lkml.org 
[lkml]   [2011]   [Nov]   [21]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    Patch in this message
    /
    Date
    From
    Subject[PATCH 8/8] readahead: dont do start-of-file readahead after lseek()
    Some applications (eg. blkid, id3tool etc.) seek around the file
    to get information. For example, blkid does

    seek to 0
    read 1024
    seek to 1536
    read 16384

    The start-of-file readahead heuristic is wrong for them, whose
    access pattern can be identified by lseek() calls.

    So test-and-set a READAHEAD_LSEEK flag on lseek() and don't
    do start-of-file readahead on seeing it. Proposed by Linus.

    Acked-by: Rik van Riel <riel@redhat.com>
    Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    fs/read_write.c | 4 ++++
    include/linux/fs.h | 1 +
    mm/readahead.c | 3 +++
    3 files changed, 8 insertions(+)
    --- linux-next.orig/mm/readahead.c 2011-11-20 22:02:01.000000000 +0800
    +++ linux-next/mm/readahead.c 2011-11-20 22:02:03.000000000 +0800
    @@ -629,6 +629,8 @@ ondemand_readahead(struct address_space
    * start of file
    */
    if (!offset) {
    + if ((ra->ra_flags & READAHEAD_LSEEK) && req_size < max)
    + goto random_read;
    ra_set_pattern(ra, RA_PATTERN_INITIAL);
    goto initial_readahead;
    }
    @@ -707,6 +709,7 @@ ondemand_readahead(struct address_space
    if (try_context_readahead(mapping, ra, offset, req_size, max))
    goto readit;

    +random_read:
    /*
    * standalone, small random read
    */
    --- linux-next.orig/fs/read_write.c 2011-11-20 22:02:01.000000000 +0800
    +++ linux-next/fs/read_write.c 2011-11-20 22:02:03.000000000 +0800
    @@ -47,6 +47,10 @@ static loff_t lseek_execute(struct file
    file->f_pos = offset;
    file->f_version = 0;
    }
    +
    + if (!(file->f_ra.ra_flags & READAHEAD_LSEEK))
    + file->f_ra.ra_flags |= READAHEAD_LSEEK;
    +
    return offset;
    }

    --- linux-next.orig/include/linux/fs.h 2011-11-20 22:02:01.000000000 +0800
    +++ linux-next/include/linux/fs.h 2011-11-20 22:02:03.000000000 +0800
    @@ -952,6 +952,7 @@ struct file_ra_state {
    /* ra_flags bits */
    #define READAHEAD_MMAP_MISS 0x000003ff /* cache misses for mmap access */
    #define READAHEAD_MMAP 0x00010000
    +#define READAHEAD_LSEEK 0x00020000 /* be conservative after lseek() */

    #define READAHEAD_PATTERN_SHIFT 28
    #define READAHEAD_PATTERN 0xf0000000



    \
     
     \ /
      Last update: 2011-11-21 10:45    [from the cache]
    ©2003-2014 Jasper Spaans. Advertise on this site