Question about CPU power draw

opelly · August 20, 2021

I have noticed that my CPU varies quite a bit in how much power it draws based on the workload, even with workloads that put 100 percent load on every thread.

I recently wrote a program in Java that generates images from the Mandelbrot set. It is multithreaded and puts a 100 percent load on my 8 core CPU while running. However when I check the power draw, its only around 85 watts. When I run something like a Blender render the CPU draws around 110 watts, and if I run a prime 95 stress test, it can draw up to 150 watts. All of these workloads report 100 percent usage across all cores, and the top boost clock is the same in each.

Can anybody explain this difference, I am just curious!

Thank you!

Levent · August 20, 2021

I am most definitely not an expert but I believe this is due to each load having different length instruction pipelines and the instruction (for example, AVX is notorious for it) used in the process. Longer it works without a wait state = more heat it generates. I too would like to learn more if anyone knows more or if I am wrong.

mariushm · August 20, 2021

It has also to do with how much cache is used, how much the cpu has to hit the ram, and what instruction sets are used.

Your program is so small it may fit inside the level 2 cache of your cpu and basically have minimal ram access and use only the basic instruction sets.

Blender will use AVX and other instruction sets, and pi is very optimized.

jaslion · August 20, 2021

19 minutes ago, Levent said:

I am most definitely not an expert but I believe this is due to each load having different length instruction pipelines and the instruction (for example, AVX is notorious for it) used in the process. Longer it works without a wait state = more heat it generates. I too would like to learn more if anyone knows more or if I am wrong.

This pretty much is it. You have a set amount of processing power but how it is used determines the power draw.

SaleGamin · August 20, 2021

and it's Java. It is not a pure compiled programming language and your memory is managed by your JVM. So with that kind of architecture, it's diffucult to take advantage of the whole computing power of your CPU.