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    Subject[PATCH 13/17] writeback: reduce per-bdi dirty threshold ramp up time
    Reduce the dampening for the control system, yielding faster
    convergence.

    Currently it converges at a snail's pace for slow devices (in order of
    minutes). For really fast storage, the convergence speed should be fine.

    It makes sense to make it reasonably fast for typical desktops.

    After patch, it converges in ~10 seconds for 60MB/s writes and 4GB mem.
    So expect ~1s for a fast 600MB/s storage under 4GB mem, or ~4s under
    16GB mem, which looks good.

    $ while true; do grep BdiDirtyThresh /debug/bdi/8:0/stats; sleep 1; done
    BdiDirtyThresh: 0 kB
    BdiDirtyThresh: 118748 kB
    BdiDirtyThresh: 214280 kB
    BdiDirtyThresh: 303868 kB
    BdiDirtyThresh: 376528 kB
    BdiDirtyThresh: 411180 kB
    BdiDirtyThresh: 448636 kB
    BdiDirtyThresh: 472260 kB
    BdiDirtyThresh: 490924 kB
    BdiDirtyThresh: 499596 kB
    BdiDirtyThresh: 507068 kB
    ...
    DirtyThresh: 530392 kB

    CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
    CC: Richard Kennedy <richard@rsk.demon.co.uk>
    CC: Martin J. Bligh <mbligh@google.com>
    Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
    ---
    mm/page-writeback.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)
    --- linux-next.orig/mm/page-writeback.c 2010-08-30 10:24:00.000000000 +0800
    +++ linux-next/mm/page-writeback.c 2010-08-30 10:27:10.000000000 +0800
    @@ -131,7 +131,7 @@ static int calc_period_shift(void)
    else
    dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
    100;
    - return 2 + ilog2(dirty_total - 1);
    + return ilog2(dirty_total - 1) - 1;
    }

    /*



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