we are facing somehow strange behavior with Turboboost:
We are running an application on top of RHEL 7.7, that, beside others, calculates some linear combination. It is single thread application, bind to one processor core. Normally, this core is running at max turboboost frequency. (4.1GHz).
When we tried to migrate the LC calculation to AVX (the original 256 AVX instructions, not AVX2.0 or AVX512), we observed strange oscillation of core frequency, when about 88% of the time the core was running at 4.1GHz, but for 12% of the time the frequency dropped to 3.5GHz. The frequency hopping happens sporadically, and then remains at the same level for minutes, even hours (e.g. 2 hours at 4.1GHz, then 15 minutes at 3.5, ...).
The core frequency was measured with turbostat tool. Booth the temperature and power usage seems ok (<=74 Celsius / <120W) for the whole duration of the test, and are basically the same as with original, non-AVX workload.
Since the frequency drops are both unpredictable, and affect the rest of the application, which is now overaly slower, we would really like to understand what is going on and how to mitigate such behavior.
Thank you for contacting Intel Customer Support.
Can you please share with us more additional information about your system configuration? i.e. server board model, amount of memory, and manufacturer.
Additionally, please let us know if you noticed this issue only with RHEL 7.7.
Intel Customer Support Technician
Motherboard is Dell 329-BDKC, with 96GB of memory (12x8GB 2666 DDR4), in Dell R640.
And yes, all our production machines run the same version of RHEL, so we even didn't try something else.
Interestingly, we didn't observe this on development machine with Gold 6154 processor (Dell R940).