Among the many results presented by VRC, the configurations that most perplexed us were the two and four virtual machine configurations, each with four vCPUs per virtual machine.  The configuration with two virtual machines looked good and matched our internal numbers.  In this configuration there are a total of eight vCPUs on the host which maps each to its own physical core on the Xeon 5500 series processor.  The problem arose when the virtual machine count was increased to four, resulting in 16 total vCPUs.  In this configuration each vCPU is paired with one logical, Hyper-Threaded core.  Project VRC showed this configuration supporting no more desktops than the two-VM configuration, which suggests no value to Hyper-Threading on this configuration.

It took us some time to understand the reason for these results, but we eventually identified a very specific condition where ESX’s scheduler enforces fairness in scheduling vCPUs at at cost of throughput.  ESX’s scheduler has long be subject of the intensive scrutiny of a large number of VMware engineers to guarantee fair access to the processor for each virtual machine.  It is because of this fairness that VMware’s customers can rely on CPU resource controls.  But, when fairness goes too far, throughput may be sub-optimal.

Hyper-Threading presents particular problems to fairness because of the non-linear performance it delivers.  A thread will run at one speed when it has full access to a physical core, at another speed when it is sharing a core, and at third speed when sharing a core with a different thread.  As a result, ESX’s scheduler will sometimes pause a thread to enforce fairness.  These pauses are more common when Hyper-Threading is present to account for its lack of uniformity in thread performance.  If the host lacks vCPUs that are ready to run, the result is CPU utilization below saturation, leaving CPU cycles unused.

There are three specific conditions that can excite this condition:

1. A Xeon 5500 series processor is present with Hyper-Threading enabled,
2. CPU utilization is near saturation, and
3. A roughly one-to-one mapping between vCPUs and logical processors.

In this scenario, VMware vSphere favors fairness over throughput and sometimes pauses one vCPU to dedicate a whole core to another vCPU, eliminating gains provided by Hyper-Threading.  In cases outside of these three conditions, the performance of VMware vSphere 4 meets the high expectations of VMware’s R&D team and its customers.  Of course production environments rarely (never?) have a one-to-one ratio of vCPUs to logical processors.  This occurs when there are only four 4-way virtual machines on a Xeon 5500 system, for example.

But environments such as Project VRC’s are simplifications of production environments meant to understand the capabilities of virtual platforms.  VMware has provided a patch to Project VRC that will allow them to improve throughput in their environment.  We are going to release this patch and its documentation to the general public within a couple of weeks.  I do not expect that any of VMware’s customers will benefit from the changes is allows, but I will later document the patch and its usage for anyone that cares to experiment.