From: Wu Fengguang 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 ==================================== stock readahead: avg 12.3 seconds ================================= The rxvt-unicode-7.7 being benchmarked is a dir with many .C/.h/.o files. Signed-off-by: Wu Fengguang Signed-off-by: Andrew Morton --- mm/readahead.c | 37 +++++++++++++++++++++++++++++++++++++ 1 file changed, 37 insertions(+) diff -puN mm/readahead.c~readahead-initial-method mm/readahead.c --- a/mm/readahead.c~readahead-initial-method +++ a/mm/readahead.c @@ -1541,6 +1541,43 @@ 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: _