Re: [PATCH 3/5] mm: Implement IO-less balance_dirty_pages()

[Vivek Goyal <vgoyal@redhat.com>]

On Wed, Mar 16, 2011 at 08:58:44PM +0100, Jan Kara wrote:

[..]
> > > > Had a query.
> > > > 
> > > > - What makes sure that flusher thread will not stop writing back till all
> > > >   the waiters on the bdi have been woken up. IIUC, flusher thread will 
> > > >   stop once global background ratio is with-in limit. Is it possible that
> > > >   there are still some waiter on some bdi waiting for more pages to finish
> > > >   writeback and that might not happen for sometime. 
> > >   Yes, this can possibly happen but once distribute_page_completions()
> > > gets called (after a given time), it will notice that we are below limits
> > > and wake all waiters.
> > > Under normal circumstances, we should have a decent
> > > estimate when distribute_page_completions() needs to be called and that
> > > should be long before flusher thread finishes it's work. But in cases when
> > > a bdi has only a small share of global dirty limit, what you describe can
> > > possibly happen.
> > 
> > So if a bdi share is small then it can happen that global background
> > threshold is fine but per bdi threshold is not. That means
> > task_bdi_threshold is also above limit and IIUC, distribute_page_completion()
> > will not wake up the waiter until bdi_task_limit_exceeded() is in control.
>   It will wake them. What you miss is the check right at the beginning of
> distribute_page_completions():
>       dirty_exceeded = check_dirty_limits(bdi, &st);
>       if (dirty_exceeded < DIRTY_MAY_EXCEED_LIMIT) {
>                /* Wakeup everybody */
> ...
> 
>   When we are globally below (background+limit)/2, dirty_exceeded is set to
> DIRTY_OK or DIRTY_BACKGROUND and thus we just wake all the waiters.

Ok, thanks. Now I see it. 

Thanks
Vivek

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