[Discuss-gnuradio] OpenCL FFT synchronization question

[address@hidden]

As I am investigating GPU and OpenCL for acceerating correlation computations, 
I thought
I'd give OpenCL-Enabled FFT block in GNU Radio Companion (still version 
3.7.13.4) a try to 
see if I can see any performance change.
When I run signal source -> throttle -> OpenCL FFT -> time sink, all goes well, 
the spectrum
*looks* similar to the GNU Radio FFT (CPU) output. So far so good.
If I now run the correlation graph found at 
http://jmfriedt.org/opencl/opencl_xcorr_grc.png
the correlation surprisingly is non existent as seen at 
http://jmfriedt.org/opencl/nosubtract.png
(top chart is CPU computation of the correlation as 
iFFT(FFT(signal).FFT(delayed signal)^* with
a fine correlation peak on the right, and bottom chart is the same through the 
OpenCL FFT).
Running the same chart but fed with a periodic signal (square wave) does show a 
set of correlation
peaks spaced by the square wave period as shown at 
http://jmfriedt.org/opencl/nosubstract_square.png
hinting at a synchronization issue with the two OpenCL blocks being fed random 
sequences sampled at
different time.
Trying to display the difference between the two correlations (output of the 
complex conjucate 
multiplication) actually seems to solve the issue as seen at 
http://jmfriedt.org/opencl/withsubtract.png 
where the correlation peak suddenly appears after a fraction of a second of 
running the flowchart, 
so the GNU Radio scheduler seems to be affected by this new subtraction chain. 
Yet, notice that 
the subtraction between the two correlation processing chains (top chart) is 
not 0, unlike running 
the same flowchart with subtraction on the periodic square wave 
(http://jmfriedt.org/opencl/withsubstract_square.png) which does show a null 
error.

I don't have an explanation, not even sure there is an obvious one, but at 
least wanted to share 
these observations with the community.

JM