AW: hyperthreading and RT latency

["Jonathan Schwender" <schwenderjonathan@gmail.com>]

Hi Alison,

> Is the advice still current?   Should we RT-users all still turn hyperthreading off?

I ran some tests as a part of my master's thesis in the beginning of this year with the 5.10-rt kernel on an Intel Broadwell-EP 2-socket server.
If you are interested, I can dig up the graphs I made, but the jist regarding wake-up latencies measured by _cyclictest_ (24 hours each) is:
1. Task-isolation (placing the RT-task on a dedicated core) + Cache allocation + disabled Hyperthreading yields the best latencies. Something around 4-5us worst-case latencies were possibly with some optimizations.
2. Placing a load (rteval) together with cyclictest increases the latencies, but worst-case latencies of I think 16us are still okay for many applications
3. Isolating a task on a dedicated CPU and placing a load (rteval) on the neighbor CPU sharing the same core yields strictly worse latencies compared to 2). I think it was around 50us worst-case.
4. Isolating a task on a dedicated core (hyperthreading disabled), but enabling hyperthreading for the non-critical cores seems to have a rather small negative impact, as long as CAT is used to reserve cache for the isolated core. I'd have to look up the details though.

I don't think the situation has improved on more modern hardware, since AFAIK the SMT hardware has no knowledge of your tasks priority.

>Thanks,
>Alison Chaiken

Best Regards,

Jonathan Schwender