[Xen-devel] Re: [PATCH 7/9] blkio-cgroup-v9: Page tracking hooks

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx>]

On Wed, 22 Jul 2009 18:40:55 +0900 (JST)
Ryo Tsuruta <ryov@xxxxxxxxxxxxx> wrote:

> KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote:
> > On Tue, 21 Jul 2009 23:23:16 +0900 (JST)
> > Ryo Tsuruta <ryov@xxxxxxxxxxxxx> wrote:
> > 
> > > This patch contains several hooks that let the blkio-cgroup framework to 
> > > know
> > > which blkio-cgroup is the owner of a page before starting I/O against the 
> > > page.
> > 
> > > @@ -464,6 +465,7 @@ int add_to_page_cache_locked(struct page
> > >                                   gfp_mask & GFP_RECLAIM_MASK);
> > >   if (error)
> > >           goto out;
> > > + blkio_cgroup_set_owner(page, current->mm);
> > >  
> > 
> > This part is doubtful...Is this necessary ? 
> > I recommend you that the caller should attach owner by itself.
> 
> I think that it is reasonable to add the hook right here rather than
> to add many hooks to a variety of places.
> 
Why ? at writing, it's will be overwriten soon, IIUC. Then, this information
is misleading. plz add a hook like this when it means something. In this case,
read/write callers.
IMO, you just increase patch's readbility but decrease easiness of maintaince.


> > IMHO, later io for swap-out is caused by the caller of swapout, not page's
> > owner. plz charge to them or,
> >   - add special BLOCK CGROUP ID for the kernel's swap out. 
> 
> I think that it is not too bad to charge the owner of a page for
> swap-out. From another perspective, it can be considered that swap-out
> is caused by a process which uses a large amount of memory.
> 
No. swap-out is caused by a thread who requests memory even while memory is
in short. IMHO, I/O by memory reqraim should work in priority of memory 
requester.

Consider following situation.

- A process "A" has big memory. When several threads requests memory, all 
  of them are caught by a blockio cgroup of "A".
- A process "B" has read big file caches. When several threads requests memory, 
  all  of them are caught by a blockio cgroup of "B".

If "A" and "B" 's threshold is small, you'll see big slow down.
But it's not _planned_ behavior in many cases.

If you charges agaisnt memory owner, the admin has to set _big_ priority of I/O
controller to "A" and "B" if it uses much memory. I think the admin can't design
his system. It's nonsense to say "plz set I/O limit propotional to memory usage 
of
your apps even if it never do I/O in usual."


If this blockio cgroup is introduced, people will see *unexpected* very
terrible slow down and the user will see heartbeat warnings/failover by cluster
management software. Please do I/O at the priority of memory reclaiming 
requester.


Thanks,
-Kame





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