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Specifying CPU Configuration
You can specify CPU configuration to improve resource management. However, if you do not customize CPU
configuration, the ESXi host uses defaults that work well in most situations.
You can specify CPU configuration in the following ways:
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Use the attributes and special features available through the vSphere Client. The vSphere Client graphical
user interface (GUI) allows you to connect to the ESXi host or a vCenter Server system.
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Use advanced settings under certain circumstances.
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Use the vSphere SDK for scripted CPU allocation.
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Use hyperthreading.
Multicore Processors
Multicore processors provide many advantages for a host performing multitasking of virtual machines.
Intel and AMD have each developed processors which combine two or more processor cores into a single
integrated circuit (often called a package or socket). VMware uses the term socket to describe a single package
which can have one or more processor cores with one or more logical processors in each core.
A dual-core processor, for example, can provide almost double the performance of a single-core processor, by
allowing two virtual CPUs to execute at the same time. Cores within the same processor are typically
configured with a shared last-level cache used by all cores, potentially reducing the need to access slower main
memory. A shared memory bus that connects a physical processor to main memory can limit performance of
its logical processors if the virtual machines running on them are running memory-intensive workloads which
compete for the same memory bus resources.
Each logical processor of each processor core can be used independently by the ESXi CPU scheduler to execute
virtual machines, providing capabilities similar to SMP systems. For example, a two-way virtual machine can
have its virtual processors running on logical processors that belong to the same core, or on logical processors
on different physical cores.
The ESXi CPU scheduler can detect the processor topology and the relationships between processor cores and
the logical processors on them. It uses this information to schedule virtual machines and optimize performance.
The ESXi 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 ESXi 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 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".
vSphere Resource Management
20 VMware, Inc.