linux-kernel - Re: [PATCH 06/13] writeback: bdi write bandwidth estimation

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Date:	Wed, 24 Nov 2010 15:31:57 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Wu Fengguang <fengguang.wu@...el.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, Jan Kara <jack@...e.cz>,
	"Li, Shaohua" <shaohua.li@...el.com>,
	Christoph Hellwig <hch@....de>,
	Dave Chinner <david@...morbit.com>,
	Theodore Ts'o <tytso@....edu>,
	Chris Mason <chris.mason@...cle.com>,
	Mel Gorman <mel@....ul.ie>, Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	linux-mm <linux-mm@...ck.org>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/13] writeback: bdi write bandwidth estimation

On Wed, 2010-11-24 at 22:21 +0800, Wu Fengguang wrote:
> 
> Hmm, but why not avoid locking at all?  With per-cpu bandwidth vars,
> each CPU will see slightly different bandwidth, but that should be
> close enough and not a big problem.

I don't think so, on a large enough machine some cpus might hardly ever
use a particular BDI and hence get very stale data.

Also, it increases the memory footprint of the whole solution.

> > +void bdi_update_write_bandwidth(struct backing_dev_info *bdi)
> > +{
> > +     unsigned long time_now, write_now;
> > +     long time_delta, write_delta;
> > +     long bw;
> > +
> > +     if (!spin_try_lock(&bdi->bw_lock))
> > +             return;
> 
> spin_try_lock is good, however is still global state and risks
> cacheline bouncing.. 

If there are many concurrent writers to the BDI I don't think this is
going to be the top sore spot, once it is we can think of something
else.
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