6.0.1

n
Use advanced settings under certain circumstances.
n
Use the vSphere SDK for scripted CPU allocation.
n
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 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".
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.
vSphere Resource Management
22 VMware, Inc.