IBM Power7 Optimization And Tuning Manual page 95

You can use the AIX Enhanced Affinity services to attach SRADs to threads and memory
ranges so that the application preferentially identifies the logical CPUs or physical memory to
use to run the application. AIX continues to support RSET attachments to identify resources
for an application.
RSET versus SRADs
When you compare RSET with SRADIDs:
1. SRADIDs can be attached to threads, shared memory segments, memory map regions,
and process memory subranges. SRADIDs may not be attached at the process level
(R_PROCESS). SRADIDs may not be attached to files (R_FILDES).
2. SRADID attachments are considered advisory. There are no mandatory SRADID
attachments. AIX might ignore advisory SRADID attachments.
3. Process and thread RSET attachments continue to be mandatory. The process and thread
resource set hierarchy continues to be enforced. Memory RSET attachments (shared
memory, file, and process subrange) continue to be advisory. This situation is unchanged
from previous affinity support.
API support
SRADIDs can be attached to threads and memory by using the following functions:
ra_attach() (new)
ra_fork()
ra_exec()
ra_mmap() and ra_mmapv()
ra_shmget() and ra_shmgetv()
SRADIDs can be detached from thread and memory by using the sra_detach()
function (new).
Hybrid thread and core
AIX provides facilities to customize simultaneous multi-threading (SMT) characteristics of
CPUs running within a partition. The features require some partition-wide CPU configuration
options, so their use is limited to specific workloads.
Background
SMT is a feature that is introduced in POWER5 and capitalized on by AIX. It allows a single
physical processor core to simultaneously dispatch instructions from more than one hardware
thread context. An overview of SMT is provided in the AIX SMT Overview.
SMT does include some performance tradeoffs:
SMT can provide a significant throughput and capacity improvement on POWER
processors. When you are in SMT mode, there is a trade-off between overall CPU
throughput and performance of each hardware thread. SMT allows multiple instruction
streams to be run simultaneously, but the concurrency can cause some resource conflict
between the instruction streams. This conflict can result in a decrease in performance for
an individual thread, but an increase in overall throughput.
Some workloads do not run well with the SMT feature. This situation is not typical for
commercial workloads, but has been observed with scientific (floating point
intensive) workloads.
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Simultaneous Multithreading, available at:
http://publib.boulder.ibm.com/infocenter/aix/v7r1/topic/com.ibm.aix.genprogc/doc/genprogc/smt.htm
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Chapter 4. AIX