On Thu, Jan 19, 2012 at 10:04 AM, ziyang <[email protected]> wrote: > > I dont recommend using the extra blocks, that would probably cause more >> overhead. Looking at gr_quadrature_demod_cf::work, it looks like you can >> vectorize the operation of the conjugate multiply, then the atan, then >> the gain scaler. So, that would be one for loop that operates on 4 >> samples at a time, and calls 3 volk functions. >> >> > Hi, Josh. I implemented a quadrature_demod_cf block (please find it in the > attachment). Since the Volk atan2 function is currently only for SSE as > Nick said, and there is no conjugate-multiply function for FC32 inputs, I > use Gnuradio's built-in conjugate and fast_atan_2f functions, plus two volk > multiply functions. The for loop is timed by high_res_timer. Besides, the > work function of gr_quadrature_demod_cf is timed for comparison purpose > (also attached). Each of these two blocks is connected to a file_source > which provides modulated data. > > I tested two blocks individually, firstly on a PC with Intel processor, > then on E100. On PC, it always take volk-based block less time to > demodulate a same-size-buffer of data (i.e. for 4096 input items, it takes > the original quadrature_demod_cf block 0.185 ms but takes volk-based block > only 0.163 ms to demodulate). > > However, the results are different on E100: sometimes the original block > runs faster, sometimes the volk-based block does. I ran the tests for > several times, although the recorded time changes by some tens > (occasionally a few handreds) of nanoseconds, but neither block is always > faster than the other. > > Now I'm confused by the results, since I expected the volk-ified > demodulator to be faster. Could you give me some help on this issue? Thanks. > > Optimizing an algorithm is a hard and sometimes counterintuitive process. You might benchmark the following:
- Gnuradio's atan2 WITHOUT any Volk multiplications (just comment out the volk mults in your block) - The Volk multiplications WITHOUT Gnuradio's atan2 (just comment out the atan2 in your block) This will let you determine where the bottleneck is. In addition, try running over a MUCH larger dataset. The clock resolution at <1ms is not very good and the scheduler will have a correspondingly larger effect at smaller timescales. I think you'll find the atan2 part takes vastly longer than the multiplications do, and that will be where you have to look for performance improvements. --n > > Best Regards, > > Terry > > _______________________________________________ > Discuss-gnuradio mailing list > [email protected] > https://lists.gnu.org/mailman/listinfo/discuss-gnuradio > >
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