*********
Threading
*********

.. _pools:

Thread Pools
============

x265 creates one or more thread pools per encoder, one pool per NUMA
node (typically a CPU socket). :option:`--pools` specifies the number of
pools and the number of threads per pool the encoder will allocate. By
default x265 allocates one thread per (hyperthreaded) CPU core on each
NUMA node.

If you are running multiple encoders on a system with multiple NUMA
nodes, it is recommended to isolate each of them to a single node in
order to avoid the NUMA overhead of remote memory access.

Work distribution is job based. Idle worker threads scan the job
providers assigned to their thread pool for jobs to perform. When no
jobs are available, the 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).  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 absolutely necessary
for data locking. If a job becomes blocked, the work function is
expected to drop that job so the worker thread may go back to the pool
and find more work.

On Windows, the native APIs offer sufficient functionality to discover
the NUMA topology and enforce the thread affinity that libx265 needs (so
long as you have not chosen to target XP or Vista), but on POSIX systems
it relies on libnuma for this functionality. If your target POSIX system
is single socket, then building without libnuma is a perfectly
reasonable option, as it will have no effect on the runtime behavior. On
a multiple-socket system, a POSIX build of libx265 without libnuma will
be less work efficient, but will still function correctly. You lose the
work isolation effect that keeps each frame encoder from only using the
threads of a single socket and so you incur a heavier context switching
cost.

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 <http://www.parabolaresearch.com/blog/2013-12-01-hevc-wavefront-animation.html>`_
which visualizes WPP very well.  It even correctly visualizes some of
WPPs key drawbacks, such as:

1. the low thread utilization at the start and end of each frame
2. a difficult block may stall the wave-front and it takes a while for
   the wave-front to recover.
3. 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 :option:`--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.

Bonded Task Groups
==================

If a worker thread job has work which can be performed in parallel by
many threads, it may allocate a bonded task group and enlist the help of
other idle worker threads from the same thread pool. Those threads will
cooperate to complete the work of the bonded task group and then return
to their idle states. The larger and more uniform those tasks are, the
better the bonded task group will perform.

Parallel Mode Analysis
~~~~~~~~~~~~~~~~~~~~~~

When :option:`--pmode` is enabled, each CU (at all depths from 64x64 to
8x8) will distribute its analysis work to the thread pool via a bonded
task group. Each analysis job will measure the cost of one prediction
for the CU: merge, skip, intra, inter (2Nx2N, Nx2N, 2NxN, and AMP).

At slower presets, the amount of increased parallelism from pmode is
often enough to be able to reduce or disable frame parallelism while
achieving the same overall CPU utilization. Reducing frame threads is
often beneficial to ABR and VBV rate control.

Parallel Motion Estimation
~~~~~~~~~~~~~~~~~~~~~~~~~~

When :option:`--pme` is enabled all of the analysis functions which
perform motion searches to reference frames will distribute those motion
searches to other worker threads via a bonded task group (if more than
two motion searches are required).

Frame Threading
===============

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. On top of this, HEVC has two loop filters:
deblocking and SAO, which must be run in series with a row lag between
them. 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). And keep in mind the wave-front progression pattern; by the
time the reference frame finishes the third row of CTUs, nearly half of
the CTUs in the frame may be compressed (depending on the display aspect
ratio).

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 less effective when frame parallelism is in
use.

:option:`--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.

.. note::

	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 :option:`--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 :option:`--frame-threads`. When
:option:`--threaded-me` is enabled, the auto-detected frame thread count
is derived from the thread fraction available after the ThreadedME
pool is allocated.

	+-------+--------+
	| Cores | Frames |
	+=======+========+
	|  > 32 |  6..8  |
	+-------+--------+
	| >= 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
:option:`--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:

1. blocked, waiting for a frame to process
2. blocked on a reference frame, waiting for a CTU row of reconstructed
   and loop-filtered reference pixels to become available
3. blocked waiting for wave-front completion
4. blocked waiting for the main thread to consume an encoded frame

Lookahead
=========

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 will use bonded task groups
to perform batches of frame cost estimates, and it may optionally use
bonded task groups to measure single frame cost estimates using slices.
(see :option:`--lookahead-slices`)

The main slicetypeDecide() function itself is also performed by a worker
thread if your encoder has a thread pool, else it runs within the
context of the thread which calls the x265_encoder_encode().

Threaded Motion Estimation
==========================

The Threaded Motion Estimation module improves parallelism by offloading
motion estimation (which often dominates encoding time) to a dedicated
thread pool for pre-processing ahead of WPP.

In the default flow, MVP derivation requires motion vectors from adjacent
CTUs. This helps compression efficiency but introduces stalls (as described
in the WPP section), which limits parallelism. Threaded ME relaxes this
dependency and uses a different algorithm to unlock additional parallelism.

In this algorithm, CTUs from different rows are processed in parallel as soon
as external row dependencies are resolved. The steps are:

1. For MVP derivation, motion vectors from neighboring PUs and colocated MVs
   from the reference frame are used when available.
2. If these are not valid, a median MV from colocated CTUs in reference
   frames is evaluated.
3. If the median MV is also unavailable (for example, when the reference
   frame is an I-slice), a 109-point diamond search is performed on the full
   CTU and, when available, on the four CU quadrants at depth=1. The
   resulting vectors are used as MVPs for their respective regions.
4. Using these MVP seeds, motion estimation is then run for every PU shape
   enabled by the active configuration, and the resulting MVs are written to
   a lookup table.
5. During inter prediction, motion vectors from this precomputed lookup table
   are used for prediction.

ThreadedME has higher threading demand because it must maintain a lead over WPP
to minimize stalls. It computes MVs for CTUs across rows within a frame and
across frames across frame encoders. It is therefore recommended primarily for
many-core systems where threading resources can be balanced.

SAO
===

The Sample Adaptive Offset loopfilter has a large effect on encode
performance because of the peculiar way it must be analyzed and coded.

SAO flags and data are encoded at the CTU level before the CTU itself is
coded, but SAO analysis (deciding whether to enable SAO and with what
parameters) cannot be performed until that CTU is completely analyzed
(reconstructed pixels are available) as well as the CTUs to the right
and below.  So in effect the encoder must perform SAO analysis in a
wavefront at least a full row behind the CTU compression wavefront.

This extra latency forces the encoder to save the encode data of every
CTU until the entire frame has been analyzed, at which point a function
can code the final slice bitstream with the decided SAO flags and data
interleaved between each CTU.  This second pass over the CTUs can be
expensive, particularly at large resolutions and high bitrates.
