Date:	Mon, 29 Sep 2008 19:40:36 +0900 (JST)
From:	Hirokazu Takahashi <taka@...inux.co.jp>
To:	righi.andrea@...il.com
Cc:	vgoyal@...hat.com, ryov@...inux.co.jp,
	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	containers@...ts.linux-foundation.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xensource.com, fernando@....ntt.co.jp,
	balbir@...ux.vnet.ibm.com, xemul@...nvz.org, agk@...rceware.org,
	jens.axboe@...cle.com
Subject: Re: dm-ioband + bio-cgroup benchmarks

Hi,

> >>>>> It's possible the algorithm of dm-ioband can be placed in the block layer
> >>>>> if it is really a big problem.
> >>>>> But I doubt it can control every control block I/O as we wish since
> >>>>> the interface the cgroup supports is quite poor.
> >>>> Had a question regarding cgroup interface. I am assuming that in a system,
> >>>> one will be using other controllers as well apart from IO-controller.
> >>>> Other controllers will be using cgroup as a grouping mechanism.
> >>>> Now coming up with additional grouping mechanism for only io-controller seems
> >>>> little odd to me. It will make the job of higher level management software
> >>>> harder.
> >>>>
> >>>> Looking at the dm-ioband grouping examples given in patches, I think cases
> >>>> of grouping based  in pid, pgrp, uid and kvm can be handled by creating right
> >>>> cgroup and making sure applications are launched/moved into right cgroup by
> >>>> user space tools. 
> >>> Grouping in pid, pgrp and uid is not the point, which I've been thinking
> >>> can be replaced with cgroup once the implementation of bio-cgroup is done.
> >>>
> >>> I think problems of cgroup are that they can't support lots of storages
> >>> and hotplug devices, it just handle them as if they were just one resource.
> >>> I don't insist the interface of dm-ioband is the best. I just hope the
> >>> cgroup infrastructure support this kind of resources.
> >>>
> >> Sorry, I did not understand fully. Can you please explain in detail what
> >> kind of situation will not be covered by cgroup interface.
> > 
> > From the concept of the cgroup, if you want control several disks
> > independently, you should make each disk have its own cgroup subsystem,
> > which only can be defined when compiling the kernel. This is impossible
> > because every linux box has various number of disks.
> 
> mmh? not true. You can define a single cgroup subsystem that implements
> the opportune interfaces to apply your type of control, and use many
> structures allocated dynamically for each controlled object (one for
> each block device, disk, partition, ... or using any kind of
> grouping/splitting policy). Actually, this is how cgroup-io-throttle, as
> well as any other cgroup subsystem, is implemented.
> 
> > So you think it may be possible to make each cgroup have lots of control
> > files for each device as a workaround. But it isn't allowed to add/remove
> > control files when some devices are hot-added or hot-removed.
> 
> Why not a single control file for all the devices?

This is possible but I wonder if this is really the way we should go.
It looks like you tried implementing another ioctl-like interface
on the cgroup control file interface. You can do anything you want
with this interface though.

I guess there should be at least some rules to implement this kind of
ioctl-like interface if they don't want to enhance the cgroup interface, 

Thank you,
Hirokazu Takahashi.



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