Re: [Xen-devel] credit2 data structures

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.10.11 at 12:11, George Dunlap <George.Dunlap@xxxxxxxxxxxxx> wrote:
> On Thu, Oct 13, 2011 at 10:42 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>> Apart from the possibility of allocating the arrays (and maybe also the
>> cpumask_t-s) separately (for which I can come up with a patch on top
>> of what I' currently putting together) - is it really necessary to have
>> all these, the more that there can be multiple instances of the structure
>> with CPU pools?
> 
> I'm not quite sure what it is that you're asking.  Do you mean, are
> all of the things in each runqueue structure necessary?  Specifically,
> I guess, the cpumask_t structures (because the rest of the structure
> isn't significantly larger than the per-cpu structure for credit1)?

No, it's really the NR_CPUS-sized array of struct csched_runqueue_data.
Credit1 otoh has *no* NR_CPUS sized arrays at all.

> At first blush, all of those cpu masks are necessary.  The assignment
> of cpus to runqueues may be arbitrary, so we need a cpu mask for that.
>  In theory, "idle" and "tickled" only need bits for the cpus actually
> assigned to this runqueue (which should be 2-8 under normal
> circumstances).  But then we would need some kind of mechanism to
> translate "mask just for these cpus" to "mask of all cpus" in order to
> use the normal cpumask mechanisms, which seems like a lot of extra
> complexity just to save a few bytes.  Surely a system with 4096
> logical cpus can afford 6 megabytes of memory for scheduling?

I'm not concerned about the total amount if run on a system that
large. I'm more concerned about this being a single chunk (possibly
allocated post-boot, where we're really aiming at having no
allocations larger than a page at all) and this size being allocated
even when running on a much smaller system (i.e. depending only
on compile time parameters).

> For one thing, the number of runqueues in credit2 is actually meant to
> be smaller than the number of logical cpus -- it's meant to be one per
> L2 cache, which should have between 2 and 8 logical cpus, depending on
> the architecture.  I just put NR_CPUS because it was easier to get
> working.  Making that an array of pointers, which is allocated on an
> as-needed basis, should reduce that requirement a great deal.

That would help, but would probably not suffice (since a NR_CPUS
sized array of pointers is still going to be larger than a page). We
may need to introduce dynamic per-CPU data allocation for this...

Jan


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