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SubjectRe: [PATCH 06/13] writeback: bdi write bandwidth estimation
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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.


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