Guest Automatic NUMA Placement in libxl and xl


NUMA (which stands for Non-Uniform Memory Access) means that the memory accessing times of a program running on a CPU depends on the relative distance between that CPU and that memory. In fact, most of the NUMA systems are built in such a way that each processor has its local memory, on which it can operate very fast. On the other hand, getting and storing data from and on remote memory (that is, memory local to some other processor) is quite more complex and slow. On these machines, a NUMA node is usually defined as a set of processor cores (typically a physical CPU package) and the memory directly attached to the set of cores.

The Xen hypervisor deals with NUMA machines by assigning to each domain a "node affinity", i.e., a set of NUMA nodes of the host from which they get their memory allocated.

NUMA awareness becomes very important as soon as many domains start running memory-intensive workloads on a shared host. In fact, the cost of accessing non node-local memory locations is very high, and the performance degradation is likely to be noticeable.

Guest Placement in xl

If using xl for creating and managing guests, it is very easy to ask for both manual or automatic placement of them across the host's NUMA nodes.

Note that xm/xend does the very same thing, the only differences residing in the details of the heuristics adopted for the placement (see below).

Manual Guest Placement with xl

Thanks to the "cpus=" option, it is possible to specify where a domain should be created and scheduled on, directly in its config file. This affects NUMA placement and memory accesses as the hypervisor constructs the node affinity of a VM basing right on its CPU affinity when it is created.

This is very simple and effective, but requires the user/system administrator to explicitly specify affinities for each and every domain, or Xen won't be able to guarantee the locality for their memory accesses.

It is also possible to deal with NUMA by partitioning the system using cpupools. Again, this could be "The Right Answer" for many needs and occasions, but has to be carefully considered and setup by hand.

Automatic Guest Placement with xl

If no "cpus=" option is specified in the config file, libxl tries to figure out on its own on which node(s) the domain could fit best. It is worthwhile noting that optimally fitting a set of VMs on the NUMA nodes of an host is an incarnation of the Bin Packing Problem. In fact, the various VMs with different memory sizes are the items to be packed, and the host nodes are the bins. As such problem is known to be NP-hard, we will be using some heuristics.

The first thing to do is find the nodes or the sets of nodes (from now on referred to as 'candidates') that have enough free memory and enough physical CPUs for accommodating the new domain. The idea is to find a spot for the domain with at least as much free memory as it has configured to have, and as much pCPUs as it has vCPUs. After that, the actual decision on which candidate to pick happens accordingly to the following heuristics:

Giving preference to candidates with fewer nodes ensures better performance for the guest, as it avoid spreading its memory among different nodes. Favoring candidates with fewer vCPUs already runnable there ensures a good balance of the overall host load. Finally, if more candidates fulfil these criteria, prioritizing the nodes that have the largest amounts of free memory helps keeping the memory fragmentation small, and maximizes the probability of being able to put more domains there.

Guest Placement within libxl

xl achieves automatic NUMA placement because that is what libxl does by default. No API is provided (yet) for modifying the behaviour of the placement algorithm. However, if your program is calling libxl, it is possible to set the numa_placement build info key to false (it is true by default) with something like the below, to prevent any placement from happening:

libxl_defbool_set(&domain_build_info->numa_placement, false);

Also, if numa_placement is set to true, the domain must not have any cpu affinity (i.e., domain_build_info->cpumap must have all its bits set, as it is by default), or domain creation will fail returning ERROR_INVAL.

Besides than that, looking and/or tweaking the placement algorithm search "Automatic NUMA placement" in libxl_internal.h.

Note this may change in future versions of Xen/libxl.


Analyzing various possible placement solutions is what makes the algorithm flexible and quite effective. However, that also means it won't scale well to systems with arbitrary number of nodes. For this reason, automatic placement is disabled (with a warning) if it is requested on a host with more than 16 NUMA nodes.