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    Subject[PATCH 7/9] readahead: sequential mmap readahead
    Auto-detect sequential mmap reads and do readahead for them.

    The sequential mmap readahead will be triggered when
    - sync readahead: it's a major fault and (prev_offset == offset-1);
    - async readahead: minor fault on PG_readahead page with valid readahead state.

    The benefits of doing readahead instead of read-around:
    - less I/O wait thanks to async readahead
    - double real I/O size and no more cache hits

    The single stream case is improved a little.
    For 100,000 sequential mmap reads:

    user system cpu total
    (1-1) plain -mm, 128KB readaround: 3.224 2.554 48.40% 11.838
    (1-2) plain -mm, 256KB readaround: 3.170 2.392 46.20% 11.976
    (2) patched -mm, 128KB readahead: 3.117 2.448 47.33% 11.607

    The patched (2) has smallest total time, since it has no cache hit overheads
    and less I/O block time(thanks to async readahead). Here the I/O size
    makes no much difference, since there's only one single stream.

    Note that (1-1)'s real I/O size is 64KB and (1-2)'s real I/O size is 128KB,
    since the half of the read-around pages will be readahead cache hits.

    This is going to make _real_ differences for _concurrent_ IO streams.

    Cc: Nick Piggin <npiggin@suse.de>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    mm/filemap.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- mm.orig/mm/filemap.c
    +++ mm/mm/filemap.c
    @@ -1471,7 +1471,8 @@ static void do_sync_mmap_readahead(struc
    if (VM_RandomReadHint(vma))
    return;

    - if (VM_SequentialReadHint(vma)) {
    + if (VM_SequentialReadHint(vma) ||
    + offset - 1 == (ra->prev_pos >> PAGE_CACHE_SHIFT)) {
    page_cache_sync_readahead(mapping, ra, file, offset, 1);
    return;
    }
    --



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