Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp uptime

[Jan Kara]

On Wed 13-04-11 16:59:41, Wu Fengguang wrote:
> Reduce the dampening for the control system, yielding faster
> convergence. The change is a bit conservative, as smaller values may
> lead to noticeable bdi threshold fluctuates in low memory JBOD setup.
> 
> CC: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
> CC: Richard Kennedy <richard@xxxxxxxxxxxxxxx>
> Signed-off-by: Wu Fengguang <fengguang.wu@xxxxxxxxx>
  Well, I have nothing against this change as such but what I don't like is
that it just changes magical +2 for similarly magical +0. It's clear that
this will lead to more rapid updates of proportions of bdi's share of
writeback and thread's share of dirtying but why +0? Why not +1 or -1? So
I'd prefer to get some understanding of why do we need to update the
proportion period and why 4-times faster is just the right amount of faster
:) If I remember right you had some numbers for this, didn't you?

								Honza
> ---
>  mm/page-writeback.c |    2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> --- linux-next.orig/mm/page-writeback.c	2011-03-02 14:52:19.000000000 +0800
> +++ linux-next/mm/page-writeback.c	2011-03-02 15:00:17.000000000 +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);
>  }
>  
>  /*
> 
> 
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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