Re: [PATCH 06/18] writeback: sync expired inodes first inbackground writeback

[Wu Fengguang]

On Fri, May 27, 2011 at 07:10:45AM +0800, Jan Kara wrote:
> On Wed 25-05-11 22:38:57, Wu Fengguang wrote:
> > > and I was wondering: Assume there is one continuously redirtied file and
> > > untar starts in parallel. With the new logic, background writeback will
> > > never consider inodes that are not expired in this situation (we never
> > > switch to "all dirty inodes" phase - or even if we switched, we would just
> > > queue all inodes and then return back to queueing only expired inodes). So
> > > the net effect is that for 30 seconds we will be only continuously writing
> > > pages of the continuously dirtied file instead of (possibly older) pages of
> > > other files that are written. Is this really desirable? Wasn't the old
> > > behavior simpler and not worse than the new one?
> > 
> > Good question! Yes sadly in this case the new behavior could be worse
> > than the old one.
> > 
> > In fact this patch do not improve the small files (< 4MB) case at all,
> > except for the side effect that less unexpired inodes will leave in
> > s_io when the background work quit and the later kupdate work will
> > write less unexpired inodes.
> > 
> > And for the mixed small/large files case, it actually results in worse
> > behavior on your mentioned case.
> > 
> > However the root cause here is the file being _actively_ written to,
> > somehow a livelock scheme. We could add a simple livelock prevention
> > scheme that works for the common case of file appending:
> > 
> > - save i_size when the range_cyclic writeback starts from 0, for
> >   limiting the writeback scope
>   Hmm, but for this we'd have to store additional 'unsigned long' (page
> index) for each inode. Not sure if it's really worth it.

Yeah, it may be considerable space cost when icache grows large.

> > - when range_cyclic writeback hits the saved i_size, quit the current
> >   inode instead of immediately restarting from 0. This will not only
> >   avoid a possible extra seek, but also redirty_tail() the inode and
> >   hence get out of possible livelock.
>   But I like the idea of doing redirty_tail() when we write out some inode
> for too long.

Then there is the hard question of "how long time is enough time to
redirty_tail()?". The time should be able to avoid livelocking the
dirty pages near EOF when the large file is being written to all over
the places (random writes or multiple sequential write streams), and
yet still be small enough to be useful. 

> Maybe we could just do redirty_tail() instead of requeue_io()
> whenever write_cache_pages() had to wrap the index? We could communicate
> this by setting a flag in wbc in write_cache_pages()...

That's the minimal required for the problem here. What do you think
about this old patch that happen to contain the required side effects?

        writeback: quit on wrap for .range_cyclic (ext4 part)
        http://lkml.org/lkml/2009/10/7/129

> > The livelock prevention scheme may not only eliminate the undesirable
> > behavior you observed for this patch, but also prevent the "some old
> > pages may not get the chance to get written to disk in an actively
> > dirtied file" data security issue discussed in an old email. What do
> > you think?
>   So my scheme would not solve this but it does not require per-inode
> overhead...

Another possible scheme is to tag the dirty inode immediate after
requeue_io(), if it's not yet tagged.  Accordingly let
write_cache_pages() write only the tagged pages as long as
mapping_tagged(TOWRITE), and to always update dirtied_when on
writeback index wrap.

Thanks,
Fengguang
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