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Subject[PATCH 21/32] readahead: initial method
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
@@ -1499,6 +1499,43 @@ 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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