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Re: Strange perfromance results on G4 and G5 FFT

Subject: Re: Strange perfromance results on G4 and G5 FFT
From: Ian Ollmann <email@hidden>
Date: Wed, 13 Oct 2004 12:41:10 -0700
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On Oct 13, 2004, at 12:23 PM, Bevis, Chris wrote:

I am benchmarking accelerated FFT's ( 512x512 real, using fft2d_zrip in vDSP ) and am getting some wierd results : 1Ghz single G4 ( codegen and instruction scheduling set for G4 ) 13msec 1Ghz single G4 ( codegen and instruction scheduling set for G5 ) 12msec 2Ghz dual G5 ( codegen and instruction scheduling set for G5 ) 17msec When I launch two instances simultaneously on the dual G5, THey both finish in 24msec. Does anyone have any idea why a 2x faster dual G5 performs slower than a G4 for a single instance and about the same for two instances ? This one has me puzzled.


Hi Chris,

A 512x512 array is about 1MB. The G5 has a 512 kB L2 cache and no L3. A G4 might have up to 2 MB L3 cache. It seems possible you are falling out of cache on the G5 and not on the G4. Does the trend go for all sizes or just that size?

Ian

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	>Strange perfromance results on G4 and G5 FFT (From: "Bevis, Chris" <email@hidden>)

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