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    SubjectRe: [PATCH 09/13] writeback: reduce per-bdi dirty threshold ramp up time
    On Wed, Nov 24, 2010 at 07:15:41PM +0800, Peter Zijlstra wrote:
    > On Wed, 2010-11-17 at 12:27 +0800, Wu Fengguang wrote:
    > > plain text document attachment
    > > (writeback-speedup-per-bdi-threshold-ramp-up.patch)
    > > 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 seems reasonable.
    > >
    > > $ 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>
    > > 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 2010-11-15 13:08:16.000000000 +0800
    > > +++ linux-next/mm/page-writeback.c 2010-11-15 13:08:28.000000000 +0800
    > > @@ -125,7 +125,7 @@ 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;
    > > }
    > >
    > > /*
    >
    > You could actually improve upon this now that you have per-bdi bandwidth
    > estimations, simply set the period to (seconds * bandwidth) to get
    > convergence in @seconds.

    I'd like to, but there is the global vs. bdi discrepancy to be
    addressed first :)

    How about doing this simple fix first, and then revisit doing per-bdi
    vm_dirties after the bandwidth estimation goes upstream?

    Thanks,
    Fengguang


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