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3.3 Details of Domain Resources
Logical Domains technology provides flexible assignment of hardware resources
to domains, with options for specifying physical resources for a corresponding
virtual resource.
3.3.1 Virtual CPUs
As mentioned in the section “Real and Virtual CPUs,” each domain is assigned ex-
clusive use of a number of CPUs, also called threads or strands . Within a domain,
these are called virtual CPUs (vCPUs).
The granularity of assignment is a single vCPU. A domain can have from one
vCPU up to all the vCPUs on the server. On UltraSPARC T1 systems (T1000
and T2000), the maximum is 8 cores with 4 threads, for a total of 32 vCPUs. On
UltraSPARC T2 and T2 Plus systems, the maximum is 8 cores with 8 threads
each, for a total of 64 vCPUs per chip. Systems with the T2 Plus chip can have
multiple chips per server: The T5140 and T5240 servers have 2 T2 Plus chips for
a total of 16 cores and 128 vCPUs, while the T5440 has 4 T2 Plus chips with 32
cores and 256 vCPUs.
Virtual CPUs should be assigned to domains on core boundaries. This strat-
egy prevents “false cache sharing,” which can reduce performance when multiple
domains share a CMT core and compete for the same L1 cache. To avoid this
problem, vCPU quantities equivalent to entire cores to each domain should be
allocated. For example, you should allocate vCPUs in units of 8 vCPUs on T2 and
T2 Plus servers. Of course, this tactic may be overkill for some workloads, and
administrators need not excessively concern themselves when defining domains
to accommodate the light CPU requirements needed to consolidate small, old, or
low utilization servers. Figure 3.3 is a simplified diagram of the threads, cores,
and caches in a SPARC CMT chip.
The number of CPUs in a domain can be dynamically and nondisruptively
changed while the domain is running. Oracle Solaris commands such as vmstat
and mpstat can be used within the domain to monitor its CPU utilization, just as
on a dedicated server. The ldm list command can be used in the control domain
to display each domain's CPU utilization. A change in the quantity of vCPUs in a
running domain takes effect immediately. The number of CPUs can be managed
automatically with the Logical Domains Dynamic Resource Manager, which is
discussed later in this chapter.
 
 
 
 
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