From: Wu Fengguang Readahead policy on read after seeking. It tries to detect sequences like: seek(), 5*read(); seek(), 6*read(); seek(), 4*read(); ... Signed-off-by: Wu Fengguang Signed-off-by: Andrew Morton --- mm/readahead.c | 43 +++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 43 insertions(+) diff -puN mm/readahead.c~readahead-seeking-reads-method mm/readahead.c --- devel/mm/readahead.c~readahead-seeking-reads-method 2006-06-09 01:22:50.000000000 -0700 +++ devel-akpm/mm/readahead.c 2006-06-09 01:22:50.000000000 -0700 @@ -1583,6 +1583,49 @@ try_read_backward(struct file_ra_state * } /* + * If there is a previous sequential read, it is likely to be another + * sequential read at the new position. + * + * i.e. detect the following sequences: + * seek(), 5*read(); seek(), 6*read(); seek(), 4*read(); ... + * + * Databases are known to have this seek-and-read-N-pages pattern. + */ +static int +try_readahead_on_seek(struct file_ra_state *ra, pgoff_t index, + unsigned long ra_size, unsigned long ra_max) +{ + unsigned long hit0 = ra->hit0; + unsigned long hit1 = ra->hit1 + hit0; + unsigned long hit2 = ra->hit2; + unsigned long hit3 = ra->hit3; + + /* There's a previous read-ahead request? */ + if (!ra_has_index(ra, ra->prev_page)) + return 0; + + /* The previous read-ahead sequences have similiar sizes? */ + if (!(ra_size < hit1 && hit1 > hit2 / 2 && + hit2 > hit3 / 2 && + hit3 > hit1 / 2)) + return 0; + + hit1 = max(hit1, hit2); + + /* Follow the same prefetching direction. */ + if ((ra->flags & RA_CLASS_MASK) == RA_CLASS_BACKWARD) + index = ((index > hit1 - ra_size) ? index - hit1 + ra_size : 0); + + ra_size = min(hit1, ra_max); + + ra_set_class(ra, RA_CLASS_SEEK); + ra_set_index(ra, index, index); + ra_set_size(ra, ra_size, 0); + + return 1; +} + +/* * ra_min is mainly determined by the size of cache memory. Reasonable? * * Table of concrete numbers for 4KB page size: _