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

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Date:	Thu, 14 Apr 2011 09:52:11 +1000
From:	Dave Chinner <david@...morbit.com>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Richard Kennedy <richard@....demon.co.uk>,
	Hugh Dickins <hughd@...gle.com>,
	Rik van Riel <riel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux Memory Management List <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH 4/4] writeback: reduce per-bdi dirty threshold ramp up
 time

On Thu, Apr 14, 2011 at 07:31:22AM +0800, Wu Fengguang wrote:
> On Thu, Apr 14, 2011 at 06:04:44AM +0800, Jan Kara wrote:
> > 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
> 
> The patch tends to make the rampup time a bit more reasonable for
> common desktops. From 100s to 25s (see below).
> 
> > 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
> 
> Yes, it will especially be a problem on _small memory_ JBOD setups.
> Richard actually has requested for a much radical change (decrease by
> 6) but that looks too much.
> 
> My team has a 12-disk JBOD with only 6G memory. The memory is pretty
> small as a server, but it's a real setup and serves well as the
> reference minimal setup that Linux should be able to run well on.

FWIW, linux runs on a lot of low power NAS boxes with jbod and/or
raid setups that have <= 1GB of RAM (many of them run XFS), so even
your setup could be considered large by a significant fraction of
the storage world. Hence you need to be careful of optimising for
what you think is a "normal" server, because there simply isn't such
a thing....

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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