IBM Power System POWER7 Series, Руководство по оптимизации и настройке

Страница: 96 / 224

80
POWER7 and Optimization and Tuning Guide
AIX provides options to allow SMT customization. The smtctl option allows the SMT feature
to be enabled, disabled, or capped (SMT2 versus SMT4 mode on POWER7). The
partition-wide tuning option,
smtctl , changes the SMT mode of all processor cores in the
partition. It is built on the AIX DR (dynamic reconfiguration) framework to allow hardware
threads (logical processors) to be added and removed in a running partition. Because of this
option’s global nature, it is normally set by system administrators. Most AIX systems
(commercial) use the default SMT settings enabled (that is, SMT2 mode on POWER5 and
POWER6, and SMT4 mode on POWER7).
When SMT is enabled (SMT2 or SMT4 mode), the AIX kernel takes advantage of the platform
feature to dynamically change SMT modes. These mode switches are done based on system
load (the number of running or waiting to run software threads) to choose the optimal SMT
mode for the CPUs in the partition. The mode switching policies optimize overall workload
throughput, but do not attempt to optimize individual software threads.
Hybrid thread features
AIX provides some basic features that allow more control in SMT mode. With these features,
specific software threads can be bound to hardware threads assigned to ST mode CPUs.
This configuration allows for an asymmetric SMT configuration where some CPUs are in high
SMT mode, while others have SMT mode disabled. This configuration allows critical software
threads within a workload to receive an ST performance boost, while it allows the remaining
threads to benefit from SMT mode. Typical reasons to take advantage of this hybrid
mode are:
򐂰 Asymmetric workload, where the performance of one thread serializes an entire workload.
For example, one master thread dispatches work to many subordinate threads.
򐂰
Software threads that are critical to a system administrator.
The ability to create hybrid SMT configurations is limited under current AIX releases and does
require administrator or privileged configuration changes. CPUs that provide ST mode
hardware threads must be placed into exclusive processor resource sets (XRSETs). XRSETs
contain logical CPUs that are segregated from the general kernel dispatching pool. Software
threads must be explicitly bound to CPUs in an XRSET. The only way to create an XRSET is
by running the
mkrset command. All of the hardware threads for logical CPUs must be
contained in the XRSET created RSET. To accomplish this task, run the
following commands:
lsrset -av Displays the RSET topology. The system CPU topology is
broken down into a hierarchy that has the form
sys/node.XX.YYYYY . The largest XX value is the CPU (core)
level. This command provides logical processor groups by
core.
mkrset -c 4-7 sysxrset/set1 Creates an XRSET sysxrset/set1 containing logical CPUs
4 - 7.
An XRSET alone can be used to ensure that only specific work uses a CPU set. There is also
the ability to restrict work execution to primary threads in an XRSET. This ability is known as
an STRSET. STRSETs allow software threads to use ST execution mode independently of
the load on the other CPUs in the system. Work can be placed onto STRSETs by running the
following commands:
execrset -S This command allows external programs to start and be bound
to an exclusive RSET.
ra_attach(R_STRSET)
This API allows a thread to be bound to an STRSET.
For more information about this topic, see 4.4, “Related publications” on page 94.