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

[Wei Liu <wei.liu2@xxxxxxxxxx>]

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.

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