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    Subject[PATCH 1/9] readahead: make context readahead more conservative
    Try to prevent negatively impact moderately dense random reads on SSD.

    Transaction-Per-Second numbers provided by Taobao:

    QPS case
    -------------------------------------------------------
    7536 disable context readahead totally
    w/ patch: 7129 slower size rampup and start RA on the 3rd read
    6717 slower size rampup
    w/o patch: 5581 unmodified context readahead

    Before, readahead will be started whenever reading page N+1 when it
    happen to read N recently. After patch, we'll only start readahead
    when *three* random reads happen to access pages N, N+1, N+2. The
    probability of this happening is extremely low for pure random reads,
    unless they are very dense, which actually deserves some readahead.

    Also start with a smaller readahead window. The impact to interleaved
    sequential reads should be small, because for a long run stream, the
    the small readahead window rampup phase is negletable.

    The context readahead actually benefits clustered random reads on HDD
    whose seek cost is pretty high. However as SSD is increasingly used for
    random read workloads it's better for the context readahead to
    concentrate on interleaved sequential reads.

    Tested-by: Tao Ma <tm@tao.ma>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    mm/readahead.c | 8 ++++----
    1 file changed, 4 insertions(+), 4 deletions(-)

    --- linux-next.orig/mm/readahead.c 2012-01-25 15:57:47.000000000 +0800
    +++ linux-next/mm/readahead.c 2012-01-25 15:57:49.000000000 +0800
    @@ -369,10 +369,10 @@ static int try_context_readahead(struct
    size = count_history_pages(mapping, ra, offset, max);

    /*
    - * no history pages:
    + * not enough history pages:
    * it could be a random read
    */
    - if (!size)
    + if (size <= req_size)
    return 0;

    /*
    @@ -383,8 +383,8 @@ static int try_context_readahead(struct
    size *= 2;

    ra->start = offset;
    - ra->size = get_init_ra_size(size + req_size, max);
    - ra->async_size = ra->size;
    + ra->size = min(size + req_size, max);
    + ra->async_size = 1;

    return 1;
    }



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