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

[Wu Fengguang <fengguang.wu@intel.com>]

On Tue, Jul 26, 2011 at 12:04:29AM +0800, Jan Kara wrote:
> 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?

The patch with a minor variable rename has been in writeback.git and
linux-next for two weeks, and two days ago I updated it to your
original patch:

http://git.kernel.org/?p=linux/kernel/git/wfg/writeback.git;a=commit;h=bcff25fc8aa47a13faff8b4b992589813f7b450a

If you have no more problems with the patchset, I'll ask Linus
to pull that branch.

Thanks,
Fengguang

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