On Fri, 2005-09-09 at 13:12 +0900, Magnus Damm wrote:
> On 9/9/05, KUROSAWA Takahiro <[email protected]> wrote:
> > On Thu, 8 Sep 2005 05:02:32 -0700
> > Paul Jackson <[email protected]> wrote:
> > > One of my passions is to avoid special cases across API boundaries.
> > >
> > > I am proposing that you don't do subcpusets like this.
> > >
> > > Consider the following alternative I will call 'cpuset meters'.
> > >
> > > For each resource named 'R' (cpu and mem, for instance):
> > > * Add a boolean flag 'meter_R' to each cpuset. If set, that R is
> > > metered, for the tasks in that cpuset or any descendent cpuset.
> > > * If a cpuset is metered, then files named meter_R_guar, meter_R_lim
> > > and meter_R_cur appear in that cpuset to manage R's usage by tasks
> > > in that cpuset and descendents.
> > > * There are no additional rules that restrict the ability to change
> > > various other cpuset properties such as cpus, mems, cpu_exclusive,
> > > mem_exclusive, or notify_on_release, when a cpuset is metered.
> > > * It might be that some (or by design all) resource controllers do
> > > not allow nesting metered cpusets. I don't know. But one should
> > > (if one has permission) be able to make child cpusets of a metered
> > > cpuset, just like one can of any other cpuset.
> > > * A metered cpuset might well have cpus or mems that are not the
> > > same as its parent, just like an unmetered cpuset ordinarly does.
> >
> > Jackson-san's idea looks good for me because users don't need
> > to create special cpusets (parents of subcpusets or subcpusets).
> > From the point of users, maybe they wouldn't like to create
> > special cpusets.
>
> Yes, from the user POV it must be good to keep the hierarchical model.
> Ckrm and cpusets both provide a tree with descendents, children and
> parents. This hierarchical model is very nice IMO and provides a
> powerful API for the user.
>
> > As for the resource controller that I've posted, it assumes
> > that there are groups of tasks that share the same cpumasks/nodemasks,
> > and that there are no hierarchy in that groups in order to make
> > things easier. I'll investigate how I can attach the resource
> > controller to the cpuset meters.
>
> Subcpusets, compared to cpusets and ckrm, gives the user a flat model.
> No hierarchy. Limited functionality compared to the hierachical model.
>
> But what I think is important to keep in mind here is that cpusets and
> subcpusets do very different things. If I understand cpusets
> correctly, each cpuset may share processors or memory nodes with other
> cpusets. One task running on a shared processor may starve other
> cpusets using the same processor. This design works well with cpusets,
> but for resource controllers that must provide some kind of guarantee,
> this starvation is unsuitable.
>
> And we already have an hierarchical alternative: ckrm. But look at the
> complexity and the amount of code. I believe that the complexity in
> ckrm mainly comes from the hierarchical model.
I've been trying to find ways to significantly reduce CKRM's kernel
footprint. I recently posted an RFC patch to CKRM-Tech describing a
5000-line reduction:
http://sourceforge.net/mailarchive/forum.php?thread_id=8132624&forum_id=35191
Feedback on the approach presented in the RFC patch would be
appreciated.
> Maybe it is possible to have an hierarchical model and keep the
> framework simple and easy to understand while providing guarantees,
> I'm not sure. But until that happens, I'm quite happy with a simple,
> limited flat model.
>
> / magnus
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