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[Libvir] CPU pinning of domains at creation time



  There are a few things I gathered on this issue. This affects 
NUMA setups, where basically if a domain must be placed on a given cell
it is not good to let the hypervisor place it first with its own heuristics
and then later migrate it to a different set of CPU, but better to 
instruct the hypervisor to start said domain on the given set.
   - For Xen it is possible to instruct the hypervisor by passing 
     (cpus '2,3') in the SExpr where the argument is a list of
     the physical processors allowed
   - For KVM I think the standard way would be to select the 
     cpuset using sched_setaffinity() between the fork of the 
     current process and the exec of the qemu process
   - there is no need (from a NUMA perspective) to do fine grained
     allocation at that point, as long as the domain can be restricted
     to a given cell at startup, then if needed virDomainPinVcpu() can be
     used later to do more precise pinning in order to try to optimize
     placement
   - to be able to instruct the hypervisor at creation time adding the
     information in the domain XML description looks the more natural way
     (another option would be to force to use virDomainDefineXML, add a
      call using the resulting virDomainPtr to define the set, and 
      then virDomainCreate would be used to do the actual start)
     + the good point of having this embedded in the XML is that
       we still have all informations about the domain settings in
       the XML, if we want to restart it later
     + the bad point is that we need to fetch and carry this extra
       information when doing XML dumps to not loose it for example
       when manipulating the domain to add or remove devices
   - extracting a cpuset can still be an heavy operation, for example
     if using xend on need one RPC per vcpu in the domain, the cpuset
     being constructed by OR'ing logically all cpumaps used by the 
     vcpus of the domain (though in most case this will be the full
     map after the first CPU and can be stopped immediately)
   - for the mapping at the XML level I suggest to use a simple extension
     to the <vcpu>n</vcpu> and extend it to
     <vcpu cpuset='2,3'>n</vcpu>
     with a limited syntax which is just the comma separated list of
     allowed CPU numbers (if the code actually detects such a cpuset is
     in effect i.e. in general this won't be added).

Internally implementing this should not be too hard, I would probably refactor
some of the existing parsing code, provide functions to get the cpuset and
the number of physical processors.

  Does this sounds okay ?

Daniel

-- 
Red Hat Virtualization group http://redhat.com/virtualization/
Daniel Veillard      | virtualization library  http://libvirt.org/
veillard redhat com  | libxml GNOME XML XSLT toolkit  http://xmlsoft.org/
http://veillard.com/ | Rpmfind RPM search engine  http://rpmfind.net/


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