linux-kernel - Re: [PATCH 18/21] blkcg: move blkio_group

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Date:	Mon, 2 Apr 2012 15:17:02 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Tao Ma <tm@....ma>
Cc:	axboe@...nel.dk, vgoyal@...hat.com, ctalbott@...gle.com,
	rni@...gle.com, linux-kernel@...r.kernel.org,
	cgroups@...r.kernel.org, containers@...ts.linux-foundation.org
Subject: Re: [PATCH 18/21] blkcg: move blkio_group_conf->weight to cfq

On Tue, Apr 03, 2012 at 06:03:03AM +0800, Tao Ma wrote:
> Currently weight is just used to calculate the time slice of different
> cfq group, right? So why can't it be used to indicate other weight? So
> say, if we are just want to use iops to indicate the difference between
> different cgroups(100 weight vs 200 weight), so one process will send
> 100 ios while the other will send 200 ios just for example.

Because it's configuring stuff which is completely unrelated.  Let's
say you added a new elevator w/ iops based proportional IO which
shares blkio.weight configuration with cfq but nothing else and in
turn your new thing would probably need some other config parameters
which don't make much sense for cfq, right?

Now, let's say there's a system which has two hard drives and sda is
using cfq and sdb is using your new elevator and you're trying to
configure cgroup blkio limits.  Now, you have blkio.weight which
applies to both elevators and other configurations which aren't and
from the looks of it there's no way to tell which configuration
controls what.

It also makes the configuration implementation hairier.  We'll need
callbacks from blkcg core layer to all policies to notify changes to
per-cgroup configuration and from there policies would have to decide
whether it has overriding per-cgroup-device configuration.  It's not
even clear we even want per-cgroup configuration.  blk-throttle only
has per-cgroup-device configuration after all.

So, again, no.  blkcg.weight isn't and won't be generic.

> We will need a new iops_weight in your option to be exported?

Yeah, just add config and stat files prefixed with the name of the new
blkcg policy.

Thanks.

-- 
tejun
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