Re: [PATCH RFC] mm: Implement balance_dirty_pages() through waiting for flusher thread

[Jan Kara <jack@suse.cz>]

On Tue 22-06-10 12:07:27, Wu Fengguang wrote:
> On Mon, Jun 21, 2010 at 03:42:39PM +0200, Jan Kara wrote:
> > On Fri 18-06-10 12:21:37, Peter Zijlstra wrote:
> > > On Thu, 2010-06-17 at 20:04 +0200, Jan Kara wrote:
> > > > +               if (bdi_stat(bdi, BDI_WRITTEN) >= bdi->wb_written_head)
> > > > +                       bdi_wakeup_writers(bdi); 
> > > 
> > > For the paranoid amongst us you could make wb_written_head s64 and write
> > > the above as:
> > > 
> > >   if (bdi_stat(bdi, BDI_WRITTEN) - bdi->wb_written_head > 0)
> > > 
> > > Which, if you assume both are monotonic and wb_written_head is always
> > > within 2^63 of the actual bdi_stat() value, should give the same end
> > > result and deal with wrap-around.
> > > 
> > > For when we manage to create a device that can write 2^64 pages in our
> > > uptime :-)
> >   OK, the fix is simple enough so I've changed it, although I'm not
> > paranoic enough ;) (I actually did the math before writing that test).
> 
> a bit more change :)
> 
> type:
> 
> -       u64 wb_written_head
> +       s64 wb_written_head
> 
> resetting:
> 
> -                       bdi->wb_written_head = ~(u64)0;
> +                       bdi->wb_written_head = 0;
> 
> setting:
> 
>                 bdi->wb_written_head = bdi_stat(bdi, BDI_WRITTEN) + wc->written;
> +               bdi->wb_written_head |= 1;
> 
> testing:
> 
>         if (bdi->wb_written_head &&
>             bdi_stat(bdi, BDI_WRITTEN) - bdi->wb_written_head > 0)
> 
> This avoids calling into bdi_wakeup_writers() pointlessly when no one
> is being throttled (which is the normal case).
  Actually, I've already changed wb_written_head to s64. I kept setting
wb_written_head to s64 maximum. That also avoids calling into
bdi_wakeup_writers() unnecessarily...

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