On Tue, Mar 29, 2011 at 10:14:58AM +0800, Dave Chinner wrote: > -printable > Content-Length: 2034 > Lines: 51 > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
