Re: [PATCH 04/35] writeback: reduce per-bdi dirty threshold rampup time

[Richard Kennedy]

On Mon, 2010-12-13 at 22:46 +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@xxxxxxxxx>
> CC: Richard Kennedy <richard@xxxxxxxxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
> ---
>  mm/page-writeback.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- linux-next.orig/mm/page-writeback.c	2010-12-13 21:46:11.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2010-12-13 21:46:11.000000000 +0800
> @@ -145,7 +145,7 @@ 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;
>  }
>  
>  /*
> 
> 
Hi Fengguang,

I've been running my test set on your v3 series and generally it's
giving good results in line with the mainline kernel, with much less
variability and lower standard deviation of the results so it is much
more repeatable.

However, it doesn't seem to be honouring the background_dirty_threshold.

The attached graph is from a simple fio write test of 400Mb on ext4.
All dirty pages are completely written in 15 seconds, but I expect to
see up to background_dirty_threshold pages staying dirty until the 30
second background task writes them out. So it is much too eager to write
back dirty pages.

As to the ramp up time, when writing to 2 disks at the same time I see
the per_bdi_threshold taking up to 20 seconds to converge on a steady
value after one of the write stops. So I think this could be speeded up
even more, at least on my setup.

I am just about to start testing v4 & will report anything interesting.

regards
Richard

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