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Subject: Re: [PATCH 09/13] writeback: reduce per-bdi dirty threshold ramp
 up time
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        Richard Kennedy <richard@rsk.demon.co.uk>,
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        "Theodore Ts'o" <tytso@mit.edu>, Chris Mason <chris.mason@oracle.com>,
        Mel Gorman <mel@csn.ul.ie>, Rik van Riel <riel@redhat.com>,
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        LKML <linux-kernel@vger.kernel.org>
In-Reply-To: <20101117042850.361893350@intel.com>
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	 <20101117042850.361893350@intel.com>
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Date: Wed, 24 Nov 2010 12:15:41 +0100
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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.