VMware vSphere vCenter Server 4.0 - Manual (Page 20)

The ESX CPU scheduler can interpret processor topology, including the relationship between sockets, cores,
and logical processors. The scheduler uses topology information to optimize the placement of virtual CPUs
onto different sockets to maximize overall cache utilization, and to improve cache affinity by minimizing
virtual CPU migrations.
In undercommitted systems, the ESX CPU scheduler spreads load across all sockets by default. This improves
performance by maximizing the aggregate amount of cache available to the running virtual CPUs. As a result,
the virtual CPUs of a single SMP virtual machine are spread across multiple sockets (unless each socket is also
a NUMA node, in which case the NUMA scheduler restricts all the virtual CPUs of the virtual machine to
reside on the same socket.)
In some cases, such as when an SMP virtual machine exhibits significant data sharing between its virtual CPUs,
this default behavior might be sub-optimal. For such workloads, it can be beneficial to schedule all of the virtual
CPUs on the same socket, with a shared last-level cache, even when the ESX/ESXi host is undercommitted. In
such scenarios, you can override the default behavior of spreading virtual CPUs across packages by including
the following configuration option in the virtual machine's .vmx configuration file:
sched.cpu.vsmpConsolidate="TRUE" .
To find out if a change in this parameter helps with performance, please do proper load testing. You cannot
easily predict the effect of a change in this parameter. If you do not see a performance boost after changing the
parameter, you have to revert the parameter to its default value.
Hyperthreading
Hyperthreading technology allows a single physical processor core to behave like two logical processors. The
processor can run two independent applications at the same time. To avoid confusion between logical and
physical processors, Intel refers to a physical processor as a socket, and the discussion in this chapter uses that
terminology as well.
Intel Corporation developed hyperthreading technology to enhance the performance of its Pentium IV and
Xeon processor lines. Hyperthreading technology allows a single processor core to execute two independent
threads simultaneously.
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.
N
OTE
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)
vSphere Resource Management Guide
20 VMware, Inc.