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

Richard Kennedy
2010-12-15T18:48:43Z

On Tue, 2010-12-14 at 21:59 +0800, Wu Fengguang wrote:
> Hi Richard,
>
> On Tue, Dec 14, 2010 at 09:37:34PM +0800, Richard Kennedy wrote:
> > On Mon, 2010-12-13 at 22:46 +0800, Wu Fengguang wrote:
> > > plain text document attachment
> > > (writeback-speedup-per-bdi-threshold-ramp-up.patch)
> > > Reduce the dampening for the control system, yielding faster
> > > convergence.
> > >
> > > Currently it converges at a snail's pace for slow devices (in order of
> > > minutes). For really fast storage, the convergence speed should be fine.
> > >
> > > It makes sense to make it reasonably fast for typical desktops.
> > >
> > > After patch, it converges in ~10 seconds for 60MB/s writes and 4GB mem.
> > > So expect ~1s for a fast 600MB/s storage under 4GB mem, or ~4s under
> > > 16GB mem, which seems reasonable.
> > >
> > > $ while true; do grep BdiDirtyThresh /debug/bdi/8:0/stats; sleep 1; done
> > > BdiDirtyThresh: 0 kB
> > > BdiDirtyThresh: 118748 kB
> > > BdiDirtyThresh: 214280 kB
> > > BdiDirtyThresh: 303868 kB
> > > BdiDirtyThresh: 376528 kB
> > > BdiDirtyThresh: 411180 kB
> > > BdiDirtyThresh: 448636 kB
> > > BdiDirtyThresh: 472260 kB
> > > BdiDirtyThresh: 490924 kB
> > > BdiDirtyThresh: 499596 kB
> > > BdiDirtyThresh: 507068 kB
> > > ...
> > > DirtyThresh: 530392 kB
> > >
> > > CC: Peter Zijlstra <a.p.zijlstra@chello.nl>
> > > CC: Richard Kennedy <richard@rsk.demon.co.uk>
> > > Signed-off-by: Wu Fengguang <fengguang.wu@intel.com>
> > > ---
> > > mm/page-writeback.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > --- linux-next.orig/mm/page-writeback.c 2010-12-13 21:46:11.000000000 +0800
> > > +++ linux-next/mm/page-writeback.c 2010-12-13 21:46:11.000000000 +0800
> > > @@ -145,7 +145,7 @@ static int calc_period_shift(void)
> > > else
> > > dirty_total = (vm_dirty_ratio * determine_dirtyable_memory()) /
> > > 100;
> > > - return 2 + ilog2(dirty_total - 1);
> > > + return ilog2(dirty_total - 1) - 1;
> > > }
> > >
> > > /*
> > >
> > >
> > Hi Fengguang,
> >
> > I've been running my test set on your v3 series and generally it's
> > giving good results in line with the mainline kernel, with much less
> > variability and lower standard deviation of the results so it is much
> > more repeatable.
>
> Glad to hear that, and thank you very much for trying it out!
>
> > However, it doesn't seem to be honouring the background_dirty_threshold.
>
> > The attached graph is from a simple fio write test of 400Mb on ext4.
> > All dirty pages are completely written in 15 seconds, but I expect to
> > see up to background_dirty_threshold pages staying dirty until the 30
> > second background task writes them out. So it is much too eager to write
> > back dirty pages.
>
> This is interesting, and seems easy to root cause. When testing v4,
> would you help collect the following trace events?
>
> echo 1 > /debug/tracing/events/writeback/balance_dirty_pages/enable
> echo 1 > /debug/tracing/events/writeback/balance_dirty_state/enable
> echo 1 > /debug/tracing/events/writeback/writeback_single_inode/enable
>
> They'll have good opportunity to disclose the bug.
>
> > 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
>
> Given that it's the typical desktop, it does seem reasonable to speed
> it up further.
>
> > I am just about to start testing v4 & will report anything interesting.
>
> Thanks!
>
> Fengguang

I just mailed the trace log to Fengguang, it is a bit big to post to
this list. If anyone wants it, let me know and I'll mail to them
directly.

I'm also seeing a write stall in some of my tests. When writing 400Mb
after about 6 seconds I'm see a few seconds when there are no reported
sectors written to sda & there are no pages under writeback although
there are lots of dirty pages. ( the graph I sent previously shows this
stall as well )

regards
Richard