[Libvir] Proposal for new Libvirt functions about virtual CPUs

[Daniel P. Berrange]

On Mon, Jun 26, 2006 at 10:43:24AM -0400, Daniel Veillard wrote:
> On Mon, Jun 26, 2006 at 04:14:59PM +0200, michel.ponceau at bull.net wrote:
> > 1) Currently the NULL virDomainPtr (meaning Domain0) is only specified in 
> > the 3 virDomain*Memory functions (GetMax, SetMax, Set). The other 
> > virDomain* functions need the actual virDomainPtr for Domain0. What is the reason for this Memory-specific 
> > option?
> 
>   I think it's problem. We should always require a virDomainPtr, I actually
> think the shortcut is not implemented.
>   I wonder if it would be better to just remove it that would be cleaner,
> however when think about other kind of virtualization, where the main
> os would not be represented as a domain, but still with memory management,
> in that case that should be kept.
> 
> > Is it applicable to virtual CPUs?
> 
>   Hum, similary, what if the main OS CPU usage could be set up even if
> not mapped as a real domain, maybe this should be kept for the vCPU APIs
> too, yes, a bit messy but more generic.
> 
> > 2) For "xm vcpu-list" equivalent function (suggested virDomainGetVcpus) I propose to return in a new structure all the information provided by 
> > Xen Daemon. I am trying the following addition in libvirt.h :
> 
>   Remember to change libvirt.h.in as libvirt.h is generated by configure.
> 
> > typedef struct _virVcpuInfo virVcpuInfo;
> > struct _virVcpuInfo {
> >     int number;                 /* virtual CPU number */
> 
>   unsigned int rather
> 
> >     unsigned char state;        /* 'r'=running, 'b'=blocked, 'p'=offline 
> > */
> 
>   Not too fond of using a letter there. I would prefer a number and a clean enum
> 
> >     unsigned long long cpuTime; /* CPU time used, in nanoseconds */
> 
> >     int cpu;                    /* last real CPU number allocated */
> 
>   That's a bit fishy to me, how do you expect to use that information ?

I guess that's to allow the 'CPU' column in the following output to be generated

# xm vcpu-list
Name                              ID  VCPU  CPU  State  Time(s)  CPU Affinity
Domain-0                           0     0    0   -b-     636.8  any cpu
Domain-0                           0     1    1   -b-     396.8  any cpu
Domain-0                           0     2    2   r--     319.5  any cpu
Domain-0                           0     3    3   -b-     369.4  any cpu
xperf1                             1     0    3   -b-     545.6  any cpu
xperf2                             3     0    2   -b-      20.0  any cpu
xperf2                             3     1    3   -b-      18.2  any cpu
xperf2                             3     2    0   -b-      18.6  any cpu
xperf2                             3     3    1   -b-      17.8  any cpu

> >     char cpumap[128];           /* affinity map of real CPUs which can be 
> > allocated */
> 
>   so we know we will never have libvirt used for a virtualization platform
> which has no more than 256 physical CPU to offer ? Remember I don't want that
> API to be limited to Xen and to me if we put this in the API this must be
> future proof. I would at least grow the CPU map to 256 entries at the API
> level. Oh and use unsigned char to go 0-255.

I expect we really need a bit-set in that field, one bit per CPU that the
VCPU can be scheduled on. One thought though, is do we want to mix up
configuration data (VCPU/VPU affinity) with performance metrics (state,
runtime, last cpu) ?

Might it make more sense to have a single API for retrieving the entire
VCPU <-> VPU mapping for all CPUs in a single call - basically return a
2-d matrix with VCPU in one axis and CPU in the other access, and the
elements are basically a bit 1/0

>   Another note is about access, do you really want to get those just from
> xend or also from the hypervisor ? The hypervisor might be way cheaper to
> reach and more accurate. Hopefully that structure still make sense with
> the associated hypervisor informations could you look up the associated
> hypervisor call and double check ?

We definitely want to get this info via the hypervisor if its available,
since the performance hit in making the multiple RPC calls will be far
too great to make this usable otherwise.

> > 3) For "xm vcpu-pin" equivalent function (suggested virDomainPinVcpu) I propose a CPU list argument in same format as virCpuInfo cpumap. Do 
> > you agree?
> 
>   I have the same worries about the hard constraints put on the number of 
> CPUs at the API level.

Whatever we do, should match up in its data structure with the API for 
retrieving CPU pining

Dan.
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