Hi Bill,
> Before completely discarding MT FFTW3 I would like to try something. Can
> you give a brief rough summary of all the FFT sizes used by jt9?
The file wisdom1.bat in .../wsjtx/lib shows the following FFT plans
being used:
rif672000 cif77175 cib77175 rif16384 rif884736 cib2048 rif8192 rif512
rib512 cib512
Several days ago the big FFT in downsam9 was changed from length 884736
to 604800. I changed the two big ones to "out of place" transforms. So
I think the new lineup is
rof672000 cif77175 cib77175 rif16384 rof604800 cib2048 rif8192 rif512
rib512 cib512
> If there are many smaller FFT being run then I think their plans should
> be limited to 1 thread and only unleash 2 or more threads for the big FFTS.
I think the only ones for which MT will help are rof672000 and
rof604800. For these, three threads (on a 4-core machine) helps
significantly:
(JTSDK-QT) C:\JTSDK\src\wsjtx\lib)timefft 1 4 or672000
Problem Threads Plan Time Gflops RMS iters
--------------------------------------------------------
or672000 1 0.005 0.004878 13.34 0.0000002 100
or672000 2 1.427 0.004469 14.55 0.0000002 100
or672000 3 1.828 0.003406 19.10 0.0000002 100
or672000 4 2.037 0.003459 18.81 0.0000002 100
(JTSDK-QT) C:\JTSDK\src\wsjtx\lib)timefft 1 4 or604800
Problem Threads Plan Time Gflops RMS iters
--------------------------------------------------------
or604800 1 0.858 0.005361 10.83 0.0000002 94
or604800 2 1.901 0.003405 17.06 0.0000002 100
or604800 3 2.509 0.002658 21.85 0.0000002 100
or604800 4 2.544 0.002618 22.19 0.0000002 100
However, these long FFTs make up only about 10% of the total running
time. Speeding them up by a factor of 2 will shave about 5% off the
running time, at best. And probably not that much, when we're already
running the two decoders in parallel.
It's not that MT FFTs won't help at all; they just won't help much.
-- Joe
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