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rakesh

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11-28-2012
08:30 PM

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computation time for 1024 pt complex FFT using MKL_FFT

HI,

i have been evaluating FFT of MKL.while computing , FFT takes approx. 1 milisecond which is too high.

i'm using following part of code to measure time of fft computation.

s_initial=dsecnd();

fft computation;

s_elapsed=dsecnd()-s_initial;

printf("time in milisec=%f",s_elapsed*1000);

can any one tell me how to reduce time compuation time gurther.

i'm using intel processor.

Thanks,

6 Replies

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This size FFT *computation* should take on the order of 10 microseconds.
If timing includes the planning/commit time, then it will take longer, of course.
Yet, 1 millisecond is very large.
Dima

Dmitry_B_Intel

Employee

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11-30-2012
04:48 AM

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Timing such short intervals is impractical in Fortran on Windows. It's possible but ugly to interface with QueryPerformanceCounter (less ugly, call a C function invoking __rdtsc()).
Under linux, I would suggest system_clock with integer(8) arguments.

TimP

Black Belt

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11-30-2012
06:32 AM

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actually dsecnd() is wrapper for __rdtsc, therefore customer can use this timing routine even for this short intervals too.

Gennady_F_Intel

Moderator

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11-30-2012
06:31 PM

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rakesh

Beginner

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11-30-2012
09:55 PM

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Thanks a lot .

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Hi,
earlier i was calculating for only one iteration once i increased the no of iteration from one to 100 the calculation time reduced in the order of microsecond. now i get computation time around 0.02 ms(approx) for 1024 pt complex FFT.

rakesh

Beginner

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12-03-2012
04:42 AM

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Bernard

Black Belt

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01-12-2013
10:41 AM

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rakesh wrote:

earlier i was calculating for only one iteration once i increased the no of iteration from one to 100 the calculation time reduced in the order of microsecond. now i get computation time around 0.02 ms(approx) for 1024 pt complex FFT.

While measuring time needed to complete 4096 sine values FFT routine I got ~121245 nanoseconds i.e 121 microseconds.Compiler intrinsic __rdtsc() was used and averaged.I hope that time interval needed to complete the FFT calculation was not influenced by the rdtsc latency which is thousand times slower.

For more complete information about compiler optimizations, see our Optimization Notice.