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

[Wu Fengguang]

On Tue, Dec 14, 2010 at 10:39:02PM +0800, Wu Fengguang wrote:
> 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..

ext2 has the same pattern:

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext2-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-01-36/dirty-pages.png

But it does not happen for btrfs!

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/btrfs-1dd-1M-8p-2952M-2.6.37-rc5-2010-12-10-21-23/vmstat-dirty.png

Seems that it's the nr_slab_reclaimable keep growing until 75s.

Looking at
http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/tests/3G/ext2-1dd-1M-8p-2952M-2.6.37-rc5+-2010-12-09-01-36/slabinfo-end

It should be the buffer heads that slowly eats the memory during the time:

buffer_head       670304 670662    104   37    1 : tunables  120   60 8 : slabdata  18117  18126    480

(670304/37)*4 = 72464KB.

The consumption seems acceptable for a 3G memory system.

Thanks,
Fengguang
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