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    SubjectRe: bdi_threshold slow to reach steady state
    On Wed, 2009-10-14 at 13:37 +0200, Peter Zijlstra wrote:
    > On Wed, 2009-10-14 at 12:09 +0100, Richard Kennedy wrote:
    > > Hi Peter,

    > Right, so we measure time in page writeback completions, and the measure
    > I used was the round up power of two of the dirty_thresh. We adjust in
    > the same time it takes to write out a full dirty_thresh amount of data.
    > The idea was that people would scale their dirty thesh according to
    > their writeout capacity, etc..
    > Martin J Bligh complained about this very same issue and I told them to
    > experiment with that same scale function. But I guess the result of that
    > got lost in the google filter (stuff goes in, nothing ever comes back
    > out).
    > Anyway, the dirty_thresh relation seems sensible still, but the exact
    > parameters could be poked at. I have no objection to reducing the period
    > with a factor of 16 like you did, except that we need some more
    > feedback, preferably from people with more than a few spindles.

    Sure, hopefully big fast machines have large amounts of memory so it
    should be a good fit.

    Yes, it would be good if someone with a big box tested this ;)
    Here's a patch just in case anyone does feel like giving it a spin.

    > (The initial ramp will be roughly twice as slow, since the steady state
    > of this approximation is half-full).
    > > I know that my machine is getting a bit old now, it's AMDX2 & only has
    > > sata 150 drives, so I'm not suggesting that this change is going to be
    > > correct for all machines but maybe we can set a better default? or take
    > > more factors in to account other than just memory size.
    > >
    > > BTW why is it ilog2(dirty_total -1) -- what does the -1 do?
    thanks for that

    (patch against 2.6.32-rc4)

    commit 11735a2336ba08cf21aebf79a706c86aca5e44b2
    Author: Richard Kennedy <>
    Date: Wed Oct 14 14:46:21 2009 +0100

    mm: speed up per bdi dirty threshold calculations

    Signed-off-by: Richard Kennedy <>

    diff --git a/mm/page-writeback.c b/mm/page-writeback.c
    index a3b1409..018024e 100644
    --- a/mm/page-writeback.c
    +++ b/mm/page-writeback.c
    @@ -144,7 +144,7 @@ static int calc_period_shift(void)
    dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
    - return 2 + ilog2(dirty_total - 1);
    + return ilog2(dirty_total - 1) - 2;


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