Re: hyperthreading and RT latency

On Fri, 6 Aug 2021, Daniel Wagner wrote:

Hi Alison,

On Thu, Aug 05, 2021 at 01:16:17PM -0700, Alison Chaiken wrote:

quoted

The advice in the RT wiki

    https://rt.wiki.kernel.org/index.php/HOWTO:_Build_an_RT-application#Hyper_threading

Note that the above wiki is defunct, the current one to read is:
https://wiki.linuxfoundation.org/realtime/start

quoted

about configuring the kernel and building RT applications was written
in 2014, when we were on the 3.x series.   That makes one wonder how
relevant some of it is for the 5.x series, especially since processors
in common use have changed some since then.  Some of the advice,
notably about power management, obviously is timeless.

In particular, Daniel Wagner added:

    https://rt.wiki.kernel.org/index.php/HOWTO:_Build_an_RT-application#Hyper_threading

    "Hyper threading and also out of order execution of CPUs introduces
    'random' latencies. As mentioned in power management, it is
     recommended to disable these feature (if possible) or carefully
     benchmark the performance."

Is the advice still current?

I don't think the situation has changed. Though, I don't run these test
on modern hardware on regular basis. The last time I played with it on a
bit more modern hardware it was still measurable by cyclictest with
hackbench as workload.

quoted

Should we RT-users all still turn hyperthreading off?

Obviously, it depends on your use case. If your application can tolerate
the added noise by SMT, you don't have to disable it.

quoted

Given the security vulnerabilities associated
with hyperthreading, there are clearly some use cases where doing so
is indicated anyway.

Again it depends on your use case.

Daniel

The safe answer is always to measure it.

John