Re: [PATCH 2/2] mm: Implement writeback livelock avoidance using page tagging

[Jan Kara <jack@suse.cz>]

On Tue 08-06-10 15:29:37, Nick Piggin wrote:
> On Mon, Jun 07, 2010 at 06:09:03PM +0200, Jan Kara wrote:
> > On Sat 05-06-10 11:38:02, Nick Piggin wrote:
> > > On Fri, Jun 04, 2010 at 08:47:11PM +0200, Jan Kara wrote:
> > > > +	if (wbc->sync_mode == WB_SYNC_ALL)
> > > > +		tag_pages_for_writeback(mapping, index, end);
> > > 
> > > I wonder if this is too much spinlock latency in a huge dirty file?
> > > Some kid of batching of the operation perhaps would be good?
> >   You mean like copy tags for 4096 pages, then cond_resched the spin lock
> > and continue? That should be doable but it will give tasks that try to
> > livelock us more time (i.e. if there were 4096 tasks creating dirty pages
> > than probably they would be able to livelock us, won't they? Maybe we don't
> > care?).
> 
> Not 100% sure. I think that if we've got the inode in I_SYNC state, it
> should stop cleaning and dirtiers will get throttled.
> 
> Even if writeback was able to continue on that inode, it would be a big
> achievement to dirty then clean pages as fast as we are able to tag them
> in batches of 4096 :)
  In practice, you are probably right that the writers will eventually get
throttled if they were aggressive enough to dirty lots of pages while
we cond_resched the lock...

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