Hello everyone,

Available as of now on github is a new script I thought the rest of you would find useful. It's called smt-manager, or Simultaneous Multithreading Manager. All it does is let you turn on and off all logical cores at once:

github.com/damentz/smt-manager

For example, executing the script in this repository with --offline returns this output:



Running smt-manager.pl with --online restores everything back to normal:



Running smt-manager.pl by itself will just show you your current topology (which you get output of before and after).

The benefits of disabling logical cores? Very workload specific - if you cannot risk two executions occurring on the same core where one task is considered high priority and the other low or idle, the only way to guarantee cpu time gets distributed accurately is to disable logical cores. With logical cores enabled, cpu time will be distributed evenly between both executions regardless of priority.

Throughput will drop but performance will be more predictable. That is the tradeoff you get when disabling hyperthreading, or the technology AMD uses on their Bulldozer cores and newer.
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This is fantastic. I was previously using the nosmt boot parameter. Having a quick and easy way to "hotplug" the logical cores is preferable. I always assumed that doing so would have adverse side-effects compared to disabling them at boot.
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Is smt-manager up to date enough to work properly on AMD Ryzen, ThreadRipper, EPYC CPU's?
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Yes, it'll work. The SMT cores are also called thread siblings in the kernel, and smt-manager uses that to find and identify the logical core.

On an Intel quad core, you'll get something like this:

Ryzen systems might number their thread siblings differently. Intel labels all physical cores before logical (at least for now).
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On my Ryzen 3 system which is 4 cores I get this:


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Most Ryzen 3 processors don't have SMT: en.m.wikipedia.org/wiki/Ryzen

What does the output of inxi tell you?
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