[Rtk-users] Difference in rtkfdk (cpu) speed/threading

[Ben Champion]

Hello,

First of all, many thanks to the RTK community for this useful toolkit!

While experimenting with different versions of the code (I'm a 
relatively new user), I've encountered large differences in rtkfdk (CPU) 
reconstruction speed between code versions (a newer version being 
substantially slower than an older version).

To test I ran rtkfdk with "--hardware 'cpu' --verbose" (as well as the 
required -g, -p, -r and -o flags, but no other flags).

Using git-bisect, I narrowed it down to a particular commit. The parent 
commit runs quite quickly, but the child commit shows nearly 4x 
reconstruction time, and less-uniform CPU utilization (it looks like a 
series of spikes).

(See below)

Looking at the diffs, it seems that in addition to adding the HannY 
functionality (which should be disabled by default?), there were some 
changes in this commit related to threading (in 
code/rtkFFTRampImageFilter.{h,txx}). However, perhaps threading is 
misleading and the substantial difference consists in changing the FFT 
Ramp Kernel.

I'm currently reading the source to try to understand those changes, but 
I thought I would post in case someone is able to point me in the right 
direction. Although these differences are unexpected to me, I doubt that 
they are unexpected to more experienced users...!

Apologies if I've left out any critical information (or if I've provided 
too much!).

Many thanks in advance,
Ben

****** Parent Commit ******
commit 9df6108ae0293f86b455a2dcd4b35801e4815718
Author: Julien Jomier <julien.jomier at kitware.com>
Date:   Fri Nov 30 09:30:59 2012 +0100

     ENH: Minimum CMake version is 2.8.3

***Partial output***

Reconstructing and writing... It took 44.3992 s
FDKConeBeamReconstructionFilter timing:
   Prefilter operations: 2.67915 s
   Ramp filter: 26.3847 s
   Backprojection: 13.0447 s

***Screenshot of CPU usage attached: 
9df6108ae0293f86b455a2dcd4b35801e4815718.png ***

****** Child Commit ******
commit e223a2ed2200bbd7d86966d4eb27319ed589ee00
Author: Simon Rit <simon.rit at creatis.insa-lyon.fr>
Date:   Wed Dec 5 16:22:47 2012 +0100

     First version of Hann windowing in the second direction 
(perpendicular to the ramp)

***Partial output***
Reconstructing and writing... It took 126.911 s
FDKConeBeamReconstructionFilter timing:
   Prefilter operations: 2.47678 s
   Ramp filter: 108.254 s
   Backprojection: 13.2973 s

***Screenshot of CPU usage attached: 
e223a2ed2200bbd7d86966d4eb27319ed589ee00.png***


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