From mboxrd@z Thu Jan 1 00:00:00 1970 From: Andrew Cooper Subject: Re: [PATCH v20 10/10] tools: CMDs and APIs for Cache Monitoring Technology Date: Mon, 6 Oct 2014 15:27:38 +0100 Message-ID: <5432A6DA.2030401@citrix.com> References: <1412337315-15609-1-git-send-email-chao.p.peng@linux.intel.com> <1412337315-15609-11-git-send-email-chao.p.peng@linux.intel.com> <20141003124956.GB7627@zion.uk.xensource.com> <20141006133253.GA25440@pengc-linux> <20141006135502.GC7627@zion.uk.xensource.com> <20141006141828.GC25440@pengc-linux> <20141006142435.GE7627@zion.uk.xensource.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <20141006142435.GE7627@zion.uk.xensource.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Wei Liu , Chao Peng Cc: keir@xen.org, Ian.Campbell@citrix.com, stefano.stabellini@eu.citrix.com, George.Dunlap@eu.citrix.com, Ian.Jackson@eu.citrix.com, xen-devel@lists.xen.org, JBeulich@suse.com, dgdegra@tycho.nsa.gov List-Id: xen-devel@lists.xenproject.org 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