Re: [Xen-devel] [PATCH 00/12] cpumask handling scalability improvements

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.10.11 at 18:04, Keir Fraser <keir@xxxxxxx> wrote:
> On 20/10/2011 16:19, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> 
>>>> 06: allow efficient allocation of multiple CPU masks at once
>>> 
>>> That is utterly hideous and for insignificant saving.
>> 
>> I was afraid you would say that, and I'm not fully convinced
>> either. But I wanted to give it a try to see how bad it is. The
>> more significant saving here really comes from not allocating
>> the CPU masks at all for unused irq_desc-s.
> 
> Aren't we planning to dynamically allocate irq_desc-s? That would seem the
> nicer solution here.

Yes, I would hope so. But irrespective of that, allocating e.g. 512 bits
(times 4) just to use, say, 20-30 of them is bad - again, not so much
from a memory wasting pov, but rather from the fact that this
needlessly causes a larger cache and TLB footprint.

I actually think that ultimately we should try to remove all
non-dynamically allocated CPU masks (including statics, per-CPU
ones, and local variables - the latter being particularly important as
they cause pretty big stack frames, despite there now being at
most one [with the rare exception of two] of them per function,
which will continue to grow with higher NR_CPUS values).

Jan


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