AIX smtctl The smtctl command controls the enabling and disabling of processor simultaneous multithreading mode.


This command is provided for privileged users and applications to control utilization of processors with simultaneous multithreading support. The simultaneous multithreading mode allows processors to have thread level parallelism at the instruction level. This mode can be enabled or disabled for all processors either immediately or on subsequent boots of the system. This command controls the simultaneous multithreading options.

Each individual Simultaneous Multi-threading (SMT) thread of a physical processor core is treated as an independent logical processor by AIX®. The AIX operating system limits the combination of physical processor cores assigned and SMT modes in order to maintain symmetry across all of the physical processor cores assigned to AIX. Due to this limitation, the number of logical processor is less than or equal to 1024 for AIX 7.1 and 256 for AIX 6.1.

The POWER8® processors are capable of SMT-8 which means up to 128 cores can be used in SMT-8 mode which yields 1024 logical processors. A lower SMT mode must be used for AIX users to be able to use more than 128 POWER8 cores.
Number of thread
When booting a P8 Logical Partition (LPAR), the default number of SMT thread is 4. To increase the default number of SMT threads dynamically, enter:
smtctl -m on
smtctl -t 8Copy
The change to SMT-8 is effective immediately and reboot is not required. If you want the setting to persist after rebooting, then you must rebuild the boot image with the bosboot command. The default SMT-4 is intended for better performance for an existing applications that are not designed or compiled for more than 4 threads.
Number of cores
If you have allocated more than 128 cores to an LPAR, by default it uses 128 cores. This is to ensure that AIX limit of maximum 1024 logical processors is not exceeded if SMT-8 is enabled (128 cores * SMT8 = 1024 total). If you want LPAR to use more than 128 cores, then you need to run a sequence of following AIX commands to establish a limit to the number of SMT threads that are available per core.
smtctl -m limit -t 4
bosboot -a
shutdown -FrCopy
Upon rebooting, AIX negotiates with the firmware to allow up to 256 cores because the operating system's limit of 1024 processors will not be exceeded with the specified limit of 4 SMT threads. You can exceed 256 cores if you run the smtctl command as stated above, but with a limit of 2 instead of 4. The following command suspends SMT capability allowing more cores.
smtctl -m suspend
bosboot -a
shutdown -Fr

My Conclusions

I should note that these were very limited tests and there are many more tests left for me to run in the benchmarking suites that I am testing. Additionally, they don’t test all of the functions of the server and are not a true mixed OLTP workload, which is what most run. However, they do provide some initial data that shows that POWER8 appears to scale as expected, both due to improvements in the memory performance as well as CPU performance. Also the jump from SMT4 to SMT8 provides around a 9 percent boost, which is on a par with what’s predicted in the published rPerf. Further, more detailed tests are planned using several test suites for memory, cpu, I/O and network performance. Although preliminary and limited, they provide a window into the performance potential of the new POWER8 scale-out servers.

Overall, the POWER8 experience has been very positive so far. Firmware and HMC updates went smoothly and the server appears to be performing as expected. This provides a level of confidence that you can move from POWER7 to POWER8 while reducing the VPs or cores in LPARs using rPerf comparisons as an approximate scaling factor.

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