src/devices/audio/latency_design.jd

page.title=Design For Reduced Latency
@jd:body

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    <h2>In this document</h2>
    <ol id="auto-toc">
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<p>
The Android 4.1 release introduced internal framework changes for
a <a href="http://en.wikipedia.org/wiki/Low_latency">lower latency</a>
audio output path. There were minimal public client API
or HAL API changes. This document describes the initial design,
which has continued to evolve over time.
Having a good understanding of this design should help device OEM and
SoC vendors implement the design correctly on their particular devices
and chipsets.  This article is not intended for application developers.
</p>

<h2 id="trackCreation">Track Creation</h2>

<p>
The client can optionally set bit <code>AUDIO_OUTPUT_FLAG_FAST</code> in the
<code>audio_output_flags_t</code> parameter of AudioTrack C++ constructor or
<code>AudioTrack::set()</code>. Currently the only clients that do so are:
</p>

<ul>
<li>Android native audio based on OpenSL ES</li>
<li><a href="http://developer.android.com/reference/android/media/SoundPool.html">android.media.SoundPool</a></li>
<li><a href="http://developer.android.com/reference/android/media/ToneGenerator.html">android.media.ToneGenerator</a></li>
</ul>

<p>
The AudioTrack C++ implementation reviews the <code>AUDIO_OUTPUT_FLAG_FAST</code>
request and may optionally deny the request at client level. If it
decides to pass the request on, it does so using <code>TRACK_FAST</code> bit of
the <code>track_flags_t</code> parameter of the <code>IAudioTrack</code> factory method
<code>IAudioFlinger::createTrack()</code>.
</p>

<p>
The AudioFlinger audio server reviews the <code>TRACK_FAST</code> request and may
optionally deny the request at server level. It informs the client
whether or not the request was accepted, via bit <code>CBLK_FAST</code> of the
shared memory control block.
</p>

<p>
The factors that impact the decision include:
</p>

<ul>
<li>Presence of a fast mixer thread for this output (see below)</li>
<li>Track sample rate</li>
<li>Presence of a client thread to execute callback handlers for this track</li>
<li>Track buffer size</li>
<li>Available fast track slots (see below)</li>
</ul>

<p>
If the client's request was accepted, it is called a "fast track."
Otherwise it's called a "normal track."
</p>

<h2 id="mixerThreads">Mixer Threads</h2>

<p>
At the time AudioFlinger creates a normal mixer thread, it decides
whether or not to also create a fast mixer thread. Both the normal
mixer and fast mixer are not associated with a particular track,
but rather with a set of tracks. There is always a normal mixer
thread. The fast mixer thread, if it exists, is subservient to the
normal mixer thread and acts under its control.
</p>

<h3 id="fastMixer">Fast mixer</h3>

<h4>Features</h4>

<p>
The fast mixer thread provides these features:
</p>

<ul>
<li>Mixing of the normal mixer's sub-mix and up to 7 client fast tracks</li>
<li>Per track attenuation</li>
</ul>

<p>
Omitted features:
</p>

<ul>
<li>Per track sample rate conversion</li>
<li>Per track effects</li>
<li>Per mix effects</li>
</ul>

<h4>Period</h4>

<p>
The fast mixer runs periodically, with a recommended period of two
to three milliseconds (ms), or a slightly higher period of five ms if needed for scheduling stability.
This number was chosen so that, accounting for the complete
buffer pipeline, the total latency is on the order of 10 ms. Smaller
values are possible but may result in increased power consumption
and chance of glitches depending on CPU scheduling predictability.
Larger values are possible, up to 20 ms, but result in degraded
total latency and so should be avoided.
</p>

<h4>Scheduling</h4>

<p>
The fast mixer runs at elevated <code>SCHED_FIFO</code> priority. It needs very
little CPU time, but must run often and with low scheduling jitter.
<a href="http://en.wikipedia.org/wiki/Jitter">Jitter</a>
expresses the variation in cycle time: it is the difference between the
actual cycle time versus the expected cycle time.
Running too late will result in glitches due to underrun. Running
too early will result in glitches due to pulling from a fast track
before the track has provided data.
</p>

<h4>Blocking</h4>

<p>
Ideally the fast mixer thread never blocks, other than at HAL
<code>write()</code>. Other occurrences of blocking within the fast mixer are
considered bugs. In particular, mutexes are avoided.
Instead, <a href="http://en.wikipedia.org/wiki/Non-blocking_algorithm">non-blocking algorithms</a>
(also known as lock-free algorithms) are used.
See <a href="avoiding_pi.html">Avoiding Priority Inversion</a> for more on this topic.
</p>

<h4>Relationship to other components</h4>

<p>
The fast mixer has little direct interaction with clients. In
particular, it does not see binder-level operations, but it does
access the client's shared memory control block.
</p>

<p>
The fast mixer receives commands from the normal mixer via a state queue.
</p>

<p>
Other than pulling track data, interaction with clients is via the normal mixer.
</p>

<p>
The fast mixer's primary sink is the audio HAL.
</p>

<h3 id="normalMixer">Normal mixer</h3>

<h4>Features</h4>

<p>
All features are enabled:
</p>

<ul>
<li>Up to 32 tracks</li>
<li>Per track attenuation</li>
<li>Per track sample rate conversion</li>
<li>Effects processing</li>
</ul>

<h4>Period</h4>

<p>
The period is computed to be the first integral multiple of the
fast mixer period that is >= 20 ms.
</p>

<h4>Scheduling</h4>

<p>
The normal mixer runs at elevated <code>SCHED_OTHER</code> priority.
</p>

<h4>Blocking</h4>

<p>
The normal mixer is permitted to block, and often does so at various
mutexes as well as at a blocking pipe to write its sub-mix.
</p>

<h4>Relationship to other components</h4>

<p>
The normal mixer interacts extensively with the outside world,
including binder threads, audio policy manager, fast mixer thread,
and client tracks.
</p>

<p>
The normal mixer's sink is a blocking pipe to the fast mixer's track 0.
</p>

<h2 id="flags">Flags</h2>

<p>
<code>AUDIO_OUTPUT_FLAG_FAST</code> bit is a hint. There's no guarantee the
request will be fulfilled.
</p>

<p>
<code>AUDIO_OUTPUT_FLAG_FAST</code> is a client-level concept. It does not appear
in server.
</p>

<p>
<code>TRACK_FAST</code> is a client -> server concept.
</p>