On 2/12/07, Sam Vilain <sam@vilain.net> wrote:
> Ask yourself this - what do you need the container structure for so
> badly, that virtualising the individual resources does not provide for?

Primarily, that otherwise every module that wants to affect/monitor
behaviour of a group of associated processes has to implement its own
process grouping abstraction.

As an example, the CPU accounting patch that in included in my patch
set as an illustration of a simple resource monitoring module is just
250 lines, almost entirely in one file; if it also had to handle
associating tasks together into groups and presenting a filesystem
interface to the user it would be far larger and would have a much
bigger footprint on the kernel.

>From the point of view of the virtual server containers, the advantage
is that you're integrated with a standard filesystem interface for
determining group membership. It does become simpler to combine
virtual servers and resource controllers, although I grant you that
you could juggle that from userspace without the additional kernel
support.

Paul
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Paul Menage wrote:
>> Ask yourself this - what do you need the container structure for so
>> badly, that virtualising the individual resources does not provide for?
>>     
> Primarily, that otherwise every module that wants to affect/monitor
> behaviour of a group of associated processes has to implement its own
> process grouping abstraction.
>   

Not every module, you just make them on sensible, planned groupings. 
The danger is that the "container" group becomes a fallback grouping for
things when people can't be bothered thinking about it properly, and
everything including the kitchen sink gets thrown in.  Then later you
find a real use case where you don't want them together, but it's too
late because it's already a part of the official API.

> As an example, the CPU accounting patch that in included in my patch
> set as an illustration of a simple resource monitoring module is just
> 250 lines, almost entirely in one file; if it also had to handle
> associating tasks together into groups and presenting a filesystem
> interface to the user it would be far larger and would have a much
> bigger footprint on the kernel.
>   

It's also less flexible.  What if I want to do CPU accounting on some
other boundaries than the "virtual server" a process is a part of?

> From the point of view of the virtual server containers, the advantage
> is that you're integrated with a standard filesystem interface for
> determining group membership. It does become simpler to combine
> virtual servers and resource controllers, although I grant you that
> you could juggle that from userspace without the additional kernel
> support.
>   

I'm not disagreeing it's a pragmatic shortcut that has been successful
for a number of projects including vserver which I use every day.  But
it reduces "synergy" by excluding the people working with virtualisation
in ways that don't fit its model.

Yes, there should be a similarity in the way that you manage namespaces
and it should be easy to develop new namespaces without constantly
re-inventing the wheel.  But why does that imply making binding
decisions about the nature of how you can virtualise?  IMHO those
decisions should be made on a per-subsystem basis.

Sam.
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