Optimization

The principle of optimization:
* Cases: What the application will typically do, and if we are more concerned with average or worst case performance before optimization.
* Order: Algorithm & Design -> Local optimization -> Assembly
* Context: The true exec time for one instruction is a time measured in a certain context, and that time is meaningful only in that context. Also it is more meaningful to optimize the segment of codes instead of only one instruction.

Algorithm & Design:
* Two kinds of computing:
-- data processing (data streaming algorithm): data is processed sequentially and only once in general. The relationship of data is not the concern of algorithms. The cache seems overkill here. The digital signal processing is a typical application. How to reduce the computing cases might be an efficient way to do optimization.
-- data transaction processing (data block algorithm): The relationship of data in one block is the focus of algorithms. The data cache is very important for them. Sorting and search are among them.

Instruction Level Optimization:
* Avoid branch hazard:
-- Unroll loops: the result might be diff for W, R, W with R. In general 8~16 times unrolling depth is good.
-- Combine loops
-- Fill-in branch delay in Assembly
* Avoid data hazard:
-- Eliminate the data dependance
-- Prefetch data and instruction: use mem/cache throughput instead of latency.
* Understand each Assembly inst details and diff. Use the best.

Memory Usage Optimization:
* Optimize data structs
-- Split reference data structures, like lists, tree, etc. into small ones and access them simultaneously.
-- Compact data structs as densely as possible to reduce access increment step and increase the utilization ratio of cache lines.
-- Separate data structs to reduce access increment step.
* Use mem/cache pipeline: The basic unit between cache/TLB and memory is one cache line. So make read step increment be the length of one cache line.
* Processing mem in Int data type instead of char, esp. for R. It seems not matter for W.
* Addr alignment for src/dst
* Combine W and R together for large data block which exceeds the size of caches. For small ones, overlapping access of cache might be harmful.
* Compact data within one mem page as densely as possible, due to TLB and page table.
* Put data into diff mem banks to avoid DRAM refreshment and access multiple banks. One page size as increment is good.

Cache Usage Optimization:
* Data and instruct size not exceed cache size: For cache, the max attainable size of cached data is CacheSize + (CacheSize / Way)
* Addr alignment and no line split data
* Put data on diff cache banks in order to access multiple cache banks.
* Increment step of mem not be multiple of the size of one cache set bank, otherwise conflict miss happens.

Write Buffer Usage Optimization:
* Access same data types, eps. R after W
* Organize the codes together to take advantage of WB since the units in it might be flushed randomly.

Multiprocessing and Multithread

Multiprocessing: multiple CPU, like CMP, SMP, etc.
Multithreading: One CPU but probably with more threading HW support, like multiple Register files and PC's.

Superscalar: HW supported dynamic instruction issue and branch predication
VLIW: Software (compiler) supported multiple instruction issue and BP

ccNUMA SMP: symmetrical multiprocessing. OS could be running on any of multiple CPUs with the help of "resource locker". cc means cache coherence and each CPU has its own memory banks (NUMA).

Inter Prediction MV Derivation

Totally three modes are used to derive inter prediction motion vector for MB partitions/Sub MB partitions.
* P skip mode
* B skip/direct mode (spatial/temporal)
* Others

The mv's could be directly calculated in the first two modes while they are derived with mvp and mvd in the third mode. There are two issues which are very important in the course of the derivation, i.e. how to locate the colPartition/colBlk in the colPic and how to locate the neighboring partition/blk in the current pic.

* ColPartition/colBlk
Note: The basic processing unit for B_skip, B_16X16_direct and B_8X8_direct are 4X4 blk. For P skip it is 16X16.
Three parameters are needed for temporal direct mode: colPic, colPart/colBlk, and refIdxL0
- colPic: different combinations of fld, frm and afrm. Table 8-6
- colPart/colBlk: The basic unit is 4X4 block in current MB and colMB. If the partition in colMB is larger than this unit, the motion inforamtion would be copied on all the composited blocks. When 8X8_direct_flag is enabled, the basic units in one 8X8 sub partition of current MB share the same mv and refidx. Otherwise, 16 blocks have their own mv and refidx. To save memory, mv and refidx are needed for only several units in colMB for the derivation. The basic unit mapping could be defined with Table 8-8. LUT could be used to implement this kind of mapping.
- refIdxL0: Keep in mind the picture structure of colRef might be different. The current MB is field MB in afrm is always a special case.

* Neighboring partition/blk
The basic unit is still 4X4 blk as the above and it is possible that multiple units share the same mv information due to copy operation. It is not necessary to store all the blk information for the derivation. For example, only 4 units are needed for B and C. It is special for the processing of A and D, however. And all the blk information is needed in the current MB.

Neighboring Location Derivation

P6.4.9 is used to define the address of neighboring MB/Partition/Sub MB partition, given one location coordinates. The derivation would be much more complicated when MBAFF is enabled and Table 6-4 would be used. Under this circumstance, the following points should be kept in mind.

- The basic unit in one slice would be a MB pair instead of a MB. Therefore the indicing order for MB is different.
- If MBAFF is disabled, only PAFF is applicable, which means slices/MBs structure is the same for the whole picture. When MBAFF is enabled the neighboring MB may be either field or frame MB.
- Only top MbAddr could be derived by using p6.4.7.
- Field MB and frame MB have different starting points and steps vertically. Keep in mind that two fields are interleaved in the FRAME grid. The starting point for top field would be the most left point of first line while bottom field be the most left point of the second line. The unit step for fields would be two instead of one for frames.
- When coordinates are negative, only one value is possible: -1.
- Note this clause just defines the derivation of neighboring MB index since MaxW and MaxH are for MB boundary. For some cases of calculating neighboring sub MB partitions, it might be possible that neighboring partition C is located on the top MB. Here the key parameter is the difference of luma/chrma location: xD, yD for MB, MB partition, subMB partition, luma8X8Blk, luma4X4Blk, chroma4X4Blk.
- The predPartWidth of xD is special for these cases: P skip, B skip, B direct 16X16 and B direct 8X8, it would be 16. Otherwise, it would be SubMbPartWidth/MbPartWidth.
- Generally the MB could be divided into 16 blocks each of which stores the motion information. The motion information might be the same for one partition or sub MB. Given one MB/Partition/Sub MB Partition, the neighboring ones could be located with the first block of this partition with the help of x, y and predPartWidth, according to Table 6-3/6-4.

Reference Picture Management

- The processing flow is reference list initialization (setup and sort) -> resorting -> decoding one picture -> reference picture marking
- Before decoding one picture, the reference pictures for every MB/partition should be ready with reference list0/1. The pictures in DPB with reference flag would be put into lists (the picture with non reference flag should not be put into DPB?). Sorting means that the short term reference pictures would be first with the decreasing PicNum order and the long term reference pictures follow with the increasing LongTermPicNum order.
- It might be possible that some reference pictures no matter if they are short or long term reference would be used more often than others by MB. The small indexes of lists for these reference pictures would reduce bitrate further. So resorting process starts on every picture of lists based on the slice header information of the current slice.
- After the current picture is decoded, this picture would be flagged as three modes: unused for reference, used for short term reference, and used for long term reference. And it is stored into DPB if it is used for reference picture. This is called reference picture marking. Also it includes the memory management of DPB. Two ways are used for this management: sliding window or adaptive_ref_pic_marking_mode with 7 commands.

Syntax elements of H.264

* Syntax for video sequence stream
- VCL represents the content of the video data. NAL is to format that data and provide header information for comm and storage.
- Big endian for video stream in byte while little endian for bits in one byte with LSBit on the right. The MSBit is always first in bit stream.
- One coded slice NAL needs to contain all the data of one slice. The data struct is slice header, slice data and trailing bits. This means NAL represents one slice of a picture instead of one picture.

- Syntax elements for NAL
1). nal_ref_idc:
For seq and pic parameter sets NAL, it shall be 1.
For slices of reference pic, it shall be 1.
For slices of non reference pic, it shall be 0.
2). nal_unit_type:

- Syntax elements for Seq Parameter Set (SPS)
0) seq_parameter_set_id: [0, 31]
1) log2_max_frame_num_minus4: [0, 12] (maximal num of MaxFrameNum is 2^16)
2) pic_order_cnt_type: specify the method to decode picture order count. [0, 2]
3) log2_max_pic_order_cnt_lsb_minus4: [0, 12] (MaxPicOrderCntLsb)
4) several elements for the decoding of picture order count
5) num_ref_frames: [0, MaxDpbSize], the sum of reference frames, complementary reference field pair and non-paired reference fields
6) frame_mbs_only_flag: indicate only frames exist in the video seq
7) mb_adaptive_frame_field_flag:

- Syntax elements for Pic Parameter Set (PPS)
0) pic_parameter_set_id: [0, 255]
1) mb to slice group map
2) QP initial value for Y/C

- Syntax elements for Slice header
0) first_mb_in_slice: MB index in general and MB pair index for MBAFF
1) slice_type: IDR only contains I/SI slices and so does the video seq when num_ref_frames is 0
2) frame_num: number reference pictures.
3) field_pic_flag: this slice is one of a coded field, i.e. the picture is field picture. The picture structure could be defined with this flag. But if it is 0 the MB structure may be either frame or field.
4) bottom_field_flag: this slice is part of a coded bottom field. The picture is bottom field.
5) pic_order_cnt_lsb: the picture order count modulo MaxPicOrderCntLsb for the top field of a coded frame or for a coded field.
6) delta_pic_order_cnt_bottom
7) delta_pic_order_cnt[0-1]?
8) idr_pic_id: identifies an IDR picture. All slices in one IDR have the same value of idr_pic_id.

- Syntax elements for slice data
0) mb_field_decoding_flag: identify if the current MB is field or frame structure in MBAFF mode

Frame Num and Picture Order Count

- The standard told me frame_num is used as an identifier for pictures and it has strong relationship with PrevRefFrameNum. However,I am not quite understanding the usage of frame_num during encoding/decoding.

The concept is simple, but it became more complicated as it was refined. It is actually primarily a loss robustness feature. It may actually sometimes be helpful for you to ignore the name of the syntax element and try to think very strictly only about how it behaves --
not what it is called. The name is only a hint -- a way to help you remember which syntax element we're talking about when we talk about some particular one. It might be better to just think about it as if its name was any_name or something like that. (This is true of all syntax elements, actually -- but it is especially true of this one.)

Primarily, the idea of the syntax element any_name was to have a counter that increments each time you decode a picture so that if there are losses of data, the decoder can detect that some picture(s) were missing and would be able to conceal the problem without losing track of what was going on.

You can see this idea reflected in the way that the behavior of any_name depends on whether the picture is a reference picture or not (i.e., on nal_ref_idc). Since the proper decoding of a non-reference picture is not necessary for the proper decoding of other pictures that arrive later, any_name was designed so that a missing non-reference picture would not cause any_name to indicate the presence of a problem when a non-reference picture is missing.

Since the value of any_name often changes from picture to picture (and does not change within a picture), it can be used (subclause 7.4.1.2.4) as part of a method to detect when a new picture begins in the bitstream.

Then there is the notion that you ought to be able to splice different coded video sequences together without changing all the any_name variables in every picture. And the decoding process for different coded video sequences is independent anyway, so the value of any_name was reset to zero whenever a new coded_video_sequence begins.

Then, we find that under some circumstances (e.g., esp. for redundant pictures that correspond to IDR primary pictures) it might be nice to be able to reset the value of any_name without necessarily using an IDR picture to do it (since IDR pictures carry a significant penalty
in rate-distortion performance relative to other types of pictures).This led to the feature embodied as memory_management_control_operation equal to 5.

We also found that if we governed the behavior of any_name within a coded video sequence too strictly, it would prevent the ability to have efficient multi-layer temporal scalability (the ability to remove some pictures from a bitstream and still have a decodable remaining sequence of pictures). This led to the features embodied in the standard as "gaps in any_name value" and "sub-sequences".

Then, finally, we get to interlace support and coded fields. Parity can be used to distinguish between a top field and a bottom field, so it is not necessary for pictures to have a different value of any_name to let you know whether an individual field is missing. So fields of different parity can share the same value of any_name.

Finally we get to the way fields are stored into memory for operation of the decoding process for PicAFF and MBAFF coding (picture- and macroblock-adaptive frame/field coding, respectively). If we let a top field be paired with a bottom field for use as a decoded reference frame, this means that we need some way for the decoder to know how to pair different fields together for that purpose. And we thought that it was probably not really necessary to allow any individual top field to be paired with any arbitrarily-selected bottom field for that purpose, since typically an encoder might not really be interested in doing that. Conceptually, it is simpler to be able to just store the data for two fields into a memory space that would ordinarily hold a frame, and not need to do extra work to be able to create an association between any arbitrary pair of fields. Then a decoder could just change the stride it uses when addressing a surface to
control whether it is accessing the samples of an individual field or a unified frame. So the decoded picture buffer (DPB) was designed to manage its memory model as a collection of frame stores, not as a collection of individual fields.

That is really essentially the entire purpose and design relating to any_name (i.e., frame_num). That is ALL it is. It is natural to want to think of any_name as essentially a numbering of source frames at the input to the encoder. Although this is what most encoders will probably do, it is not a strictly correct understanding sufficient to build a well-designed decoder. (It is important to keep in mind that we do not specify how encoders or displays will operate -- only
decoders.) For example, that thinking could lead to some incorrect assumptions about the allowed timing relationship of pictures at the output of the decoder. The syntax element is not really for that purpose. Instead, it is a way to achieve picture loss robustness without sacrificing too much flexibility for the way the video can be used, and a way to simplify the picture buffering model management in decoders for frame/field adaptive coding.


- The standard says "Picture order counts are used to determine initial picture orderings for reference pictures in the decoding of B slices",which means we don't need to consider pic_order_cnt_type when dealing with baseline profile?

The basic concept of POC is to provide a counter that specifies the relative order of the pictures in the bitstream in output order (which may differ from the relative order in which the coded pictures appear in the data of the bitstream, which is referred to as the decoding order).
The relative order of the pictures is indicated in POC, rather than the timing of the pictures. This allows systems that carry the video bitstream to control the exact timing of the processing and output of the video bitstream without affecting the decoding process for the values of the samples in the luma and chroma sample arrays of the pictures. In some cases, the values of the samples in the luma and chroma sample arrays will depend on POC values. However, the values of
the samples in the luma and chroma sample arrays will never depend on the timing of the pictures.

There are three modes of POC operation:

In POC type 0, each slice header contains a simple fixed-length counter syntax element (pic_order_cnt_lsb) that provides the LSBs of the current POC. The MSBs of the current POC are calculated by the decoder by tracking modulus wrapping in the LSBs.

In POC type 1, each slice header contains one or two variable-length-encoded syntax elements that provide the difference to apply to a prediction of the current POC to compute the actual current
POC. This POC type provides the encoder with the ability to encode the POC values using significantly fewer bits per slice than what would otherwise be needed when using POC type 0 in cases where the encoder will usually be using a repetitive pattern of POC behavior.

In POC type 2, no data is carried in the slice header to compute the current POC. When POC type 2 is in use, the output order of the pictures in the bitstream will be the same as the order in which the coded pictures appear in the data of the bitstream. This POC type eliminates the need for the encoder to send any syntax data in the slice header for POC derivation. However, it provides no flexibility to allow the output order of the pictures in the bitstream to differ from their decoding order.

That statement would ordinarily be true. However, picture order count can also be used to determine the output order of pictures. The decoder ought to have other sources of information to determine that (e.g., timestamps on pictures carried at a systems level), so a Baseline decoder may not need to pay attention to picture order count. But it does need to figure out the output order of pictures one way or another.

Picture order count is also used to determine weights for temporal weighted prediction. Of course, that's not part of the Baseline profile either.

I think the only dependencies between picture order count and the processes for determining the values of decoded picture samples are the following:
1) The ordering of the initial reference picture lists in B slices
2) Temporal weighted prediction in B slices
3) Temporal direct prediction in B slices

So the summary is that if you're not supporting B slices you don't need picture order count for for determining the values of decoded picture samples.

The only other issue is how to determine the output order of pictures. But a system may provide that information in some way that doesn't depend on picture order count.

-- From mpegif.org