19-rc5-mm2/mm/readahead.c
@@ -822,6 +822,22 @@ out:
 }
 
 /*
+ * Update `backing_dev_info.ra_thrash_bytes' to be a _biased_ average of
+ * read-ahead sizes. Which makes it an a-bit-risky(*) estimation of the
+ * _minimal_ read-ahead thrashing threshold on the device.
+ *
+ * (*) Note that being a bit risky can _help_ overall performance.
+ */
+static void update_ra_thrash_bytes(struct backing_dev_info *bdi,
+							unsigned long ra_size)
+{
+	ra_size <<= PAGE_CACHE_SHIFT;
+	bdi->ra_thrash_bytes = (bdi->ra_thrash_bytes < ra_size) ?
+				(ra_size + bdi->ra_thrash_bytes * 127) / 128:
+				(ra_size + bdi->ra_thrash_bytes *   7) /   8;
+}
+
+/*
  * Some helpers for querying/building a read-ahead request.
  *
  * Diagram for some variable names used frequently:
@@ -1118,6 +1134,10 @@ state_based_readahead(struct address_spa
 
 	limit_rala(growth_limit, la_old, &ra_size, &la_size);
 
+	/* ra_size in its _steady_ state reflects thrashing threshold */
+	if (page && ra_old + ra_old / 8 >= ra_size)
+		update_ra_thrash_bytes(mapping->backing_dev_info, ra_size);
+
 	ra_set_class(ra, RA_CLASS_STATE);
 	ra_set_index(ra, offset, ra->readahead_index);
 	ra_set_size(ra, ra_size, la_size);

--
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