I received a question last week from a customer. They have a fairly big VDI environment and are researching the migration to vSphere 5.1. One of the changes they made in the 4.1 time frame was the advanced setting “HaltingIdleMsecPenalty” in order to optimize hyper threading fairness for their specific desktop environment. I knew that this was no longer needed but didn’t have an official reference for them (There is a blog post by Tech Marketing performance guru Mark A. that mentions it though). Today I noticed it was mentioned in a whitepaper which was recently released titled “The CPU Scheduler in VMware vSphere 5.1“. I recommend everyone to read this whitepaper as it gives you a better understanding of how the scheduler works and how it has been improved over time.
The following section is an outtake from that white paper.
Improvement in Hyper-Threading Utilization
In vSphere 4.1, a strict fairness enforcement policy on HT systems might not allow achieving full utilization of all logical processors in a situation described in KB article 1020233 . This KB also provides a work-around based on an advanced ESX host attribute, “HaltingIdleMsecPenalty”. While such a situation should be rare, a recent change in the HT fairness policy described in “Policy on Hyper-Threading,” obviates the need for the work-around. Figure 8 illustrates the effectiveness of the new HT fairness policy for VDI workloads. In the experiments, the number of VDI users without violating the quality of service (QoS) requirement is measured on vSphere 4.1, vSphere 4.1 with “HaltingIdleMsecPenalty” tuning applied, and vSphere 5.1. Without the tuning, vSphere 4.1 supports 10% fewer users. On vSphere 5.1 with the default setting, it slightly exceeds the tuned performance of vSphere 4.1.