linux-kernel - Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up time

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Date:	Thu, 14 Apr 2011 00:04:44 +0200
From:	Jan Kara <jack@...e.cz>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Richard Kennedy <richard@....demon.co.uk>,
	Jan Kara <jack@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	Dave Chinner <david@...morbit.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up
 time

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@...llo.nl>
> CC: Richard Kennedy <richard@....demon.co.uk>
> Signed-off-by: Wu Fengguang <fengguang.wu@...el.com>
  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@...e.cz>
SUSE Labs, CR
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