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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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....

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.

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx
--
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


[Index of Archives]     [Linux Ext4 Filesystem]     [Union Filesystem]     [Filesystem Testing]     [Ceph Users]     [Ecryptfs]     [AutoFS]     [Kernel Newbies]     [Share Photos]     [Security]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux Cachefs]     [Reiser Filesystem]     [Linux RAID]     [Samba]     [Device Mapper]     [CEPH Development]
  Powered by Linux