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Subject: Re: [PATCH 06/13] writeback: bdi write bandwidth estimation
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Wu Fengguang <fengguang.wu@intel.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Jan Kara <jack@suse.cz>,
        "Li, Shaohua" <shaohua.li@intel.com>, Christoph Hellwig <hch@lst.de>,
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Date: Wed, 24 Nov 2010 15:31:57 +0100
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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.