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Old 2021-03-23, 18:49   #1
bhelmes's Avatar
Mar 2016

14416 Posts
Default huge of the 3.level cache for optimal testing Mp

A peaceful day,

I have an amd cpu with 32MB 3. level cache and make double checking.

Is the 3. level cache big enough for first prime checking resp.
how much space does the implemented fft needs ?

Thanks for a short answer.

ps: I can feel it, we are aproaching to the next mersenne prime.
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Old 2021-03-23, 18:59   #2
Viliam Furik
"Viliam Furík"
Jul 2018
Martin, Slovakia

22·113 Posts

The current FTC wavefront takes up about 44 MiB (mebibytes - basically megabytes, but counted in base 2, so it's 220 bytes, instead of 106) of L3 cache.
You can calculate the usage with this rule:

Take the FFT size, e.g. 5.5M, and multiply it by 8 B (bytes). 5.5M FFT uses 5.5*8 = 44 MiB of cache.

That means that with 32 MiB of L3 cache, you are in the RAM territory and are bottlenecked by its speed.

If you want to go faster, either change out the CPU for one of the Ryzen 9 CPUs or change out the memory for a faster one (3600 MHz for Ryzen 3000; 4000 MHz is said to be the best for Ryzen 5000).
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