Section 4.5. NUMA and ProcessorMemory Affinity

4.5. NUMA and Processor/Memory Affinity

ESX Server supports physical hosts that are of Non-Uniform Memory Access (NUMA) architecture. Each NUMA machine can be comprised of multiple nodes that contain the processors, memory cache, and so on. Running a virtual machine's resources locally on one of these nodes without having to access resources remotely or on a different NUMA node increases performance, and that's where affinity comes in.

By default, ESX Server utilizes all NUMA nodes equally, balancing virtual machines between each node by using local memory when possible for VMs running on the same node. ESX Server designates every virtual machine as a NUMA nodethe CPUs of which the virtual machine will only run on. According to VMware documentation, ESX Server "periodically…compares the utilization levels of all NUMA nodes and attempts to 'rebalance' the nodes if one node has a higher utilization level than the other nodes. ESX Server rebalances the nodes by changing a virtual machine's 'home' NUMA node from the overutilized node to the underutilized node. "This is an automatic process. However, you can also tweak NUMA settings yourself.

Since ESX automatically tries to balance virtual machines on NUMA nodes, you may never have to manually set NUMA options. However, if you find it necessary, you can set both CPU and memory affinity. Under the CPU and Memory tabs of your virtual machine, select the affinity for the virtual machine, similar to what was shown previously in this chapter.