Re: [PATCH 09/13] writeback: reduce per-bdi dirty threshold ramp up time

[Wu Fengguang <fengguang.wu@intel.com>]

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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