Re: NFS page states & writeback

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

> However there is another problem. Look at the below graph. Even though
> the commits are sent to NFS server in relatively small size and evenly
> distributed in time (the green points), the commit COMPLETION events
> from the server are observed to be pretty bumpy over time (the blue
> points sitting on the red lines). This may not be easily fixable.. So
> we still have to live with bumpy NFS commit completions...
> 
> http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/NFS/nfs-1dd-1M-8p-2945M-20%25-2.6.38-rc6-dt6+-2011-02-22-21-09/nfs-commit.png

The bumpy commit completions result in large fluctuations in estimated
write bandwidth (the red line):

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/NFS/nfs-1dd-1M-8p-2945M-20%25-2.6.38-rc6-dt6+-2011-02-22-21-09/balance_dirty_pages-bandwidth.png

and fluctuated number of dirty pages (the red line):

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/NFS/nfs-1dd-1M-8p-2945M-20%25-2.6.38-rc6-dt6+-2011-02-22-21-09/balance_dirty_pages-pages.png

And you can see the v6 patches still manages to keep the pause times
(the red points) under 100ms (hands down!):

http://www.kernel.org/pub/linux/kernel/people/wfg/writeback/dirty-throttling-v6/NFS/nfs-1dd-1M-8p-2945M-20%25-2.6.38-rc6-dt6+-2011-02-22-21-09/balance_dirty_pages-pause.png

Thanks,
Fengguang