Re: [PATCH 11/11] blkcg: implement per-blkg request allocation

[Vivek Goyal <vgoyal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On Fri, Apr 27, 2012 at 08:02:17AM -0700, Tejun Heo wrote:
> Hello,
> 
> On Fri, Apr 27, 2012 at 10:54:01AM -0400, Jeff Moyer wrote:
> > > This patch implements per-blkg request_list.  Each blkg has its own
> > > request_list and any IO allocates its request from the matching blkg
> > > making blkcgs completely isolated in terms of request allocation.
> > 
> > So, nr_requests is now actually nr_requests * # of blk cgroups.  Is that
> > right?  Are you at all concerned about the amount of memory that can be
> > tied up as the number of cgroups increases?
> 
> Yeah, I thought about it and I don't think there's a single good
> solution here.  The other extreme would be splitting nr_requests by
> the number of cgroups but that seems even worse - each cgroup should
> be able to hit maximum throughput.  Given that a lot of workloads tend
> to regulate themselves before hitting nr_requests, I think it's best
> to leave it as-is and treat each cgroup as having separate channel for
> now.  It's a configurable parameter after all.

So on a slow device a malicious application can easily create thousands
of group, queue up tons of IO and create unreclaimable memory easily?
Sounds little scary. 

I had used two separate limits. Per queue limit and per group limit
(nr_requests and nr_group_requests). That had made implementation 
complex and relied on user doing the right configuration so that one
cgroup does not get serialized behind other once we hit nr_requests.
I am not advocating that solution as it was not very nice either.

Hmm.., tricky...

Thanks
Vivek