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Old 2019-06-15, 04:59   #1
Bulldozer
 
Jun 2019

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Question Why does prime95 use K10 FFT on a Bulldozer CPU?

As I said in the title, prime95 uses K10 FFT on a Bulldozer CPU. My CPU is A10-6800k(codenamed Richland) which is based on Bulldozer GEN2(Piledriver). It supports AVX, FMA3, XOP and even FMA4(probably better than FMA3), but it seems the application is not using them ignoring them. AMD K10 doesn't support ALL of these instructions listed, so I guess the program is not using it. I hope for further optimizations for Bulldozer.

Last fiddled with by Bulldozer on 2019-06-15 at 05:29
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Old 2019-06-15, 06:34   #2
Prime95
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Quote:
Originally Posted by Bulldozer View Post
As I said in the title, prime95 uses K10 FFT on a Bulldozer CPU. My CPU is A10-6800k(codenamed Richland) which is based on Bulldozer GEN2(Piledriver). It supports AVX, FMA3, XOP and even FMA4(probably better than FMA3), but it seems the application is not using them ignoring them. AMD K10 doesn't support ALL of these instructions listed, so I guess the program is not using it. I hope for further optimizations for Bulldozer.
The original Bulldozer implementation was so bad that programs were better off not using AVX and FMA. Yes, it was that bad.

Rumor has it that later Bulldozers improved somewhat. Some may even be faster using AVX.

Try adding "CpuArchitecture=5" to local.txt. Prime95 will treat your CPU like an Intel Core 3/5/7. See if your CPU is faster using AVX/FMA.
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Old 2019-06-15, 07:30   #3
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Thank for your reply! Besides, does prime95 support XOP/FMA4?

Last fiddled with by Bulldozer on 2019-06-15 at 07:32
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Old 2019-06-15, 14:07   #4
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Nope.
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Old 2019-08-21, 05:16   #5
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I've tested it on my Excavator CPU, A10-9700. It get much faster with FMA enabled. On 3.9GHZ, it is a bit faster then my 3.4GHZ I5-8250u. It seems like the rumor is true.
Hope to make FMA instructions as a default of Excavators.

Last fiddled with by Bulldozer on 2019-08-21 at 05:18
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