On 03/15/2012 04:16 AM, KAMEZAWA Hiroyuki wrote:

(2012/03/14 18:46), Glauber Costa wrote:

quoted

On 03/14/2012 04:28 AM, KAMEZAWA Hiroyuki wrote:

quoted

IIUC, in general, even in the processes are in a tree, in major case
of servers, their workloads are independent.
I think FLAT mode is the dafault. 'heararchical' is a crazy thing which
cannot be managed.

Better pay attention to the current overall cgroups discussions being
held by Tejun then. ([RFD] cgroup: about multiple hierarchies)

The topic of whether of adapting all cgroups to be hierarchical by
deafult is a recurring one.

I personally think that it is not unachievable to make res_counters
cheaper, therefore making this less of a problem.

I thought of this a little yesterday. Current my idea is applying following
rule for res_counter.

1. All res_counter is hierarchical. But behavior should be optimized.

2. If parent res_counter has UNLIMITED limit, 'usage' will not be propagated
   to its parent at _charge_.

That doesn't seem to make much sense. If you are unlimited, but your 
parent is limited,
he has a lot more interest to know about the charge than you do. So the 
logic should rather be the opposite: Don't go around getting locks and 
all that if you are unlimited. Your parent might, though.

I am trying to experiment a bit with billing to percpu counters for 
unlimited res_counters. But their inexact nature is giving me quite a 
headache.

3. If a res_counter has UNLIMITED limit, at reading usage, it must visit
    all children and returns a sum of them.

Then,
	/cgroup/
		memory/                       (unlimited)
			libivirt/             (unlimited)
				 qeumu/       (unlimited)
				        guest/(limited)

All dir can show hierarchical usage and the guest will not have
any lock contention at runtime.

If we are okay with summing it up at read time, we may as well
keep everything in percpu counters at all times.

By this
  1. no runtime overhead if the parent has unlimited limit.
  2. All res_counter can show aggregate resource usage of children.

To do this
  1. res_coutner should have children list by itself.

Implementation problem
  - What should happens when a user set new limit to a res_counter which have
    childrens ? Shouldn't we allow it ? Or take all locks of children and
    update in atomic ?

Well, increasing the limit should be always possible.

As for the kids, how about:

- ) Take their locks
- ) scan through them seeing if their usage is bellow the new allowance
     -) if it is, then ok
     -) if it is not, then try to reclaim (*). Fail if it is not possible.

(*) May be hard to implement, because we already have the res_counter 
lock taken, and the code may get nasty. So maybe it is better just fail 
if any of your kids usage is over the new allowance...

  - memory.use_hierarchy should be obsolete ?

If we're going fully hierarchical, yes.

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