This blog reviews a few of the key features of POWER7 that were initially supported in the IBM i 6.1.1 release. While this blog is a bit dated, some of the key concepts of SMT, energy management, and processor frequency remain applicable to current current generations of POWER hardware and IBM i releases. Links have been updated to reference current information on these topics.
The announcement of the IBM POWER7 systems is really exciting! This announcement is primarily a hardware announcement though. Although the announcement is about IBM’s new POWER7 servers, there’s IBM i support for these new systems in the 6.1.1 release.
SMT Threads per Core
POWER7 supports Simultaneous Multithreading (SMT) with four hardware threads, which provides more capacity per core. Power hardware has had SMT support for many years. Initially, it was two hardware threads of execution on a processor core. With POWER7, the number of hardware threads has been increased to four. POWER8 and POWER9 both support SMT8.
In addition, with POWER7 we’ve also added the capability to support “dynamic switching.” Dynamic switching allows the hardware to allocate resources to optimize the processing capability. So while the core may be configured to use four threads, if the work to be done only takes one thread, the other three threads can free up resources, giving that single thread greater performance. POWER8 and POWER9 have extended the intelligent threads technology which has been enhanced to adapt more quickly and with greater efficiency to changes in the workload.
You control the SMT behavior with Processor multitasking (QPRCMLTTSK) system value. The default for this system value is “system controlled,” which means the operating system determines the optimal setting; for POWER7 systems, this is dynamic switching with four hardware threads. For POWER8 and POWER9, this is dynamic switching with eight hardware threads. You can explicitly turn off SMT support and always run the cores in a single-threaded mode by setting QPRCMLTTSK to *OFF.
In general, the default setting will be the best for most applications; in fact, IBM doesn’t recommend the use of single-threaded mode on POWER7 or later since the system automatically switches each core to single-threaded mode if there’s just one task executing on the core. The white paper “Simultaneous Multi-Threading on POWER7 Processors” has a lot more information for those of you interested in the details.
Energy Management Features
Energy management features were introduced with the POWER6 systems.
While the energy management features in POWER7 are functionally similar as POWER6, both IBM i and the underlying hardware have been enhanced to allow for increased energy efficiency and more intelligent power savings. Release 6.1.1 of IBM i is now more tightly coupled to EnergyScale functions built-in to PowerVM, the system hardware and the POWER processor itself. For more information on POWER7 and energy management, see the POWER7 EneryScale Whitepaper. IBM has also published the POWER8 EnergyScale Whitepaper and the POWER9 EnergyScale Whitepaper.
What About That Processor Frequency?
Historically, the one thing about the ever-increasing performance of processors has been that the processor frequency always improves. However, POWER7’s design improves performance per core as well as a massive improvement in capacity per chip with a processor core frequency roughly 30 percent less than that of POWER6. We’ve all gotten so used to frequency being the performance touchstone in processors that all the other intriguing things that make a processor fast tend to get overlooked. And some of them—like processor cache, memory bandwidth, and Non-Uniform Memory Access (NUMA)—matter. POWER7 has a number of advantages over POWER6, including a much faster L2 cache, a large on-chip L3 cache and twice the memory bandwidth. And POWER7, like POWER5, uses “out of order” execution.
Check out the article “Of Gigahertz and CPWs – P7” for a discussion of what else is going on in these modern processors and what it means to the execution of your programs.
POWER7 has eight cores on a single chip. That’s a tremendous amount of processing capacity on a single chip. See “What’s This Multi-Core Computing Really?” for some great information on multi-core computing.
“Of POWER7 and NUMA” discusses POWER7’s memory access design in much more detail.
While the above references and papers were written for POWER7, these concepts exist in POWER8 and POWER9 and have been enhanced in each hardware generation.
I’d like to thank Mark Funk on the IBM i Systems Performance team, Chris Francois and Darcy Koch from the IBM i LIC Development team, and Michael Hollinger on the IBM Power Firmware Development team for their assistance in providing content and reviewing this blog article.
This blog post was edited on January 31, 2020 to reflect applicability to current hardware and software features.
This blog post was originally published on IBMSystemsMag.com and is reproduced here by permission of IBM Systems Media.