x265 creates a pool of worker threads and shares this thread pool with all encoders within the same process (it is process global, aka a singleton). The number of threads within the thread pool is determined by the encoder which first allocates the pool, which by definition is the first encoder created within each process.
--threads specifies the number of threads the encoder will try to allocate for its thread pool. If the thread pool was already allocated this parameter is ignored. By default x265 allocated one thread per (hyperthreaded) CPU core in your system.
Work distribution is job based. Idle worker threads ask their parent pool object for jobs to perform. When no jobs are available, idle worker threads block and consume no CPU cycles.
Objects which desire to distribute work to worker threads are known as job providers (and they derive from the JobProvider class). When job providers have work they enqueue themselves into the pool’s provider list (and dequeue themselves when they no longer have work). The thread pool has a method to poke awake a blocked idle thread, and job providers are recommended to call this method when they make new jobs available.
Worker jobs are not allowed to block except when abosultely necessary for data locking. If a job becomes blocked, the worker thread is expected to drop that job and go back to the pool and find more work.
x265_cleanup() frees the process-global thread pool, allowing it to be reallocated if necessary, but only if no encoders are allocated at the time it is called.
Wavefront Parallel Processing¶
New with HEVC, Wavefront Parallel Processing allows each row of CTUs to be encoded in parallel, so long as each row stays at least two CTUs behind the row above it, to ensure the intra references and other data of the blocks above and above-right are available. WPP has almost no effect on the analysis and compression of each CTU and so it has a very small impact on compression efficiency relative to slices or tiles. The compression loss from WPP has been found to be less than 1% in most of our tests.
WPP has three effects which can impact efficiency. The first is the row starts must be signaled in the slice header, the second is each row must be padded to an even byte in length, and the third is the state of the entropy coder is transferred from the second CTU of each row to the first CTU of the row below it. In some conditions this transfer of state actually improves compression since the above-right state may have better locality than the end of the previous row.
Parabola Research have published an excellent HEVC animation which visualizes WPP very well. It even correctly visualizes some of WPPs key drawbacks, such as:
- the low thread utilization at the start and end of each frame
- a difficult block may stall the wave-front and it takes a while for the wave-front to recover.
- 64x64 CTUs are big! there are much fewer rows than with H.264 and similar codecs
Because of these stall issues you rarely get the full parallelisation benefit one would expect from row threading. 30% to 50% of the theoretical perfect threading is typical.
In x265 WPP is enabled by default since it not only improves performance at encode but it also makes it possible for the decoder to be threaded.
If WPP is disabled by --no-wpp the frame will be encoded in scan order and the entropy overheads will be avoided. If frame threading is not disabled, the encoder will change the default frame thread count to be higher than if WPP was enabled. The exact formulas are described in the next section.
Frame threading is the act of encoding multiple frames at the same time. It is a challenge because each frame will generally use one or more of the previously encoded frames as motion references and those frames may still be in the process of being encoded themselves.
Previous encoders such as x264 worked around this problem by limiting the motion search region within these reference frames to just one macroblock row below the coincident row being encoded. Thus a frame could be encoded at the same time as its reference frames so long as it stayed one row behind the encode progress of its references (glossing over a few details).
x265 has the same frame threading mechanism, but we generally have much less frame parallelism to exploit than x264 because of the size of our CTU rows. For instance, with 1080p video x264 has 68 16x16 macroblock rows available each frame while x265 only has 17 64x64 CTU rows.
The second extenuating circumstance is the loop filters. The pixels used for motion reference must be processed by the loop filters and the loop filters cannot run until a full row has been encoded, and it must run a full row behind the encode process so that the pixels below the row being filtered are available. When you add up all the row lags each frame ends up being 3 CTU rows behind its reference frames (the equivalent of 12 macroblock rows for x264)
The third extenuating circumstance is that when a frame being encoded becomes blocked by a reference frame row being available, that frame’s wave-front becomes completely stalled and when the row becomes available again it can take quite some time for the wave to be restarted, if it ever does. This makes WPP many times less effective when frame parallelism is in use.
--merange can have a negative impact on frame parallelism. If the range is too large, more rows of CTU lag must be added to ensure those pixels are available in the reference frames. Similarly --sao-lcu-opt 0 will cause SAO to be performed over the entire picture at once (rather than being CTU based), which prevents any motion reference pixels from being available until the entire frame has been encoded, which prevents any real frame parallelism at all.
Even though the merange is used to determine the amount of reference pixels that must be available in the reference frames, the actual motion search is not necessarily centered around the coincident block. The motion search is actually centered around the motion predictor, but the available pixel area (mvmin, mvmax) is determined by merange and the interpolation filter half-heights.
When frame threading is disabled, the entirety of all reference frames are always fully available (by definition) and thus the available pixel area is not restricted at all, and this can sometimes improve compression efficiency. Because of this, the output of encodes with frame parallelism disabled will not match the output of encodes with frame parallelism enabled; but when enabled the number of frame threads should have no effect on the output bitstream except when using ABR or VBV rate control or noise reduction.
When --nr is enabled, the outputs of each number of frame threads will be deterministic but none of them will match becaue each frame encoder maintains a cumulative noise reduction state.
VBV introduces non-determinism in the encoder, at this point in time, regardless of the amount of frame parallelism.
By default frame parallelism and WPP are enabled together. The number of frame threads used is auto-detected from the (hyperthreaded) CPU core count, but may be manually specified via --frame-threads
Cores Frames > 32 6 >= 16 5 >= 8 3 >= 4 2
If WPP is disabled, then the frame thread count defaults to min(cpuCount, ctuRows / 2)
Over-allocating frame threads can be very counter-productive. They each allocate a large amount of memory and because of the limited number of CTU rows and the reference lag, you generally get limited benefit from adding frame encoders beyond the auto-detected count, and often the extra frame encoders reduce performance.
Given these considerations, you can understand why the faster presets lower the max CTU size to 32x32 (making twice as many CTU rows available for WPP and for finer grained frame parallelism) and reduce --merange
Each frame encoder runs in its own thread (allocated separately from the worker pool). This frame thread has some pre-processing responsibilities and some post-processing responsibilities for each frame, but it spends the bulk of its time managing the wave-front processing by making CTU rows available to the worker threads when their dependencies are resolved. The frame encoder threads spend nearly all of their time blocked in one of 4 possible locations:
- blocked, waiting for a frame to process
- blocked on a reference frame, waiting for a CTU row of reconstructed and loop-filtered reference pixels to become available
- blocked waiting for wave-front completion
- blocked waiting for the main thread to consume an encoded frame
The lookahead module of x265 (the lowres pre-encode which determines scene cuts and slice types) uses the thread pool to distribute the lowres cost analysis to worker threads. It follows the same wave-front pattern as the main encoder except it works in reverse-scan order.
The function slicetypeDecide() itself may also be performed by a worker thread if your system has enough CPU cores to make this a beneficial trade-off, else it runs within the context of the thread which calls the x265_encoder_encode().