IBM p5 550, Technical Overview And Introduction

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IBM System p5 550 and 550Q Technical Overview and Introduction
POWER Hypervisor calculates a partition’s processing
entitlement
based on its desired
processing units and logical processor settings, sharing mode, and also based on other
active partitions’ requirements. The actual entitlement is never smaller than the desired
processing units value and can exceed the desired processing units value if the LPAR is an
uncapped partition.
A partition can be defined with a processor capacity as small as 0.10 processing units. This
represents one-tenth of a physical processor. Each physical processor can be shared by up to
10 shared processor partitions and partition’s entitlement can be incremented fractionally by
as little as one-hundredth of the processor. The shared processor partitions are dispatched
and time-sliced on the physical processors under control of the POWER Hypervisor. The
shared processor partitions are created and managed by the HMC or Integrated Virtualization
Management (included with Virtual I/O Server software version 1.2 or later). There is only one
pool of shared processors at the time of the writing of this IBM Redbook and all shared
partitions are dispatched by Hypervisor within this pool. Dedicated partitions and
Micro-partitions can coexist on the same processor-based server as long as
enough processors are available.
The p5-550Q supports up to a 8-core configuration, therefore up to eight dedicated partitions,
or up to 80 micro-partitions can be created. It is important to point out that the maximums
stated are supported by the hardware. Practical limits depend on the application workload
demands.
2.12.2 Logical, virtual, and physical processor mapping
The meaning of the term
physical processor
in this section is a
processor core
. For example,
in a 2-core server with a DCM (dual-core module), there are two physical processors; in a
4-core configuration with a QCM (quad-core module), there are four physical processors.
In dedicated mode, physical processors are assigned as a whole to partitions. The
simultaneous multithreading feature in the processor core allows the core to
execute instructions from two independent software threads simultaneously. To support this
feature, the concept of
logical processors
was introduced. The operating system (AIX 5L or
Linux) sees one physical processor as two logical processors if the simultaneous
multithreading feature is on. It can be turned off while the operating system is executing (for
AIX 5L, use the
smtctl command). If simultaneous multithreading is off, then each physical
processor is presented as one logical processor and thus only one thread is executed on the
physical processor at the time.
In a micro-partitioned environment with shared mode partitions an additional concept of
virtual processors
was introduced. 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, the virtual processors represent
dispatching objects (for example, the POWER Hypervisor dispatches virtual processors to
physical processors according to the partition’s processing units entitlement). At the end of
the POWER Hypervisor’s dispatch cycle, all partitions should receive total CPU time equal to
their processing units entitlement. Virtual processors are either running (dispatched) on a
physical processor or standby (waiting). The operating system is able to dispatch its software
threads to these virtual processors and is completely screened from the actual number of
physical processors. The logical processors are defined on top of the virtual processors as
though they are physical processors. So, even with a virtual processor, the concept of a
logical processor exists and the number of logical processor depends on whether the
simultaneous multithreading is turned on or off.