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 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?).

If you insist randomization is necessary (as I cannot justify how
valuable this randomization could be), I think the above explanation in
your previous email should become comment of this function.

Also this function itself seems to be useful to PSR CMT only. I think
you can safely make it a static function in libxl_psr.c without
exporting it to other parts of libxl.

Wei.

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