Phillip J. Windley and his student Terry Wilcox published a very interesting 32-pages paper about VMware ESX Server 2.5.2 virtual machines performances depending on amount of assigned virtual RAM and enabling of Intel HyperThreading or vSMP:
We present a summary of our evaluation of VMWare ESX Server 2.5.2. In particular we confirm and work around known timing issues with guest operating systems running on ESX server. Our work validates and adds to the work of other groups modeling the behavior of ESX Server during CPU intensive workloads by exploring in more detail the effects of Hyper-Threading and the overhead of Virtual SMP.
We report and measure a previously unknown performance penalty for allocating too much RAM in virtual machines with Linux as the guest operating system.
This paper also describes the testbed we used to manage and run our tests including a virtualization test management system we developed to run the tests we performed.
We describe timing issues that affect performance testing on ESX Server and a method for measuring runtimes that gives accurate results.
Reported conclusion are extremely appealing:
- Single CPU virtual machines scale better than virtual machines using Virtual SMP
- Hyper-Threading increases throughput if there are a large number of virtual CPUs, but makes no difference if the number of virtual CPUs is less than or equal to the number of physical CPUs
- Do not allocate excessive resources to virtual machines. Additional resources may hurt performance
Read the whole paper at source.