6.0.1

While hyperthreading does not double the performance of a system, it can increase performance by better
utilizing idle resources leading to greater throughput for certain important workload types. An application
running on one logical processor of a busy core can expect slightly more than half of the throughput that it
obtains while running alone on a non-hyperthreaded processor. Hyperthreading performance
improvements are highly application-dependent, and some applications might see performance degradation
with hyperthreading because many processor resources (such as the cache) are shared between logical
processors.
NOTE On processors with Intel Hyper-Threading technology, each core can have two logical processors
which share most of the core's resources, such as memory caches and functional units. Such logical
processors are usually called threads.
Many processors do not support hyperthreading and as a result have only one thread per core. For such
processors, the number of cores also matches the number of logical processors. The following processors
support hyperthreading and have two threads per core.
n
Processors based on the Intel Xeon 5500 processor microarchitecture.
n
Intel Pentium 4 (HT-enabled)
n
Intel Pentium EE 840 (HT-enabled)
Hyperthreading and ESXi Hosts
A host that is enabled for hyperthreading should behave similarly to a host without hyperthreading. You
might need to consider certain factors if you enable hyperthreading, however.
ESXi hosts manage processor time intelligently to guarantee that load is spread smoothly across processor
cores in the system. Logical processors on the same core have consecutive CPU numbers, so that CPUs 0 and
1 are on the first core together, CPUs 2 and 3 are on the second core, and so on. Virtual machines are
preferentially scheduled on two different cores rather than on two logical processors on the same core.
If there is no work for a logical processor, it is put into a halted state, which frees its execution resources and
allows the virtual machine running on the other logical processor on the same core to use the full execution
resources of the core. The VMware scheduler properly accounts for this halt time, and charges a virtual
machine running with the full resources of a core more than a virtual machine running on a half core. This
approach to processor management ensures that the server does not violate any of the standard ESXi
resource allocation rules.
Consider your resource management needs before you enable CPU affinity on hosts using hyperthreading.
For example, if you bind a high priority virtual machine to CPU 0 and another high priority virtual machine
to CPU 1, the two virtual machines have to share the same physical core. In this case, it can be impossible to
meet the resource demands of these virtual machines. Ensure that any custom affinity settings make sense
for a hyperthreaded system.
Enable Hyperthreading
To enable hyperthreading, you must first enable it in your system's BIOS settings and then turn it on in the
vSphere Web Client. Hyperthreading is enabled by default.
Consult your system documentation to determine whether your CPU supports hyperthreading.
Procedure
1 Ensure that your system supports hyperthreading technology.
2 Enable hyperthreading in the system BIOS.
Some manufacturers label this option Logical Processor, while others call it Enable Hyperthreading.
Chapter 4 Administering CPU Resources
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