src/devices/audio/latency.jd

page.title=Audio Latency
@jd:body

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<p>Audio latency is the time delay as an audio signal passes through a system.
  For a complete description of audio latency for the purposes of Android
  compatibility, see <em>Section 5.5 Audio Latency</em>
  in the <a href="{@docRoot}compatibility/index.html">Android CDD</a>.
  See <a href="latency_design.html">Design For Reduced Latency</a> for an 
  understanding of Android's audio latency-reduction efforts.
</p>

<p>
  This page focuses on the contributors to output latency,
  but a similar discussion applies to input latency.
</p>
<p>
  Assuming the analog circuitry does not contribute significantly, then the major 
  surface-level contributors to audio latency are the following:
</p>

<ul>
  <li>Application</li>
  <li>Total number of buffers in pipeline</li>
  <li>Size of each buffer, in frames</li>
  <li>Additional latency after the app processor, such as from a DSP</li>
</ul>

<p>
  As accurate as the above list of contributors may be, it is also misleading.
  The reason is that buffer count and buffer size are more of an
  <em>effect</em> than a <em>cause</em>.  What usually happens is that
  a given buffer scheme is implemented and tested, but during testing, an audio
  underrun or overrun is heard as a "click" or "pop."  To compensate, the
  system designer then increases buffer sizes or buffer counts.
  This has the desired result of eliminating the underruns or overruns, but it also
  has the undesired side effect of increasing latency.
</p>

<p>
  A better approach is to understand the causes of the
  underruns and overruns, and then correct those.  This eliminates the
  audible artifacts and may permit even smaller or fewer buffers
  and thus reduce latency.
</p>

<p>
  In our experience, the most common causes of underruns and overruns include:
</p>
<ul>
  <li>Linux CFS (Completely Fair Scheduler)</li>
  <li>high-priority threads with SCHED_FIFO scheduling</li>
  <li>long scheduling latency</li>
  <li>long-running interrupt handlers</li>
  <li>long interrupt disable time</li>
  <li>power management</li>
  <li>security kernels</li>
</ul>

<h3 id="linuxCfs">Linux CFS and SCHED_FIFO scheduling</h3>
<p>
  The Linux CFS is designed to be fair to competing workloads sharing a common CPU
  resource. This fairness is represented by a per-thread <em>nice</em> parameter.
  The nice value ranges from -19 (least nice, or most CPU time allocated)
  to 20 (nicest, or least CPU time allocated). In general, all threads with a given
  nice value receive approximately equal CPU time and threads with a
  numerically lower nice value should expect to
  receive more CPU time. However, CFS is "fair" only over relatively long
  periods of observation. Over short-term observation windows,
  CFS may allocate the CPU resource in unexpected ways. For example, it
  may take the CPU away from a thread with numerically low niceness
  onto a thread with a numerically high niceness.  In the case of audio,
  this can result in an underrun or overrun.
</p>

<p>
  The obvious solution is to avoid CFS for high-performance audio
  threads. Beginning with Android 4.1, such threads now use the
  <code>SCHED_FIFO</code> scheduling policy rather than the <code>SCHED_NORMAL</code> (also called
  <code>SCHED_OTHER</code>) scheduling policy implemented by CFS.
</p>

<h3 id="schedFifo">SCHED_FIFO priorities</h3>
<p>
  Though the high-performance audio threads now use <code>SCHED_FIFO</code>, they
  are still susceptible to other higher priority <code>SCHED_FIFO</code> threads.
  These are typically kernel worker threads, but there may also be a few
  non-audio user threads with policy <code>SCHED_FIFO</code>. The available <code>SCHED_FIFO</code>
  priorities range from 1 to 99.  The audio threads run at priority
  2 or 3.  This leaves priority 1 available for lower priority threads,
  and priorities 4 to 99 for higher priority threads.  We recommend 
  you use priority 1 whenever possible, and reserve priorities 4 to 99 for
  those threads that are guaranteed to complete within a bounded amount
  of time, execute with a period shorter than the period of audio threads,
  and are known to not interfere with scheduling of audio threads.
</p>

<h3 id="rms">Rate-monotonic scheduling</h3>
<p>
  For more information on the theory of assignment of fixed priorities,
  see the Wikipedia article
  <a href="http://en.wikipedia.org/wiki/Rate-monotonic_scheduling">Rate-monotonic scheduling</a> (RMS).
  A key point is that fixed priorities should be allocated strictly based on period,
  with higher priorities assigned to threads of shorter periods, not based on perceived "importance."
  Non-periodic threads may be modeled as periodic threads, using the maximum frequency of execution
  and maximum computation per execution.  If a non-periodic thread cannot be modeled as
  a periodic thread (for example it could execute with unbounded frequency or unbounded computation
  per execution), then it should not be assigned a fixed priority as that would be incompatible
  with the scheduling of true periodic threads.
</p>

<h3 id="schedLatency">Scheduling latency</h3>
<p>
  Scheduling latency is the time between when a thread becomes
  ready to run, and when the resulting context switch completes so that the
  thread actually runs on a CPU. The shorter the latency the better, and 
  anything over two milliseconds causes problems for audio. Long scheduling
  latency is most likely to occur during mode transitions, such as
  bringing up or shutting down a CPU, switching between a security kernel
  and the normal kernel, switching from full power to low-power mode,
  or adjusting the CPU clock frequency and voltage.
</p>

<h3 id="interrupts">Interrupts</h3>
<p>
  In many designs, CPU 0 services all external interrupts.  So a
  long-running interrupt handler may delay other interrupts, in particular
  audio direct memory access (DMA) completion interrupts. Design interrupt handlers
  to finish quickly and defer lengthy work to a thread (preferably
  a CFS thread or <code>SCHED_FIFO</code> thread of priority 1).
</p>

<p>
  Equivalently, disabling interrupts on CPU 0 for a long period
  has the same result of delaying the servicing of audio interrupts.
  Long interrupt disable times typically happen while waiting for a kernel
  <i>spin lock</i>.  Review these spin locks to ensure that
  they are bounded.
</p>

<h3 id="power">Power, performance, and thermal management</h3>
<p>
  <a href="http://en.wikipedia.org/wiki/Power_management">Power management</a>
  is a broad term that encompasses efforts to monitor
  and reduce power consumption while optimizing performance.
  <a href="http://en.wikipedia.org/wiki/Thermal_management_of_electronic_devices_and_systems">Thermal management</a>
  and <a href="http://en.wikipedia.org/wiki/Computer_cooling">computer cooling</a>
  are similar but seek to measure and control heat to avoid damage due to excess heat.
  In the Linux kernel, the CPU
  <a href="http://en.wikipedia.org/wiki/Governor_%28device%29">governor</a>
  is responsible for low-level policy, while user mode configures high-level policy.
  Techniques used include:
</p>

<ul>
  <li>dynamic voltage scaling</li>
  <li>dynamic frequency scaling</li>
  <li>dynamic core enabling</li>
  <li>cluster switching</li>
  <li>power gating</li>
  <li>hotplug (hotswap)</li>
  <li>various sleep modes (halt, stop, idle, suspend, etc.)</li>
  <li>process migration</li>
  <li><a href="http://en.wikipedia.org/wiki/Processor_affinity">processor affinity</a></li>
</ul>

<p>
  Some management operations can result in "work stoppages" or
  times during which there is no useful work performed by the application processor.
  These work stoppages can interfere with audio, so such management should be designed
  for an acceptable worst-case work stoppage while audio is active.
  Of course, when thermal runaway is imminent, avoiding permanent damage
  is more important than audio!
</p>

<h3 id="security">Security kernels</h3>
<p>
  A <a href="http://en.wikipedia.org/wiki/Security_kernel">security kernel</a> for
  <a href="http://en.wikipedia.org/wiki/Digital_rights_management">Digital rights management</a>
  (DRM) may run on the same application processor core(s) as those used
  for the main operating system kernel and application code.  Any time
  during which a security kernel operation is active on a core is effectively a
  stoppage of ordinary work that would normally run on that core.
  In particular, this may include audio work.  By its nature, the internal
  behavior of a security kernel is inscrutable from higher-level layers, and thus
  any performance anomalies caused by a security kernel are especially
  pernicious.  For example, security kernel operations do not typically appear in
  context switch traces.  We call this "dark time" &mdash; time that elapses
  yet cannot be observed.  Security kernels should be designed for an
  acceptable worst-case work stoppage while audio is active.
</p>