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

Wu Fengguang
2010-12-14T14:33:49Z

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

Thanks,
Fengguang