IBM 8687 - Eserver xSeries 440, Planning And Installation Manual

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IBM
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xSeries 440 Planning and Installation Guide
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
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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
.