Boinc issue

[fosf0r]

Per the spec for the big.LITTLE CPU cluster architecture, only one "side" (big, or little) is allowed to be active at one time.

The "sides" share one memory space and can easily toggle tasks between the two sides, which is what it was designed for.
But the tasks either toggle all the way to "big", or all the way to "LITTLE", and cannot straddle the two.

It allows a developer to swap tasks between lower power consumption and higher power consumption, on purpose, when needed, to achieve more granular power management, than say forcefully frequency-cycling the individual cores within the 2 clusters.

So anyway, always having either: "4 out of 6", or "2 out of 6", CPUs active at any given time, is 100% correct.
You will never, ever see even 5 out of 6.

The "big" cores are Cortex A57 and "LITTLE" cores are Cortex A53, and that probably has something to do with it.

The "S" in "SMP" ("symmetric" multiprocessing) is the reason for this - this SoC is asymmetrical.
This big.LITTLE architecture is more like forced "AMP", and is wholly incompatible with the concept of "SMP".

Sorry, that's incorrect as a general statement. Only the very first big.LITTLE implementations couldn't run all big and little cores at the same time, that was Exynos 5410 and Tegra 3 with its low-power companion core, some chips from Apple and maybe something else. Nowadays the kernel decides what cores to power at what frequency and it may be even all big and all cores if the need arises.

RK3399 should very well be able to run all 6 cores at the same time.

I suspect that the original poster did not have any heatsink attached which caused throttling and shutdown of some cores.

Thanks for clarifying this.
I must have been reading outdated information.
(I don't remember where I got all that)