[PATCH] readahead even for FMODE_RANDOM

[Jens Axboe <jens.axboe@oracle.com>]

Hi,

I got a problem report with fio where larger block size random reads
where markedly slower with buffered IO than with O_DIRECT, and the
initial thought was that perhaps this was some fio oddity. The reporter
eventually discovered that turning off the fadvise hint made it work
fine. So I took a look, and it seems we never do readahead for
FMODE_RANDOM even if the request size is larger than 1 page. That seems
like a bug, if an application is doing eg 16kb random reads, you want to
readahead the 12kb remaining data. On devices where smaller transfer
sizes are slower than larger ones, this can make a large difference.

This patch makes us readahead even for FMODE_RANDOM, iff we'll be
reading more pages in that single read. I ran a quick test here, and it
appears to fix the problem (no difference with fadvise POSIX_FADV_RANDOM
being passed in or not).

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

diff --git a/mm/readahead.c b/mm/readahead.c
index 337b20e..d4b201c 100644
--- a/mm/readahead.c
+++ b/mm/readahead.c
@@ -501,8 +501,11 @@ void page_cache_sync_readahead(struct address_space *mapping,
 	if (!ra->ra_pages)
 		return;
 
-	/* be dumb */
-	if (filp->f_mode & FMODE_RANDOM) {
+	/*
+	 * Be dumb for files marked as randomly accessed, but do readahead
+	 * inside the original request (req_size > 1).
+	 */
+	if ((filp->f_mode & FMODE_RANDOM) && req_size == 1) {
 		force_page_cache_readahead(mapping, filp, offset, req_size);
 		return;
 	}

-- 
Jens Axboe