4405ch03 Virtualization.fm
Draft Document for Review September 2, 2008 5:05 pm
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IBM Power 570 Technical Overview and Introduction
Figure 3-1 Logical partitioning concepts
Dedicated mode
In dedicated mode, physical processors are assigned as a whole to partitions. The
simultaneous multithreading feature in the POWER6 processor core allows the core to
execute instructions from two independent software threads simultaneously. To support this
feature we use the concept of
logical processors
. The operating system (AIX or Linux) sees
one physical processor as two logical processors if the simultaneous multithreading feature is
on. It can be turned off and on dynamically while the operating system is executing (for AIX,
use the
smtctl
command). If simultaneous multithreading is off, then each physical processor
is presented as one logical processor and thus only one thread at a time is executed on the
physical processor.
Shared dedicated mode
On POWER6 servers, you can configure dedicated partitions to become processor donors for
idle processors they own. Allowing for the donation of spare CPU cycles from dedicated
processor partitions to a Shared Processor Pool. The dedicated partition maintains absolute
priority for dedicated CPU cycles. Enabling this feature may help to increase system
utilization, without compromising the computing power for critical workloads in a dedicated
processor.
Shared mode
In shared mode, logical partitions use virtual processors to access fractions of physical
processors. Shared partitions can define any number of virtual processors (maximum number
is 10 times the number of processing units assigned to the partition). From the POWER
Hypervisor point of view, virtual processors represent dispatching objects. The POWER
Hypervisor dispatches virtual processors to physical processors according to partition’s
processing units entitlement. One Processing Unit represents one physical processor’s
processing capacity. At the end of the POWER Hypervisor’s dispatch cycle (10 ms), all
partitions should receive total CPU time equal to their processing units entitlement. The
logical processors are defined on top of virtual processors. So, even with a virtual processor,
the concept of logical processor exists and the number of logical processor depends whether
the simultaneous multithreading is turned on or off.
