Introduction to Virtualization - Oracle Solaris 10 System Virtualization Essentials - page 13

provides predictable performance and avoids performance overhead. A work-
load assigned to a set of CPUs will always have access to its assigned CPUs,
and will never be required to wait until another VE completes its time slice.
A resource manager can reduce wasted capacity by reassigning idle CPUs.
The amount of waste will be determined by two factors: (1) reconfiguration
latency—the time it takes to shift a CPU from one partition to another and
(2) resource granularity—the unconsumed partition of, at most, a single
CPU. This model of CPU control is shown in Figure 1.4.

Figure 1.4 CPU Partitioning

A software scheduler such as FSS may allow the administrator to enforce
minimum response times either directly or via VE prioritization. Early im-
plementations included software schedulers for VM/XA on mainframes and
BSD UNIX on VAX 11/780s in the 1980s. This approach is often the best
general-purpose solution. It is very flexible, in that the minimum amount of
processing power assigned to each VE can be changed while the VE is run-
ning. Moreover, a software scheduler does not force workloads to wait while
unused CPU cycles are wasted.
System administrators can use an FSS to enforce the assignment of
a particular minimum portion of compute capacity to a specific workload.
A quantity of shares—a unitless value—is assigned to each workload, as
depicted in Figure 1.5. The scheduler sums the shares assigned to all of the
current workloads, and divides each workload's share quantity by the sum to
obtain the intended minimum portion.

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