Re: [Lsf] IO less throttling and cgroup aware writeback (Was: Re: Preliminary Agenda and Activities for LSF)

Jan Kara
2011-04-19T14:48:34Z

On Tue 19-04-11 10:34:23, Vivek Goyal wrote:
> On Tue, Apr 19, 2011 at 10:17:17PM +0800, Wu Fengguang wrote:
> > [snip]
> > > > > > For throttling case, apart from metadata, I found that with simple
> > > > > > throttling of data I ran into issues with journalling with ext4 mounuted
> > > > > > in ordered mode. So it was suggested that WRITE IO throttling should
> > > > > > not be done at device level instead try to do it in higher layers,
> > > > > > possibly balance_dirty_pages() and throttle process early.
> > > > >
> > > > > The problem with doing it at the page cache entry level is that
> > > > > cache hits then get throttled. It's not really a an IO controller at
> > > > > that point, and the impact on application performance could be huge
> > > > > (i.e. MB/s instead of GB/s).
> > > >
> > > > Agreed that throttling cache hits is not a good idea. Can we determine
> > > > if page being asked for is in cache or not and charge for IO accordingly.
> > >
> > > You'd need hooks in find_or_create_page(), though you have no
> > > context of whether a read or a write is in progress at that point.
> >
> > I'm confused. Where is the throttling at cache hits?
> >
> > The balance_dirty_pages() throttling kicks in at write() syscall and
> > page fault time. For example, generic_perform_write(), do_wp_page()
> > and __do_fault() will explicitly call
> > balance_dirty_pages_ratelimited() to do the write throttling.
>
> This comment was in the context of what if we move block IO controller read
> throttling also in higher layers. Then we don't want to throttle reads
> which are already in cache.
>
> Currently throttling hook is in generic_make_request() and it kicks in
> only if data is not present in page cache and actual disk IO is initiated.
You can always throttle in readpage(). It's not much higher than
generic_make_request() but basically as high as it can get I suspect
(otherwise you'd have to deal with lots of different code paths like page
faults, splice, read, ...).

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