NFS page states & writeback

[Jan Kara <jack@suse.cz>]

  Hi,

  while working on changes to balance_dirty_pages() I was investigating why
NFS writeback is *so* bumpy when I do not call writeback_inodes_wb() from
balance_dirty_pages(). Take a single dd writing to NFS. What I can
see is that we quickly accumulate dirty pages upto limit - ~700 MB on that
machine. So flusher thread starts working and in an instant all these ~700
MB transition from Dirty state to Writeback state. Then, as server acks
writes, Writeback pages slowly change to Unstable pages (at 100 MB/s rate
let's say) and then at one moment (commit to server happens) all pages
transition from Unstable to Clean state - the cycle begins from the start.

The reason for this behavior seems to be a flaw in the logic in
over_bground_thresh() which checks:
global_page_state(NR_FILE_DIRTY) +
      global_page_state(NR_UNSTABLE_NFS) > background_thresh
So at the moment all pages are turned Writeback, flusher thread goes to
sleep and doesn't do any background writeback, until we have accumulated
enough Stable pages to get over background_thresh. But NFS needs to have
->write_inode() called so that it can sent commit requests to the server.
So effectively we end up sending commit only when background_thresh Unstable
pages have accumulated which creates the bumpyness. Previously this wasn't
a problem because balance_dirty_pages() ended up calling ->write_inode()
often enough for NFS to send commit requests reasonably often.

Now I wouldn't write so long email about this if I knew how to cleanly fix
the check ;-). One way to "fix" the check would be to add there Writeback
pages:
NR_FILE_DIRTY + NR_WRITEBACK + NR_UNSTABLE_NFS > background_thresh

This would work in the sense that it would keep flusher thread working but
a) for normal filesystems it would be working even if there's potentially
nothing to do (or it is not necessary to do anything)
b) NFS is picky when it sends commit requests (inode has to have more
Stable pages than Writeback pages if I'm reading the code in
nfs_commit_unstable_pages() right) so flusher thread may be working but
nothing really happens until enough stable pages accumulate.

A check which kind of works but looks a bit hacky and is not perfect when
there are multiple files is:
NR_FILE_DIRTY + NR_UNSTABLE_NFS > background_thresh ||
NR_UNSTABLE_NFS > NR_WRITEBACK (to match what NFS does)

Any better idea for a fix?

								Honza
-- 
Jan Kara <jack@suse.cz>
SUSE Labs, CR