This time, a less serious article, meant to be treated as a fun fact 😉
Every AIX administrator knows the available multithreading settings, which are:
- SMT=off
- SMT=2
- SMT=4
- SMT=8
But how does this look in the case of Linux on a Power10 server?
It turns out that SMT settings can be configured not only to 1, 2, 4, and 8 threads but also to 3, 5, 6, and 7 threads (!!)
Many people might ask, “Why do this??”
I have no idea. 😊
I don’t think anyone has a practical need for such an unconventional configuration, but I’m sharing this information as an interesting fact. I was quite surprised to learn that such a setting is even possible.
Maybe it’s also doable in AIX? Below is an example of my attempt:
aix_test:[//]# smtctl -t 3
smtctl: The value given for the -t flag is invalid.
aix_test:[//]# smtctl -t 5
smtctl: The value given for the -t flag is invalid.
aix_test:[//]# smtctl -t 6
smtctl: The value given for the -t flag is invalid.
aix_test:[//]# smtctl -t 7
smtctl: The value given for the -t flag is invalid.
aix_test:[//]# smtctl -t 4
smtctl: SMT is now enabled. It will persist across reboots if
you run the bosboot command before the next reboot.
aix_test:[//]# smtctl -t 8
smtctl: SMT is now enabled. It will persist across reboots if
you run the bosboot command before the next reboot.I only confirmed what was already known.
So, how do you change the setting in the case of RHEL9 on a Power10 server?
To change the setting, I used the ppc64_cpu command, which can be installed along with the powerpc-utils-core package (# dnf install powerpc-utils-core).
# dnf install powerpc-utils-coreTo change the SMT setting, you can simply run the following command:
[root@rhel9_test~]# ppc64_cpu --smt=XFirst, let’s compare the standard settings for SMT=8 (default)
Default settings: SMT=8
[root@rhel9_test ~]# grep 'processor' /proc/cpuinfo
processor : 0
processor : 1
processor : 2
processor : 3
processor : 4
processor : 5
processor : 6
processor : 7
[root@rhel9_test ~]#
[root@rhel9_test ~]# lscpu | grep -e "Model name" -e Thread -e list
On-line CPU(s) list: 0-7
Model name: POWER10 (architected), altivec supported
Thread(s) per core: 8Below is an example of changing SMT to an “unusual” value and checking the current configuration.
SMT=7
[root@rhel9_test~]# ppc64_cpu --smt=7
[root@rhel9_test~]# grep 'processor' /proc/cpuinfo
processor : 0
processor : 1
processor : 2
processor : 3
processor : 4
processor : 5
processor : 6
[root@rhel9_test ~]# lscpu | grep -e "Model name" -e Thread -e list
On-line CPU(s) list: 0-6
Off-line CPU(s) list: 7
Model name: POWER10 (architected), altivec supported
Thread(s) per core: 7For testing purposes, I put a bit of load on the system.
In the screenshot below, you can see the utilization of all threads in the NMON program. The current SMT setting (SMT=7) is also visible.
SMT=6
[root@rhel9_test~]# ppc64_cpu --smt=6
[root@rhel9_test~]# grep 'processor' /proc/cpuinfo
processor : 0
processor : 1
processor : 2
processor : 3
processor : 4
processor : 5
[root@rhel9_test~]# lscpu | grep -e "Model name" -e Thread -e list
On-line CPU(s) list: 0-5
Off-line CPU(s) list: 6,7
Model name: POWER10 (architected), altivec supported
Thread(s) per core: 6SMT=5
[root@rhel9_test~]# ppc64_cpu --smt=3
[root@rhel9_test~]# grep 'processor' /proc/cpuinfo
processor : 0
processor : 1
processor : 2
processor : 3
processor : 4
[root@rhel9_test~]# lscpu | grep -e "Model name" -e Thread -e list
On-line CPU(s) list: 0-4
Off-line CPU(s) list: 5-7
Model name: POWER10 (architected), altivec supported
Thread(s) per core: 5
SMT=3
[root@rhel9_test~]# ppc64_cpu --smt=3
[root@rhel9_test~]# grep 'processor' /proc/cpuinfo
processor : 0
processor : 1
processor : 2
[root@rhel9_test~]# lscpu | grep -e "Model name" -e Thread -e list
On-line CPU(s) list: 0-2
Off-line CPU(s) list: 3-7
Model name: POWER10 (architected), altivec supported
Thread(s) per core: 3Summary
This publication is not particularly useful, and I don’t expect it to help you in your work with Power servers, but I hope at least it was something a little interesting 😊
Thank you for reading!