• Wu Fengguang's avatar
    readahead: introduce FMODE_RANDOM for POSIX_FADV_RANDOM · 0141450f
    Wu Fengguang authored
    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: default avatarQuentin Barnes <qbarnes+nfs@yahoo-inc.com>
    Signed-off-by: default avatarWu 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: <stable@kernel.org>			[2.6.33.x]
    Cc: <qbarnes+nfs@yahoo-inc.com>
    Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
fadvise.c 3.54 KB