Scheduler Cell Size on Six-Core Processors

The VMware ESX scheduler logically partitions a host's physical processor cores into cells. In ESX 3.5, four physical cores are grouped into a cell.

 

The Intel® Xeon® 7400-series processors (codename Dunnington), with six cores per package, are an exception to the trend of cores per socket being a power of two (1, 2, 4, 8...). On a Dunnington processor the ESX default cell size of four causes some cells to be entirely within one socket (using four of that socket's six cores) and some cells to span sockets (using two cores from one socket and two from a different socket). This asymmetry in cell construction could result in variations in the performance of virtual machines due to on-chip cache access behavior, inter-vCPU communication, etc. The performance variation could be two virtual machines performing differently (if they are scheduled to different cell types) or the performance of a single virtual machine varying over time (as it is scheduled first to one cell type and then to the other). The impact would depend on your virtual machine configuration and workloads. Note that occasional rebalancing happens across cells so that no cells are unfairly overloaded.