Software Tuning, Performance Optimization & Platform Monitoring
Discussion around monitoring and software tuning methodologies, Performance Monitoring Unit (PMU) of Intel microprocessors, and platform monitoring

Xeon E3-1285 v6 CPU throttles uncore for no reason

MartinMoe
Beginner
311 Views

Hello,

I need to know why the Xeon E3-1285 v6 CPU throttles its uncore (cache, ring) after several minutes of runtime although I set it to fixed min/max of 42/42 in MSR_UNCORE_RATIO_LIMIT (0x620). The cores are set to P-State 44 (Turbo, nominal is 41) and stay there all the time (cores do not throttle). MSR_RING_PERF_LIMIT_REASONS (0x6B1) does not indicate any reason. Power and Thermal limits are definitely not reached.

Writing MSR_UNCORE_RATIO_LIMIT (0x620) two times with 41/42 and 42/42 again resets the condition, but the perf drop will happen again after some minutes. I cannot write to the MSR all the time as it increases jitter (application is very time critical).

I tried measuring the uncore frequency using  MSR_UNC_PERF_GLOBAL_CTRL (0xE01), MSR_UNC_PERF_FIXED_CTRL (0x394) and MSR_UNC_PERF_FIXED_CTR (0x395), but the counter does not give me any meaningful value for UCLK.

The OS is proprietary and does not perform any power management, so it is not relevant.

Xeon-E3-1285-v6-PerfDrop-Uncore.png

 

I already posted this topic in "Processors". They couldn't help me there.

0 Kudos
0 Replies
Reply