Author Topic: using bigger CPUs with some cores disabled on Blackbird?  (Read 460 times)

pocock

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using bigger CPUs with some cores disabled on Blackbird?
« on: July 20, 2020, 06:50:51 am »

I started a separate thread about disabling cores in the CPUs in a Talos II

It occurred to me that disabling cores could be a way to reduce CPU power consumption on Blackbird, for example, if you have an 18 core CPU (which is not supported) and you use the operating system to disable 4 cores, leaving just 14 cores, does the power consumption fall within the supported range for the motherboard?

This is probably not a good way to design a new system but if somebody has a spare CPU and they want to temporarily use it in a Blackbird, maybe this would be a useful hack that will reduce the risk of unexpected behavior.
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MPC7500

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Re: using bigger CPUs with some cores disabled on Blackbird?
« Reply #1 on: July 21, 2020, 10:45:07 am »
Haven't tried, but would be interesting if a change in SMT (ppc64_cpu --smt=n) level influences power consumption also.

ClassicHasClass

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Re: using bigger CPUs with some cores disabled on Blackbird?
« Reply #2 on: July 26, 2020, 11:23:25 pm »
A better way might be to manually guard out some of the cores. This is supported by the firmware, even, but I haven't played with doing so intentionally.

pocock

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Re: using bigger CPUs with some cores disabled on Blackbird?
« Reply #3 on: September 07, 2020, 12:40:51 pm »


I tried using the ppc64_cpu command to disable all but 2 out of 16 cores.

It reports the cores are disabled but I didn't see any reduction in the power consumption when the OS is idle.

I didn't test making the cores work but I assume that the power consumption and fan noise would remain much closer to idle when it can only use limited cores.

It would be really interesting to know about any runtime options to reduce power consumption even further when the system is idle.

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ejfluhr

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Re: using bigger CPUs with some cores disabled on Blackbird?
« Reply #4 on: September 09, 2020, 05:50:08 pm »
Can you tell what Stop state the cores are put into before and after your command?    Technically the cores can be power-gated hence you can make larger-core-count modules behave similarly to the small-core-count variants.   The frequencies won't be identical, as the boost tables may adjust frequencies commensurate with extra regulator headroom but it won't line up identically to lower-core-count-tuned modules.  Higher frequency could affect your power readings, but taking cores offline should reduce power far faster than raising voltage/frequency, especially with 16->2 cores.

I'm not sure how well Linux supports all of the power management features of POWER9, though.   Are all cores active when at idle, or has it already power-gated many such that formally disabling them doesn't actually change their state?

May be worth posting your power/current readings if you have them.  VDD current is what matters, if you can separate that out.

Regards, Eric

surf

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Re: using bigger CPUs with some cores disabled on Blackbird?
« Reply #5 on: November 21, 2020, 08:22:22 am »
Has anyone had success with this?  Since there is no 'sleep' with these computers, perhaps this could get close?