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="http://source.android.com/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 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, 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 then correct those.  This eliminates the
  audible artifacts and may even permit even smaller or fewer buffers
  and thus reduce latency.
</p>

<p>
  In our experience, the most common causes of underruns 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>
</ul>

<h3>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.
</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>

<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 and are known to not interfere with scheduling of audio threads.
</p>

<h3>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>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 any 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>