Re: [PATCH 00/17] [RFC] soft and dynamic dirty throttling limits

[Wu Fengguang]

Hi Dave,

On Mon, Nov 01, 2010 at 02:24:46PM +0800, Dave Chinner wrote:
> On Wed, Oct 13, 2010 at 08:26:27PM +1100, Dave Chinner wrote:
> > On Wed, Oct 13, 2010 at 04:26:12PM +0800, Wu Fengguang wrote:
> > > On Wed, Oct 13, 2010 at 11:07:33AM +0800, Dave Chinner wrote:
> > > > On Tue, Oct 12, 2010 at 10:17:16AM -0400, Christoph Hellwig wrote:
> > > > > Wu, what's the state of this series?  It looks like we'll need it
> > > > > rather sooner than later - try to get at least the preparations in
> > > > > ASAP would be really helpful.
> > > > 
> > > > Not ready in it's current form. This load (creating millions of 1
> > > > byte files in parallel):
> > > > 
> > > > $ /usr/bin/time ./fs_mark -D 10000 -S0 -n 100000 -s 1 -L 63 \
> > > > > -d /mnt/scratch/0 -d /mnt/scratch/1 \
> > > > > -d /mnt/scratch/2 -d /mnt/scratch/3 \
> > > > > -d /mnt/scratch/4 -d /mnt/scratch/5 \
> > > > > -d /mnt/scratch/6 -d /mnt/scratch/7
> > > > 
> > > > Locks up all the fs_mark processes spinning in traces like the
> > > > following and no further progress is made when the inode cache
> > > > fills memory.
> > > 
> > > I reproduced the problem on a 6G/8p 2-socket 11-disk box.
> > > 
> > > The root cause is, pageout() is somehow called with low scan priority,
> > > which deserves more investigation.
> > > 
> > > The direct cause is, balance_dirty_pages() then keeps nr_dirty too low,
> > > which can be improved easily by not pushing down the soft dirty limit
> > > to less than 1-second worth of dirty pages.
> > > 
> > > My test box has two nodes, and their memory usage are rather unbalanced:
> > > (Dave, maybe you have NUMA setup too?)
> > 
> > No, I'm running the test in a single node VM.
> > 
> > FYI, I'm running the test on XFS (16TB 12 disk RAID0 stripe), using
> > the mount options "inode64,nobarrier,logbsize=262144,delaylog".
> 
> Any update on the current status of this patchset?

The last 3 patches to dynamically lower the 20% dirty limit seem
to hurt writeback throughput when it goes too small. That's not
surprising. I tried moderately increase the low bound of dynamic
dirty limit but tests show that it's still not enough. Days ago I
came up with another low bound scheme, however the test box has
been running LKP (and other) benchmarks for the new -rc1 release..

Anyway I see some tricky points in deciding the low bound for dynamic
dirty limit. It seems reasonable to bypass this feature for now, and
to test/submit the other important parts first.

I'm feeling relatively good about the first 14 patches to do IO-less
balance_dirty_pages() and larger writeback chunk size. I'll repost
them separately as v2 after returning to Shanghai.

Some days ago I prepared some slides which has some figures on the old
and new dirty throttling schemes. Hope it helps.

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling.pdf

Thanks,
Fengguang
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