20210121, 16:50  #1 
Jan 2021
Osijek, Croatia
2 Posts 
Prime95 FFT log meaning
Hi,
I am trying to figure out meaning behind some logged parameters behind Prime95 logs while doing LL test. Concrete log line is: Code:
Resuming primality test of M63351487 using FMA3 FFT length 3360K, Pass1=448, Pass2=7680, clm=4, 6 threads FMA3 is a CPU instruction set emloyed FFT is Fast Fourier Transform algorithm 1. What is meaning behind Pass1, Pass2 and clm? 2. What is meaning of FFT length? My understanding of FFT is basic. 
20210121, 18:49  #2 
6809 > 6502
"""""""""""""""""""
Aug 2003
101×103 Posts
256F_{16} Posts 
The FFT length is the size of the FFT that is used. If an FFT is done that is too close to the size of the number that needs to be tested, then the 'noise' at the end of the calculation can cause problems. But, if one uses an FFT that is too large, then the data take too many cycles to complete. For any given exponent, software, and hardware there will be an ideal FFT size.

20210121, 23:03  #3  
P90 years forever!
Aug 2002
Yeehaw, FL
7492_{10} Posts 
Quote:
Welcome to GIMPS! It is not expected that these values would be meaningful to you. These values let me identify exactly which assembly code is being executed. The FFT is split into two "passes". This is not a requirement of FFTs in general, but for best performance one usually wants to load as much FFT data into the L2 or L3 cache and do as much work as possible, then write it back to memory. Internally, a number of different pass sizes are available which means there are several different ways to implement an FFT size. For example, a 1M FFT could be 512x2K or 1Kx1K or 2Kx512, etc. In 25 years, I don't think anyone has asked what clm is. clm is a prime95internalsonly abbreviation for cachelinemultiplier. The assembly code can process 1, 2, or 4 cache lines at a time, which sometimes results in a few percent speed difference on various CPU architectures. 

20210122, 07:09  #4 
Romulan Interpreter
Jun 2011
Thailand
2^{2}·17·139 Posts 
Thanks for explanation George. I was wondering sometimes too, but never dared to ask... Now you gave enough material to kriesel to write another of his interminable tutorials

20210122, 14:22  #5  
Just call me Henry
"David"
Sep 2007
Cambridge (GMT/BST)
2×5×587 Posts 
Quote:


20210122, 15:42  #6 
P90 years forever!
Aug 2002
Yeehaw, FL
2^{2}×1,873 Posts 
Carry propagation is handled the same way for all FFT implementations.

20210125, 14:03  #7  
Jan 2021
Osijek, Croatia
2_{8} Posts 
Quote:
Quote:
I have checked the source code a bit, but with all optimizations and assembly code it is a bit overwhelming to be honest. I'll deep dive into forum and literature for time being, and ask further questions on FFT details in appropriate subforum. Cheers! 

20210127, 13:42  #8  
Just call me Henry
"David"
Sep 2007
Cambridge (GMT/BST)
2×5×587 Posts 
Quote:


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