Choosing a CPU

The type of CPU in your compute node is a very important decision. You must ensure that the CPU supports virtualization by way of VT-x for Intel chips and AMD-v for AMD chips.

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Consult the vendor documentation to check for virtualization support. For Intel CPUs, see Does my processor support Intel® Virtualization Technology?. For AMD CPUs, see AMD Virtualization. Your CPU may support virtualization but it may be disabled. Consult your BIOS documentation for how to enable CPU features.

The number of cores that the CPU has also affects your decision. It is common for current CPUs to have up to 24 cores. Additionally, if an Intel CPU supports hyper-threading, those 24 cores are doubled to 48 cores. If you purchase a server that supports multiple CPUs, the number of cores is further multiplied.

As of the Kilo release, key enhancements have been added to the OpenStack code to improve guest performance. These improvements allow the Compute service to take advantage of greater insight into a compute host’s physical layout and therefore make smarter decisions regarding workload placement. Administrators can use this functionality to enable smarter planning choices for use cases like NFV (Network Function Virtualization) and HPC (High Performance Computing).

Considering non-uniform memory access (NUMA) is important when selecting CPU sizes and types, as there are use cases that use NUMA pinning to reserve host cores for operating system processes. These reduce the available CPU for workloads and protects the operating system.

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When CPU pinning is requested for a guest, it is assumed there is no overcommit (or, an overcommit ratio of 1.0). When dedicated resourcing is not requested for a workload, the normal overcommit ratios are applied.

Therefore, we recommend that host aggregates are used to separate not only bare metal hosts, but hosts that will provide resources for workloads that require dedicated resources. This said, when workloads are provisioned to NUMA host aggregates, NUMA nodes are chosen at random and vCPUs can float across NUMA nodes on a host. If workloads require SR-IOV or DPDK, they should be assigned to a NUMA node aggregate with hosts that supply the functionality. More importantly, the workload or vCPUs that are executing processes for a workload should be on the same NUMA node due to the limited amount of cross-node memory bandwidth. In all cases, the NUMATopologyFilter must be enabled for nova-scheduler.

Additionally, CPU selection may not be one-size-fits-all across enterprises, but more of a list of SKUs that are tuned for the enterprise workloads.

For more information about NUMA, see CPU topologies in the Administrator Guide.

In order to take advantage of these new enhancements in the Compute service, compute hosts must be using NUMA capable CPUs.

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Multithread Considerations

Hyper-Threading is Intel’s proprietary simultaneous multithreading implementation used to improve parallelization on their CPUs. You might consider enabling Hyper-Threading to improve the performance of multithreaded applications.

Whether you should enable Hyper-Threading on your CPUs depends upon your use case. For example, disabling Hyper-Threading can be beneficial in intense computing environments. We recommend performance testing with your local workload with both Hyper-Threading on and off to determine what is more appropriate in your case.

In most cases, hyper-threading CPUs can provide a 1.3x to 2.0x performance benefit over non-hyper-threaded CPUs depending on types of workload.