Re: [PATCH 04/35] writeback: reduce per-bdi dirty threshold ramp up time

Wu Fengguang
2010-12-14T14:39:10Z

On Tue, Dec 14, 2010 at 10:33:25PM +0800, Wu Fengguang wrote:
> On Tue, Dec 14, 2010 at 09:59:10PM +0800, Wu Fengguang wrote:
> > On Tue, Dec 14, 2010 at 09:37:34PM +0800, Richard Kennedy wrote:
>
> > > As to the ramp up time, when writing to 2 disks at the same time I see
> > > the per_bdi_threshold taking up to 20 seconds to converge on a steady
> > > value after one of the write stops. So I think this could be speeded up
> > > even more, at least on my setup.
> >
> > I have the roughly same ramp up time on the 1-disk 3GB mem test:
> >
> > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext4-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-00-37/dirty-pages.png
> >
>
> Interestingly, the above graph shows that after about 10s fast ramp
> up, there is another 20s slow ramp down. It's obviously due the
> decline of global limit:
>
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext4-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-00-37/vmstat-dirty.png
>
> But why is the global limit declining? The following log shows that
> nr_file_pages keeps growing and goes stable after 75 seconds (so long
> time!). In the same period nr_free_pages goes slowly down to its
> stable value. Given that the global limit is mainly derived from
> nr_free_pages+nr_file_pages (I disabled swap), something must be
> slowly eating memory until 75 ms. Maybe the tracing ring buffers?
>
> free file reclaimable pages
> 50s 369324 + 318760 => 688084
> 60s 235989 + 448096 => 684085
>
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext4-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-00-37/vmstat

The log shows that ~64MB reclaimable memory is stoled. But the trace
data only takes 1.8MB. Hmm..

Thanks,
Fengguang