Re: Xen Platform QoS design discussion

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 29/05/2014 08:31, Xu, Dongxiao wrote:
>> -----Original Message-----
>> From: Jan Beulich [mailto:jbeulich@suse.com]
>> Sent: Thursday, May 29, 2014 3:02 PM
>> To: george.dunlap@eu.citrix.com; Xu, Dongxiao
>> Cc: andrew.cooper3@citrix.com; Ian.Campbell@citrix.com;
>> xen-devel@lists.xen.org
>> Subject: Re: RE: [Xen-devel] Xen Platform QoS design discussion
>>
>>>>> "Xu, Dongxiao" <dongxiao.xu@intel.com> 05/29/14 2:46 AM >>>
>>> I think Jan's opinion here is similar to what I proposed in the beginning of this
>> thread.
>>> The only difference is that, Jan prefers to get the CQM data per-socket and
>> per-domain
>>> with data copying, while I proposed to get the CQM data per-domain for all
>> sockets
>>> that can reduce the amount of hypercalls.
>> I don't think I ever voiced any preference between these two. All I said it
>> depends on
>> prevalent usage models, and to date I don't think I've seen a proper analysis of
>> what
>> the main usage model would be - it all seems guesswork and/or taking random
>> examples.
>>
>> What I did say I'd prefer is to have all this done outside the hypervisor, with the
>> hypervisor just providing fundamental infrastructure (MSR accesses).
> Okay. If I understand correctly, you prefer to implement a pure MSR access hypercall for one CPU, and put all other CQM things in libxc/libxl layer.
>
> In this case, if libvert/XenAPI is trying to query a domain's cache utilization in the system (say 2 sockets), then it will trigger _two_ such MSR access hypercalls for CPUs in the 2 different sockets.
> If you are okay with this idea, I am going to implement it.
>   
> Thanks,
> Dongxiao

While I can see the use and attraction of a generic MSR access 
hypercalls, using this method for getting QoS data is going to have 
subsantitally higher overhead than even the original domctl suggestion.

I do not believe it will be an effective means of getting large 
quantities of data from ring0 MSRs into dom0 userspace.  This is not to 
say that having a generic MSR interface is a bad thing, but I don't 
think it should be used for this purpose.

~Andrew