[PATCH 22/33] readahead: initial method

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

Aggressive readahead policy for read on start-of-file.

Instead of selecting a conservative readahead size,
it tries to do large readahead in the first place.

However we have to watch on two cases:
	- do not ruin the hit rate for file-head-checkers
	- do not lead to thrashing for memory tight systems

It benefits the many-small-files case:

adaptive readahead: avg 10.3 seconds
====================================
diff -r work/rxvt-unicode-7.7 /tmp/rxvt-unicode-7.7  0.10s user 0.32s system 4% cpu 10.211 total
diff -r work/rxvt-unicode-7.7 /tmp/rxvt-unicode-7.7  0.09s user 0.31s system 3% cpu 10.389 total

stock readahead: avg 12.3 seconds
=================================
diff -r work/rxvt-unicode-7.7 /tmp/rxvt-unicode-7.7  0.12s user 0.30s system 3% cpu 12.274 total
diff -r work/rxvt-unicode-7.7 /tmp/rxvt-unicode-7.7  0.09s user 0.33s system 3% cpu 12.403 total

The rxvt-unicode-7.7 being benchmarked is a dir with many .C/.h/.o files.

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

 mm/readahead.c |   37 +++++++++++++++++++++++++++++++++++++
 1 files changed, 37 insertions(+)

--- linux-2.6.17-rc4-mm3.orig/mm/readahead.c
+++ linux-2.6.17-rc4-mm3/mm/readahead.c
@@ -1537,6 +1537,43 @@ try_context_based_readahead(struct addre
 }
 
 /*
+ * Read-ahead on start of file.
+ *
+ * We want to be as aggressive as possible, _and_
+ * 	- do not ruin the hit rate for file-head-peekers
+ * 	- do not lead to thrashing for memory tight systems
+ */
+static unsigned long
+initial_readahead(struct address_space *mapping, struct file *filp,
+		struct file_ra_state *ra, unsigned long req_size)
+{
+	struct backing_dev_info *bdi = mapping->backing_dev_info;
+	unsigned long thrash_pages = bdi->ra_thrash_bytes >> PAGE_CACHE_SHIFT;
+	unsigned long expect_pages = bdi->ra_expect_bytes >> PAGE_CACHE_SHIFT;
+	unsigned long ra_size;
+	unsigned long la_size;
+
+	ra_size = req_size;
+
+	/* be aggressive if the system tends to read more */
+	if (ra_size < expect_pages)
+		ra_size = expect_pages;
+
+	/* no read-ahead thrashing */
+	if (ra_size > thrash_pages)
+		ra_size = thrash_pages;
+
+	/* do look-ahead on large(>= 32KB) read-ahead */
+	la_size = ra_size / LOOKAHEAD_RATIO;
+
+	ra_set_class(ra, RA_CLASS_INITIAL);
+	ra_set_index(ra, 0, 0);
+	ra_set_size(ra, ra_size, la_size);
+
+	return ra_dispatch(ra, mapping, filp);
+}
+
+/*
  * ra_min is mainly determined by the size of cache memory. Reasonable?
  *
  * Table of concrete numbers for 4KB page size:

--
-
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