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    Subject[PATCH] btrfs: lower metadata writeback threshold on low dirty threshold
    This helps write performance when setting the dirty threshold to tiny numbers.

    3.4.0-rc2 3.4.0-rc2-btrfs4+
    ------------ ------------------------
    96.92 -0.4% 96.54 bay/thresh=1000M/btrfs-100dd-1-3.4.0-rc2
    98.47 +0.0% 98.50 bay/thresh=1000M/btrfs-10dd-1-3.4.0-rc2
    99.38 -0.3% 99.06 bay/thresh=1000M/btrfs-1dd-1-3.4.0-rc2
    98.04 -0.0% 98.02 bay/thresh=100M/btrfs-100dd-1-3.4.0-rc2
    98.68 +0.3% 98.98 bay/thresh=100M/btrfs-10dd-1-3.4.0-rc2
    99.34 -0.0% 99.31 bay/thresh=100M/btrfs-1dd-1-3.4.0-rc2
    ==> 88.98 +9.6% 97.53 bay/thresh=10M/btrfs-10dd-1-3.4.0-rc2
    ==> 86.99 +13.1% 98.39 bay/thresh=10M/btrfs-1dd-1-3.4.0-rc2
    ==> 2.75 +2442.4% 69.88 bay/thresh=1M/btrfs-10dd-1-3.4.0-rc2
    ==> 3.31 +2634.1% 90.54 bay/thresh=1M/btrfs-1dd-1-3.4.0-rc2

    Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
    ---
    fs/btrfs/disk-io.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    --- linux-next.orig/fs/btrfs/disk-io.c 2012-05-02 14:04:00.989262395 +0800
    +++ linux-next/fs/btrfs/disk-io.c 2012-05-02 14:04:01.773262414 +0800
    @@ -930,7 +930,8 @@ static int btree_writepages(struct addre

    /* this is a bit racy, but that's ok */
    num_dirty = root->fs_info->dirty_metadata_bytes;
    - if (num_dirty < thresh)
    + if (num_dirty < min(thresh,
    + global_dirty_limit << (PAGE_CACHE_SHIFT-2)))
    return 0;
    }
    return btree_write_cache_pages(mapping, wbc);

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