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Old 2019-09-19, 23:10   #10
nomead
 
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"Sam Laur"
Dec 2018
Turku, Finland

317 Posts
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Quote:
Originally Posted by lavalamp View Post
Unfortunate then.

Since each CCD has 32 MB L3 cache, this would be good for an FFT of 4096K, which is the assigned FFT for exponents up to 78M. However the first-time test front is now approaching 90M.
Yes I agree, for a single CCD, the performance really sinks at that point, or actually even a bit before that. The efficiency actually starts dropping after about 65M, or 3456K FFT size. But on the dual CCD models (3900X, the future 3960X) the rolloff point is double that, still quite capable of running at its optimum rate at the current wavefront.

Quote:
Originally Posted by lavalamp View Post
It's surprisingly difficult to find information about what size numbers correspond to a given FFT size, but assuming vaguely linear scaling, an 8192K FFT should just about fill the L3 cache and be good for exponents up to 150M in size.
Try the CPU credit calculator at mersenne.ca - it gives some sort of info for the expected FFT size for the L-L exponent to be tested. But yes, it's vaguely linear.
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