September 30, 2013
Simultaneous Multi-Threading (SMT) allows sharing of
processor facilities to run two or more threads of execution at the same time
on a single processing core. On POWER7, each processing core supports up to 4
threads of execution. SMT generally increases the performance capacity of a
system and can improve the responsiveness of multithreaded applications. Running
multiple instruction streams at the same time does not, however, improve the
performance of any given task.
The Processor Multitasking system value (QPRCMLTTSK) is
used to control how SMT is used on the partition. The system value has 3
options:
• Off (0) – Each processor supports the processing of only
one task at any moment.
• On (1) – Processors can concurrently process multiple
tasks.
• System controlled (2) – Processors can concurrently
process multiple tasks. The system-controlled state also enables the operating
system to occasionally limit a processor to processing one task at most.
In the past, changes to this system value required an IPL
to take effect. But now with enhanced SMT controls in IBM i 7.1 with TR5, you
can switch between SMT modes immediately without an IPL. We have also
introduced a new set of APIs to allow for additional controls to limit the
number of hardware threads per processor. The Change Processor Multitasking
Information (QWCCHGPR) API can be used to change the maximum number of
secondary hardware threads per processor and the Retrieve Processor
Multitasking Information (QWCRTVPR) API can be used to view the current setting
on the partition.
In most cases, the system value should be set to 2 (system
controlled) and there should be no need to change it. But if a situation arises
in which IBM support determines your workload could benefit from limiting the
number of processing threads, the system value along with the additional API
controls can be used to make an immediate change.
For more information, see the following links in the IBM i
Information Center and IBM developerWorks:
QPRCMLTTSK
system value
Change
Processor Multitasking Information (QWCCHGPR) API
Retrieve
Processor Multitasking Information (QWCRTVPR) API
“Enhanced Administrative SMT controls for POWER7” section in the IBM i Functional Enhancements
article.
I’d like to thank
Angela Newton for writing this blog article. Angela is the team leader of the
IBM i work management team and is the IBM i expert for the development of APIs
within the operating system. Thanks, Angela!
Posted September 30, 2013| Permalink
