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

[Wu Fengguang]

On Wed, Nov 24, 2010 at 10:31:57PM +0800, Peter Zijlstra wrote:
> 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.

Good point!

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

Yeah, maybe not a good trade off.

> > > +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.

When there are lots of concurrent writers, we'll target at ~100ms
pause time, hence the update frequency will be lowered accordingly.

Thanks,
Fengguang
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/