Re: [PATCH RFC 0/5] IO-less balance_dirty_pages() v2 (simple approach)

[Wu Fengguang <fengguang.wu@intel.com>]

On Tue, Mar 29, 2011 at 10:14:58AM +0800, Dave Chinner wrote:
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> On Mon, Mar 28, 2011 at 10:44:45AM +0800, Wu Fengguang wrote:
> > On Sat, Mar 26, 2011 at 07:05:44AM +0800, Jan Kara wrote:
> > > And actually the NFS traces you pointed to originally seem to be different
> > > problem, in fact not directly related to what balance_dirty_pages() does...
> > > And with local filesystem the results seem to be reasonable (although there
> > > are some longer sleeps in your JBOD measurements I don't understand yet).
> > 
> > Yeah the NFS case can be improved on the FS side (for now you may just
> > reuse my NFS patches and focus on other generic improvements).
> > 
> > The JBOD issue is also beyond my understanding.
> > 
> > Note that XFS will also see one big IO completion per 0.5-1 seconds,
> > when we are to increase the write chunk size from the current 4MB to
> > near the bdi's write bandwidth. As illustrated by this graph:
> > 
> > http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/4G/xfs-1dd-1M-8p-3927M-20%25-2.6.38-rc6-dt6+-2011-02-27-22-58/global_dirtied_written-500.png
> 
> Which is _bad_.
> 
> Increasing the writeback chunk size simply causes dirty queue
> starvation issues when there are lots of dirty files and lots more
> memory than there is writeback bandwidth. Think of a machine with
> 1TB of RAM (that's a 200GB dirty limit) and 1GB/s of disk
> throughput. Thats 3 minutes worth of writeback and increasing the
> chunk size to ~1s worth of throughput means that the 200th dirty
> file won't get serviced for 3 minutes....
> 
> We used to have behaviour similar to this this (prior to 2.6.16, IIRC),
> and it caused all sorts of problems where people were losing 10-15
> minute old data when the system crashed because writeback didn't
> process the dirty inode list fast enough in the presence of lots of
> large files....
 
Yes it is a problem, and can be best solved by automatically lowering
bdi dirty limit to (bdi->write_bandwidth * dirty_expire_interval/100).
Then we reliably control the lost data size to < 30s by default.

> A small writeback chunk size has no adverse impact on XFS as long as
> the elevator does it's job of merging IOs (which in 99.9% of cases
> it does) so I'm wondering what the reason for making this change
> is.

It's explained in this changelog (is the XFS paragraph still valid?)

        https://patchwork.kernel.org/patch/605151/

The larger write chunk size generally helps ext4 and RAID setups.

Thanks,
Fengguang

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