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</o:shapelayout></xml><![endif]--></head><body lang=FR link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>That’s is why on MJPEG there is no latency ?<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Frame N doesn’t depend on N-1 nor N+1 so each thread is independent ?<o:p></o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'>De :</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif"'> libav-user-bounces@ffmpeg.org [mailto:libav-user-bounces@ffmpeg.org] <b>De la part de</b> Robin Stevens<br><b>Envoyé :</b> mardi 10 février 2015 10:33<br><b>À :</b> This list is about using libavcodec, libavformat, libavutil, libavdevice and libavfilter.<br><b>Objet :</b> Re: [Libav-user] Real time camera IP RTSP H264<o:p></o:p></span></p></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal><br><br>On Tuesday, February 10, 2015, Maziar Mehrabi <<a href="mailto:maziar.mehrabi@gmail.com">maziar.mehrabi@gmail.com</a>> wrote:<o:p></o:p></p><div><div><div><div><div><p class=MsoNormal style='margin-bottom:12.0pt'>Hi,<o:p></o:p></p></div><p class=MsoNormal>I wonder why multithreading causes latency.<o:p></o:p></p></div><p class=MsoNormal>My understanding is that having concurrent threads should improve performance.<o:p></o:p></p></div><p class=MsoNormal style='margin-bottom:12.0pt'>Is there any explanation for this?<o:p></o:p></p></div><p class=MsoNormal>Thanks,<o:p></o:p></p><div><div><p class=MsoNormal><o:p> </o:p></p></div></div></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Without any knowledge of the code in question: concurrency can increase overall throughput (for particular kinds of task) but increased latency is a common payoff.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>Imagine each thread as a separate decoder with carefully controlled access to some common data/state. To fully decode the Nth frame probably requires at least a partial decode (possibly a complete decode) of the N-1 frame. Therefore although the decoding for the Nth frame may happen on a separate processor core, it's likely not truly independent of the other cores.<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>So if you have 4 threads, T4 is waiting on T3, which is waiting on T2 and on T1. Even though T1 may have finished frame 1, it probably can't fully decode frame 5 until 4 is available from T4...<o:p></o:p></p></div><div><p class=MsoNormal><o:p> </o:p></p></div><div><p class=MsoNormal>But that's just my guess!<o:p></o:p></p></div></div>
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