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Messages - ejfluhr

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General CPU Discussion / Re: IBM POWER9+ ?
« on: September 24, 2020, 02:06:37 am »
Even "simple" die shrinks are very, very costly, and POWER sales volumes cannot justify such an expense.  IBM succeeds due to its system sales, and OpenPOWER support is an offshoot of whatever the system roadmap drives for processor development.


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Blackbird / Re: using bigger CPUs with some cores disabled on Blackbird?
« 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

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Do the cores remain powered on but idle when doing that, or does the OS+firmware actually power-gate the cores and caches?   Ideally the cores and caches are power gated for maximum savings, but I'm not sure if the open-source setups fully implement all the stop states.

Note that the maximum boost frequency may also go up, offsetting the power reduction slightly until the technology-limited frequency is reached.  Although given the plots in this link, it may be that the procs naturally run at Fmax/Vmax most of the time as their active power tends to run toward idle:
   https://forums.raptorcs.com/index.php/topic,99.0.html

Some other good posts on power readings that seem relevant:
   https://forums.raptorcs.com/index.php/topic,199.0.html
   https://forums.raptorcs.com/index.php/topic,135.0.html
   
(I know a lot about the processor, not so much about Linux and I don't have a Raptor system to play with so can't test anything.)

Regards, Eric

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