Towards Efficient Wavefront Parallel Encoding of HEVC: Parallelism Analysis and Improvement Keji Chen, Yizhou Duan, Jun Sun, Zongming Guo 2014 IEEE 16th International Workshop on Multimedia Signal Processing (MMSP)

Outline  Introduction  Parallelism Evaluation Of HEVC Encoding  Proposed Method  Experimental Results  Conclusion 2

Introduction  Great increment of computational complexity introduced by the enhanced coding tools makes HEVC difficult for application.  By developing the parallelism among the encoding tasks, the encoding speed can be significantly improved. 3

Introduction  Compared with slices, WPP can achieve similar parallelism with less loss of coding efficiency.  In [11], Chi et al. proposed an Overlapped WaveFront (OWF) method based on WPP. [11] C. C. Chi, M. Alvarez-Mesa, B. Juurlink, G. Clare, F. Henry, S. Pateux, and T. Schierl, “Parallel Scalability and Efficiency of HEVC Parallelization Approaches,” IEEE Trans. Circuits Syst. Video Technol., vol. 22, pp , Dec

Parallelism Evaluation Of HEVC Encoding(1/3)  T i,j,k : Self Encoding Complexity (SEC) of C i,j,k.  SEC can be evaluated by the encoding time.  Determined by the frame content and RDO design and does not change with parallel methods.  ETF(C i,j,k ) : Required Encoding Complexity (REC) to encode C i,j,k using parallel method F.  REC can be regarded as the earliest ending time.  Affected by the data dependence. 5

Parallelism Evaluation Of HEVC Encoding(2/3) i, j, k : order of frame, line, and CTU. DEP F,inter (C i,j,k ) : CTBs that C i,j,k depends on when using parallel encoding method F. 6

Parallelism Evaluation Of HEVC Encoding(3/3)  From (1) and (2), it is clear that the parallelism of different parallel methods can be evaluated:  This criterion is easy to be proved with (1) and (2) and can be simply explained as the less dependence in HEVC encoding, the higher parallelism can be obtained. 7

Data Dependence Analysis of WPP and OWF Method(1/4)  For intra : 8

Data Dependence Analysis of WPP and OWF Method(2/4)  SEC of each CTB is of significant difference.  Variance of the SEC in inter frame is much greater than that of intra frame.  Under the given encoding algorithm, the unbalanced SEC is determined, thus being the bottleneck of intra-frame parallelism. 9

Data Dependence Analysis of WPP and OWF Method(3/4) 10

Data Dependence Analysis of WPP and OWF Method(4/4)  For inter : i, j, k : order of frame, line, and CTU. W : the width of a frame measured by CTB. L_OWF : a positive integer parameter denoting the safe range. In [11], L_OWF is roughly set to the upper round of 1/4 height of a frame measured by CTB. 11

Proposed Method(1/5)  To best exploit the inter-frame parallelism, we designed a new Inter-frame Wavefront (IFW) coding order. 12

Proposed Method(2/5)  For intra :  For inter : 13

Proposed Method(3/5)  Frame Thread (FT) is assigned to each frame to develop inter-frame parallelism.  Wavefront Thread (WT) is assigned to each frame to develop intra-frame parallelism. 14

Proposed Method(4/5)  If L_IFW is no greater than L_OWF, for any i, j, k we can deduce that: 15

Proposed Method(5/5)  It is also confirmed that the unbalanced SEC is a bottleneck for intra-frame parallelism.  Parallelism of IFW significantly increases as B-frames increase, because the effectively reduced inter-frame dependence makes much greater contribution in improving the overall parallelism. 16

Experimental Results  The common test conditions and software reference configurations [12].  The hardware platform is a shared memory system with two AMD Opteron 6272 processors. 17

Experimental Results(2/) 18

Experimental Results  Frame Thread = 9, Wavefront Thread = 8 19

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Conclusion  A parallelism evaluation criterion and an IFW method are proposed to improve the encoding speed of HEVC.  IFW method achieves significant speedup on various sequences, being a promising technology for large-scale HEVC video applications. 22