EE398 Project Presentation

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EE398 Project Presentation

• Performance-Complexity Tradeoff H.264 Motion
  Search
• Ionut Hristodorescu
• ionuth_at_stanford.edu

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Outline

• H.264 motion search algorithm
• Mapping of the motion compensation algorithm on a
  memory hierarchy subsystem
• Cache organization and its impact on the motion
  compensation speed
• Making the internal H.264 data structures more
  cache friendly

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H.264 Motion Search Algorithm

• Block Matching Algorithm
• Computes the SADs for all the targets in a given
  area (exhaustive search)
• Computationally intensive
• Its complexity is equal or greater than the rest
  of the encoding steps
• Takes most of the encoding time

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Mapping on a memory hierarchy subsystem

• The luma/chroma is represented internally in the
  motion compensation algorithm as a line-by-line
  matrix
• So, each line of a macroblock will be separated
  by (size(pel)width) bytes
• This means that accessing pels that are sitting
  on the same row will generate 1 cache miss/pel !!!

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Mapping on a memory hierarchy subsystem

• To overcome the above,
• we could arrange the information so that
  consecutive block lines will sit in consecutive
  memory locations

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Mapping on a memory hierarchy subsystem

• So, a natural representation of the chroma/luma
  matrixes would be as a sequential macroblock line
  by macroblock line
• This way, the needed information is loaded into
  the cache quicker

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Mapping on a memory hierarchy subsystem

• The advantages are immediate
• Each macroblock line is 16 pels
• So, we could fit 2 16-pels consecutive lines in a
  cache line
• The macroblock is accessed now in a natural,
  sequential order

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Mapping on a memory hierarchy subsystem
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Mapping on a memory hierarchy subsystem

• The biggest problem that arises now is with
  non-macroblock line boundary access
• Each macroblock line sits at a 16-pel boundary in
  our representation so far
• For macroblock line aligned access, this is great
• How about non-macroblock line aligned access ?

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Mapping on a memory hierarchy subsystem

• We have problems imagine we want to access 16
  pixels, but starting from position 4 in a
  macroblock line
• In the original representation, this is no
  problem, since the original picture lines are
  sequential in memory
• In our case, we will end up in the next
  consecutive macroblock line

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Mapping on a memory hierarchy subsystem

• Solutions 1 pretend we dont know about this
  problem and let the encoder access the wrong pels
• Solution 2 check each time if we are crossing a
  macroblock line boundary and proceed accordingly
• Solution 3 keep two blocked versions of the
  picture the original picture blocked and the
  shifted-by-32pels blocked

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Mapping on a memory hierarchy subsystem

• We prefer solution 3 (even if it is more
  expensive in terms of memory) because this way
  the pels are accessed quicker
• If pel_pos32 lt 16, we are going to pick up the
  pels from the blocked version of the original
  picture
• Else, we are going to pick up the pels from the
  blocked versions of the original picture
  shifted-by-32

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Mapping on a memory hierarchy subsystem

• 32pels will fit exactly in one cache line (or,
  for better processors, even 64 pels)
• So, each time we access two macroblock lines, we
  will have no cache miss since the two macroblock
  lines will fit into a cache line

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Results

• MET time decreased by approx. 8 compared to the
  non-blocked exhaustive search
• Cache misses/pixel decreased to approx. 15 from
  600-800 !!!
• Rate-distortion ratio was preserved

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Further optimizations

• Assembly language coding of the SAD computation
  and in particular usage of the PSADW MMX
  instruction
• Multi-threading of the motion compensation
  algorithm
• By using Performance API (PAPI), we could measure
  the runtime behavior of the cache and introduce
  the cache misses into the motion cost function,
  much like in 1

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Further optimizations

• Intelligent prefetching of data
• Extend the blocking algorithm to the entire
  motion estimation engine