]> git.karo-electronics.de Git - karo-tx-linux.git/commit
readahead: introduce FMODE_RANDOM for POSIX_FADV_RANDOM
authorWu Fengguang <fengguang.wu@intel.com>
Fri, 5 Mar 2010 21:42:03 +0000 (13:42 -0800)
committerGreg Kroah-Hartman <gregkh@suse.de>
Mon, 15 Mar 2010 15:49:37 +0000 (08:49 -0700)
commit7c083ba91b1fead40edfe8397606e04e92a0fc79
tree4964d760a59a0265c65f2000fada58b3d78588dd
parentd9ee70916b2f1bb17ebdf993e591d2fd84e7e775
readahead: introduce FMODE_RANDOM for POSIX_FADV_RANDOM

commit 0141450f66c3c12a3aaa869748caa64241885cdf upstream.

This fixes inefficient page-by-page reads on POSIX_FADV_RANDOM.

POSIX_FADV_RANDOM used to set ra_pages=0, which leads to poor performance:
a 16K read will be carried out in 4 _sync_ 1-page reads.

In other places, ra_pages==0 means
- it's ramfs/tmpfs/hugetlbfs/sysfs/configfs
- some IO error happened
where multi-page read IO won't help or should be avoided.

POSIX_FADV_RANDOM actually want a different semantics: to disable the
*heuristic* readahead algorithm, and to use a dumb one which faithfully
submit read IO for whatever application requests.

So introduce a flag FMODE_RANDOM for POSIX_FADV_RANDOM.

Note that the random hint is not likely to help random reads performance
noticeably.  And it may be too permissive on huge request size (its IO
size is not limited by read_ahead_kb).

In Quentin's report (http://lkml.org/lkml/2009/12/24/145), the overall
(NFS read) performance of the application increased by 313%!

Tested-by: Quentin Barnes <qbarnes+nfs@yahoo-inc.com>
Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Cc: Chuck Lever <chuck.lever@oracle.com>
Cc: <qbarnes+nfs@yahoo-inc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
include/linux/fs.h
mm/fadvise.c
mm/readahead.c