Re: Summary of resource management discussion

[Herbert Poetzl <herbert@13thfloor.at>]

On Thu, Mar 15, 2007 at 10:34:35PM +0530, Srivatsa Vaddagiri wrote:
> On Thu, Mar 15, 2007 at 04:24:37AM -0700, Paul Menage wrote:
> > If there really was a grouping that was always guaranteed to match the
> > way you wanted to group tasks for e.g. resource control, then yes, it
> > would be great to use it. But I don't see an obvious candidate. The
> > pid namespace is not it, IMO.
> 
> In vserver context, what is the "normal" case then? Atleast for Linux
> Vserver pid namespace seems to be normal unit of resource control (as per 
> Herbert). 

it is, but mainly because a 'context' in the Linux-VServer
case is a struct, which defines all the properties for a
'guest' excluding separate (name)spaces. as the pid space
doesn't exist yet, it is part of the task grouping context

> Even if one wanted to manage a arbitrary group of tasks in vserver
> context, IMHO its still possible to construct that arbitrary group using 
> the existing pointer, ns[/task]proxy, and not break existing namespace 
> semantics/functionality. 
> 
> So the normal case I see is:
> 
>     pid_ns1  uts_ns1  cpu_ctl_space1   pid_ns2   uts_ns2  cpu_ctl_space2
>       ^        ^           (50%)          ^          ^          (50%)
>       |        |	     ^	          |          |           ^
>       |	       |	     |            |          |           |
>      ---------------------------        -------------------------------
>     |      task_proxy1          |      |         task_proxy2           |
>     |       (Vserver1)          |      |          (Vserver2)           |
>      ---------------------------        -------------------------------
> 
> 
> But, if someone wanted to manage cpu resource differently, and say that
> postgres tasks from both vservers should be in same cpu resource class, 
> the above becomes:
> 
> 
>     pid_ns1 uts_ns1 cpu_ctl_space1      pid_ns1 uts_ns1 cpu_ctl_space2
>        ^       ^	  (25%)              ^        ^        (50%)
>        |       |           ^                 |        |          ^
>        |       |           |                 |        |          |
>      ---------------------------       -------------------------------
>     |      task_proxy1          |     |          task_proxy2          |
>     |       (Vserver1)          |     |  (postgres tasks in VServer1) |
>      ---------------------------       -------------------------------
>     
>     
>     pid_ns2 uts_ns2 cpu_ctl_space3      pid_ns2 uts_ns2 cpu_ctl_space2
>        ^       ^	 (25%)              ^        ^        (50%)
>        |       |          ^                 |        |          ^
>        |       |          |                 |        |          |
>      ---------------------------       ------------------------------
>     |      task_proxy3          |     |          task_proxy4         |
>     |       (Vserver2)          |     |  (postgres tasks in VServer2 |
>      ---------------------------       ------------------------------
> 
> (the best I could draw using ASCII art!)
> 
> The benefit I see of this approach is it will avoid introduction of 
> additional pointers in struct task_struct and also additional structures
> (struct container etc) in the kernel, but we will still be able to retain 
> same user interfaces you had in your patches.
> 
> Do you see any drawbacks of doing like this? What will break if we do
> this?

looks good to me, except for the potential issue with
the double indirection introducing too much overhear
(compared to something like this:

       ipc uts
        ^   ^   .
        |   |   ¦
    +---+---+---+---+  
    |    nsproxy    | 
    +------------+--+   cpu pid
                 ^       ^   ^
                 |       |   |	
             +---+-------+---+--+
             |       task       |
             +------------------+

don't forget, accounting for cpu is probably very closely
tied to tasks, while this doesn't matter much for other
resources like number of tasks or file handles ...

> > Resource control (and other kinds of task grouping behaviour) shouldn't 
> > require virtualization.
> 
> Certainly. AFAICS, nsproxy[.c] is unconditionally available in the
> kernel (even if virtualization support is not enabled). When reused for 
> pure resource control purpose, I see that as a special case of virtualization
> where only resources are virtualized and namespaces are not.
> 
> I think an interesting question would be : what more task-grouping
> behavior do you want to implement using an additional pointer that you
> can't reusing ->task_proxy?
> 
> > >a. Paul Menage's patches:
> > >
> > >        (tsk->containers->container[cpu_ctlr.subsys_id] - X)->cpu_limit
> > 
> > So what's the '-X' that you're referring to
> 
> Oh ..that's to seek pointer to begining of the cpulimit structure (subsys
> pointer in 'struct container' points to a structure embedded in a larger
> structure. -X gets you to point to the larger structure).
> 
> > >6. As tasks move around namespaces/resource-classes, their
> > >   tsk->nsproxy/containers object will change. Do we simple create
> > >   a new nsproxy/containers object or optimize storage by searching
> > >   for one which matches the task's new requirements?
> > 
> > I think the latter.
> 
> Yes me too. But maybe to keep in simple in initial versions, we should
> avoid that optimisation and at the same time get statistics on duplicates?.

I agree here, just let us keep some way to actually 
check _how_ much overhead we add with nsproxy, etc

best,
Herbert

> -- 
> Regards,
> vatsa
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