1 page.title=Design For Reduced Latency 2 @jd:body 3 4 <div id="qv-wrapper"> 5 <div id="qv"> 6 <h2>In this document</h2> 7 <ol id="auto-toc"> 8 </ol> 9 </div> 10 </div> 11 12 <p> 13 Android 4.1 (Jelly Bean) release introduced internal framework changes for 14 a lower latency audio output path. There were no public client API 15 or HAL API changes. This document describes the initial design, 16 which is expected to evolve over time. 17 Having a good understanding of this design should help device OEM and 18 SoC vendors to implement the design correctly on their particular devices 19 and chipsets. This article is not intended for application developers. 20 </p> 21 22 <h2 id="trackCreation">Track Creation</h2> 23 24 <p> 25 The client can optionally set bit <code>AUDIO_OUTPUT_FLAG_FAST</code> in the 26 <code>audio_output_flags_t</code> parameter of AudioTrack C++ constructor or 27 <code>AudioTrack::set()</code>. Currently the only clients that do so are: 28 </p> 29 30 <ul> 31 <li>OpenSL ES</li> 32 <li>SoundPool</li> 33 <li>ToneGenerator</li> 34 </ul> 35 36 <p> 37 The AudioTrack C++ implementation reviews the <code>AUDIO_OUTPUT_FLAG_FAST</code> 38 request and may optionally deny the request at client level. If it 39 decides to pass the request on, it does so using <code>TRACK_FAST</code> bit of 40 the <code>track_flags_t</code> parameter of the <code>IAudioTrack</code> factory method 41 <code>IAudioFlinger::createTrack()</code>. 42 </p> 43 44 <p> 45 AudioFlinger (server) reviews the <code>TRACK_FAST</code> request and may 46 optionally deny the request at server level. It informs the client 47 whether or not the request was accepted, via bit <code>CBLK_FAST</code> of the 48 shared memory control block. 49 </p> 50 51 <p> 52 The factors that impact the decision include: 53 </p> 54 55 <ul> 56 <li>presence of a fast mixer thread for this output (see below)</li> 57 <li>track sample rate</li> 58 <li>presence of a client thread to execute callback handlers for this track</li> 59 <li>track buffer size</li> 60 <li>available fast track slots (see below)</li> 61 </ul> 62 63 <p> 64 If the client's request was accepted, it is called a "fast track", 65 otherwise it's called a "normal track". 66 </p> 67 68 <h2 id="mixerThreads">Mixer Threads</h2> 69 70 <p> 71 At the time AudioFlinger creates a normal mixer thread, it decides 72 whether or not to also create a fast mixer thread. Both the normal 73 mixer and fast mixer are not associated with a particular track, 74 but rather with a set of tracks. There is always a normal mixer 75 thread. The fast mixer thread, if it exists, is subservient to the 76 normal mixer thread and acts under its control. 77 </p> 78 79 <h3 id="fastMixer">Fast mixer</h3> 80 81 <h4>Features</h4> 82 83 <p> 84 The fast mixer thread provides these features: 85 </p> 86 87 <ul> 88 <li>mixing of the normal mixer's sub-mix and up to 7 client fast tracks</li> 89 <li>Per track attenuation</li> 90 </ul> 91 92 <p> 93 Omitted features: 94 </p> 95 96 <ul> 97 <li>Per track sample rate conversion</li> 98 <li>Per track effects</li> 99 <li>Per mix effects</li> 100 </ul> 101 102 <h4>Period</h4> 103 104 <p> 105 The fast mixer runs periodically, with a recommended period of 2 106 to 3 ms, or slightly higher if needed for scheduling stability. 107 This number was chosen so that, accounting for the complete 108 buffer pipeline, the total latency is on the order of 10 ms. Smaller 109 values are possible but may result in increased power consumption 110 and chance of glitches depending on CPU scheduling predictability. 111 Larger values are possible, up to 20 ms, but result in degraded 112 total latency and so should be avoided. 113 </p> 114 115 <h4>Scheduling</h4> 116 117 <p> 118 The fast mixer runs at elevated <code>SCHED_FIFO</code> priority. It needs very 119 little CPU time, but must run often and with low scheduling jitter. 120 Running too late will result in glitches due to underrun. Running 121 too early will result in glitches due to pulling from a fast track 122 before the track has provided data. 123 </p> 124 125 <h4>Blocking</h4> 126 127 <p> 128 Ideally the fast mixer thread never blocks, other than at HAL 129 <code>write()</code>. Other occurrences of blocking within the fast mixer are 130 considered bugs. In particular, mutexes are avoided. 131 </p> 132 133 <h4>Relationship to other components</h4> 134 135 <p> 136 The fast mixer has little direct interaction with clients. In 137 particular, it does not see binder-level operations, but it does 138 access the client's shared memory control block. 139 </p> 140 141 <p> 142 The fast mixer receives commands from the normal mixer via a state queue. 143 </p> 144 145 <p> 146 Other than pulling track data, interaction with clients is via the normal mixer. 147 </p> 148 149 <p> 150 The fast mixer's primary sink is the audio HAL. 151 </p> 152 153 <h3 id="normalMixer">Normal mixer</h3> 154 155 <h4>Features</h4> 156 157 <p> 158 All features are enabled: 159 </p> 160 161 <ul> 162 <li>Up to 32 tracks</li> 163 <li>Per track attenuation</li> 164 <li>Per track sample rate conversion</li> 165 <li>Effects processing</li> 166 </ul> 167 168 <h4>Period</h4> 169 170 <p> 171 The period is computed to be the first integral multiple of the 172 fast mixer period that is >= 20 milliseconds. 173 </p> 174 175 <h4>Scheduling</h4> 176 177 <p> 178 The normal mixer runs at elevated <code>SCHED_OTHER</code> priority. 179 </p> 180 181 <h4>Blocking</h4> 182 183 <p> 184 The normal mixer is permitted to block, and often does so at various 185 mutexes as well as at a blocking pipe to write its sub-mix. 186 </p> 187 188 <h4>Relationship to other components</h4> 189 190 <p> 191 The normal mixer interacts extensively with the outside world, 192 including binder threads, audio policy manager, fast mixer thread, 193 and client tracks. 194 </p> 195 196 <p> 197 The normal mixer's sink is a blocking pipe to the fast mixer's track 0. 198 </p> 199 200 <h2 id="flags">Flags</h2> 201 202 <p> 203 <code>AUDIO_OUTPUT_FLAG_FAST</code> bit is a hint. There's no guarantee the 204 request will be fulfilled. 205 </p> 206 207 <p> 208 <code>AUDIO_OUTPUT_FLAG_FAST</code> is a client-level concept. It does not appear 209 in server. 210 </p> 211 212 <p> 213 <code>TRACK_FAST</code> is a client -> server concept. 214 </p> 215 216
