On Fri, Apr 08, 2016 at 04:11:35PM -0400, Tejun Heo wrote:

quoted

quoted

Widely diverging from
CPU's behavior, IO grouped all internal tasks into an internal leaf
node and used to assign a fixed weight to it.

That's just plain broken... That is not how a proportional weight based
hierarchical controller works.

That's a strong statement. 

No its plain fact.

If you modify a graph, it is not the same graph.

Even if you argue by merit of the function on this graph, and state that
only the result of this function is important, and any modification to
the graph that leaves this result in tact is good; ie. a modification
invariant to the function, this fails.

Because for proportional controllers all that matters is the number and
weight of edges leaving a node.

The modification described above does clearly change the outcome and is
not invariant under the proportional weight distribution function.

When the hierarchy is composed of
equivalent objects as in CPU, not distinguishing internal and leaf
nodes would be a more natural way to organize; however, it isn't
necessarily true in all cases.  For example, while a writeback IO
would be issued by some task, the task itself might not have done
anything to cause that IO and the IO would essentially be anonymous in
the resource domain.  Also, different controllers use different units
of organization - CPU sees threads, IO sees IO contexts which are
usually shared in a process.  The difference would lead to differing
scaling behaviors in proportional distribution.

While the separate buckets and entities model may not be as elegant as
tree of uniform objects, it is far from uncommon and more robust when
dealing with different types of objects.

The graph does not care about the type of objects the nodes represent,
and proportional weight distribution only cares about the edges.

With cpu-cgroup the nodes are not of uniform type either, they can be a
group or a task. You get runtime type identification and make it work.

There just isn't an excuse for crazy crap like this. Its wrong, no two
ways about it.