Currently ESX Server does not support Hyper-Threading technology. For support information including supported servers, see:
http://www.pc.ibm.com/ww/eserver/xseries/vmware.html
For more information on VMware, refer to the redbook Server Consolidation with the IBM ^xSeries 440 and VMware ESX Server, SG24-6852.
3.6 Application considerations
As well as operating systems, there are enterprise applications currently available that are licensed on a per-processor basis. You should be aware that enabling Hyper-Threading or adding physical processors on systems running these applications may have licensing implications for your organization. This will need to be considered in the planning phase of your deployment and, if required, additional licenses purchased prior to enabling Hyper-Threading.
Microsoft has stated that current server products licensed on a per-processor basis will require one license per physical processor. For example, on a two-way system with Hyper-Threading enabled and running Microsoft SQL Server, a two-processor license is required, even though the application may process threads on four logical processors.
Performance benefits versus licensing costs may be a consideration before enabling Hyper-Threading or adding processors and may require testing to confirm that there will be a substantial benefit to application performance. In most applications there will be a performance gain as processors are added; however, this gain does not generally remain linear with the continued addition of processors. The performance improvements seen will depend largely on application scalability, which is discussed in more detail in the next section.
3.6.1 Scalability and performance considerations
Adding processors improves server performance because software instruction execution can be shared among the additional processors. However, this requires software to detect the additional CPUs and generates additional work in the form of threads or processes, which execute on the additional processors. This does not happen automatically. The operating system provides a platform that enables the capability of multiprocessing, but it is up to the application to generate the additional threads and processes to execute on all processors. This is referred to as application scalability.
100IBM ^xSeries 440 Planning and Installation Guide
