Re: [RFC] writeback and cgroup

[Jan Kara <jack@suse.cz>]

  Hello,

On Tue 17-04-12 15:01:06, Tejun Heo wrote:
> On Wed, Apr 11, 2012 at 09:22:31PM +0200, Jan Kara wrote:
> > > So all the metadata IO will happen thorough journaling thread and that
> > > will be in root group which should remain unthrottled. So any journal
> > > IO going to disk should remain unthrottled.
> >
> >   Yes, that is true at least for ext3/ext4 or btrfs. In principle we don't
> > have to have the journal thread (as is the case of reiserfs where random
> > writer may end up doing commit) but let's not complicate things
> > unnecessarily.
> 
> Why can't journal entries keep track of the originator so that bios
> can be attributed to the originator while committing?  That shouldn't
> be too difficult to implement, no?
  I think I was just describing the current state but yes, in future we
can track which cgroup first attached a buffer to a transaction.

> > > Now, IIRC, fsync problem with throttling was that we had opened a
> > > transaction but could not write it back to disk because we had to
> > > wait for all the cached data to go to disk (which is throttled). So
> > > my question is, can't we first wait for all the data to be flushed
> > > to disk and then open a transaction for metadata. metadata will be
> > > unthrottled so filesystem will not have to do any tricks like bdi is
> > > congested or not.
> >
> >   Actually that's what's happening. We first do filemap_write_and_wait()
> > which syncs all the data and then we go and force transaction commit to
> > make sure all metadata got to stable storage. The problem is that writeout
> > of data may need to allocate new blocks and that starts a transaction and
> > while the transaction is started we may need to do some reads (e.g. of
> > bitmaps etc.) which may be throttled and at that moment the whole
> > filesystem is blocked. I don't remember the stack traces you showed me so
> > I'm not sure it this is what your observed but it's certainly one possible
> > scenario. The reason why fsync triggers problems is simply that it's the
> > only place where process normally does significant amount of writing. In
> > most cases flusher thread / journal thread do it so this effect is not
> > visible. And to precede your question, it would be rather hard to avoid IO
> > while the transaction is started due to locking.
> 
> Probably we should mark all IOs issued inside transaction as META (or
> whatever which tells blkcg to avoid throttling it).  We're gonna need
> overcharging for metadata writes anyway, so I don't think this will
> make too much of a difference.
  Agreed.

								Honza