Re: [PATCH] mm: Properly reflect task dirty limits in dirty_exceeded logic

[Jan Kara <jack@suse.cz>]

On Sat 23-07-11 15:43:45, Wu Fengguang wrote:
> On Fri, Jul 15, 2011 at 05:34:09AM +0800, Jan Kara wrote:
> > > - tasks dirtying close to 25% pages probably cannot be called light
> > >   dirtier and there is no need to protect such tasks
> >   The idea is interesting. The only problem is that we don't want to set
> > dirty_exceeded too late so that heavy dirtiers won't push light dirtiers
> > over their limits so easily due to ratelimiting. It did some computations:
> > We normally ratelimit after 4 MB. Take a low end desktop these days. Say
> > 1 GB of ram, 4 CPUs. So dirty limit will be ~200 MB and the area for task
> > differentiation ~25 MB. We enter balance_dirty_pages() after dirtying
> > num_cpu * ratelimit / 2 pages on average which gives 8 MB. So we should
> > set dirty_exceeded at latest at bdi_dirty / TASK_LIMIT_FRACTION / 2 or
> > task differentiation would have no effect because of ratelimiting.
> > 
> > So we could change the limit to something like:
> > bdi_dirty - min(bdi_dirty / TASK_LIMIT_FRACTION, ratelimit_pages *
> > num_online_cpus / 2 + bdi_dirty / TASK_LIMIT_FRACTION / 16)
> 
> Good analyze!
> 
> > But I'm not sure setups where this would make difference are common...
> 
> I think I'd prefer the original simple patch given that the common
> 1-dirtier is not impacted.
  OK, thanks. So will you merge the patch please?

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR

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