Re: [Xen-devel] [PATCH v20 10/10] tools: CMDs and APIs for Cache Monitoring Technology

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 06/10/14 15:24, Wei Liu wrote:
> On Mon, Oct 06, 2014 at 10:18:28PM +0800, Chao Peng wrote:
>> On Mon, Oct 06, 2014 at 02:55:02PM +0100, Wei Liu wrote:
>>> On Mon, Oct 06, 2014 at 09:32:53PM +0800, Chao Peng wrote:
>>>> On Fri, Oct 03, 2014 at 01:49:56PM +0100, Wei Liu wrote:
>>>>> Thanks for this quick turnaround.
>>>>>
>>>>> Overall this looks good to me. Just some more questions on one thing I
>>>>> don't understand.
>>>>>
>>>>> On Fri, Oct 03, 2014 at 07:55:15PM +0800, Chao Peng wrote:
>>>>> [...]
>>>>>> +int libxl__pick_random_socket_cpu(libxl__gc *gc, uint32_t socketid)
>>>>>> +{
>>>>> This name is clearer.
>>>>>
>>>>> But still, why randomization is required?
>>>>>
>>>>> Does this mean picking arbitrary CPU returns the same result to library
>>>>> user? If so, why randomization is required?
>>>> The background here is that the L3 cache info we want to get in this
>>>> patch serial is a per-socket resource. To get it, we need to run the
>>>> related RDMSR from a cpu in that socket. So our real purpose of this
>>>> routine is to pick up a cpu number in that socket. From function
>>>> perspective, any cpu in that socket should work.
>>>>
>>>> But for different domains we may have more than one
>>>> getting-l3-cache-info operations for a certain socket. We want to avoid
>>>> to run all these operations always on a same cpu every time. So the
>>>> randomization is used for load-balance among all the cpus in the same
>>>> socket.
>>>>
>>> I'm not sure how much we can get from this randomization. Are you
>>> implying this operation is quite heavy workload for a cpu and / or
>>> there's potentially hundreds or thousands of parallel operations
>>> executed at the same time? FWIW in order to get cpu topology you need to
>>> issue hypercall, which is quite expensive (perhaps not that expensive
>>> compared to the CMT operation itself?).
>> For CMT itself, I don't think we gain much benifit from this. But we
>> introduced a new generic resource_op hypercall which can be used for 
>> potential
>> heavy workload in the future. So we add this function in the tools side.
> If so, I would rather avoid doing pre-mature optimization. We can
> always add it in later when it's necessary.
>
> Wei.

The core randomisation per socket started when all of this was
implemented in Xen, and there would be repeated IPIs to core 0 on each
socket for the information.  At that point, it was far more likely to
repeatedly bounce the same VM in and out of non-root mode.

Now this is all in the toolstack, it is far less likely to happen, and
is fine to drop the optimisation.

~Andrew

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