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Trying Intel OpenCL On Linux For Video Encoding

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  • Trying Intel OpenCL On Linux For Video Encoding

    Phoronix: Trying Intel OpenCL On Linux For Video Encoding

    Following my testing and reporting last weekend about Intel Beignet starting to provide very usable open-source OpenCL support on Linux, one of the most common requests was to next see if this Intel OpenCL Linux supprot benefits x264 encoding at all...

    http://www.phoronix.com/vr.php?view=MTc3ODc

  • #2
    After a quick google the Haswell GPU seems (theoretically) to have 2,5-3x more GFLOPS than the CPU. In this task the GPU was slower, but it would be nice if they could work together.

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    • #3
      Well, at least it runs and doesn't simply fall back to non-OpenCL, looks like.

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      • #4
        Not too surprising given the amount of effort that goes into x264's SIMD/avx support (which is very fast for Intel). Also opencl, currently, really needs its kernels targeted to particular hardware in order to get best performance. My guess is that x264 was aiming for compatibility rather than speed.

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        • #5
          Originally posted by opensource View Post
          After a quick google the Haswell GPU seems (theoretically) to have 2,5-3x more GFLOPS than the CPU. In this task the GPU was slower, but it would be nice if they could work together.
          AFAIK opencl in x264 is used only for part of the ecnoding process, so CPU and GPU actually work together here.

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          • #6
            You has to use a 2 pass encoding for best benefits. And set a high lookahead value (max 250 and is the default settings its 40 )

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            • #7
              OpenCL option only works with 8bit profiles, donth work with 10bits(Hi10p) (--bit-depth=10)

              greetings

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              • #8
                the opencl support in x264 does not actually run video encoding via opencl. all it does is offload the lookahead functionality, which at best gains like 2-3% overall encoding speed increase. i think it was basically added as "well we could try this just for the hell of it and see how miniscule the gains are...yep just as miniscule as we thought..."

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                • #9
                  For Haswell machine, currently we need a patch to be applied to make beignet fully functional.
                  You can get the details at:
                  https://01.org/zh/beignet/downloads/...ch-hsw-support
                  You can also get the above link in the wiki page.

                  There is also another issue which hasn't been documented:

                  For linux kernel 3.16, there is a cmdparser issue which will bring problems on both IVB and HSW.
                  The work around is to disable it as below:
                  echo 0 > /sys/module/i915/parameters/enable_cmd_parser
                  We will update the document shortly.

                  Thanks for the testing and reporting.

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                  • #10
                    I for one only get this segfault: http://pastebin.com/raw.php?i=GTirsFBb
                    Possibly since the 3.17 rcs. I haven't really checked again with 3.16.

                    Also libgbe seems to be compiled without debug symbols no matter what.

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