en/devices/tech/power/performance.html

page.title=Performance Management
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    <h2>In this document</h2>
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<p>Managing the power and performance of Android devices can help ensure
applications run consistently and smoothly on a wide range of hardware. In
Android 7.0, OEMs can implement support for sustained performance hints that
enable apps to maintain a consistent device performance and specify an exclusive
core to improve performance for CPU-intensive, foreground applications.</p>

<h2 id=sustained_performance>Sustained performance</h2>
<p>For long-running applications (games, camera, RenderScript, audio
processing), performance can vary dramatically as device temperature limits are
reached and system on chip (SoC) engines are throttled. App developers creating
high-performance, long-running apps are limited because the capabilities of the
underlying platform are a moving target when the device begins to heat up.</p>

<p>To address these limitations, Android 7.0 includes support for sustained
performance, enabling OEMs to provide hints on device performance capabilities
for long-running applications. App developers can use these hints to tune
applications for a predictable, consistent level of device performance over long
periods of time.</p>

<h3 id=architecture>Architecture</h3>
<p>An Android application can request the platform to enter a sustained
performance mode where the Android device can keep a consistent level of
performance for prolonged periods of time.</p>

<p><img src="../images/power_sustained_perf.png"></p>
<p class="img-caption"><strong>Figure 1.</strong> Sustained performance mode
architecture</p>

<h3 id=implementation>Implementation</h3>
<p>To support sustained performance in Android 7.0, OEMs must:</p>
<ul>
<li>Make device-specific changes to the power HAL to either lock the maximum
CPU/GPU frequencies <strong>or</strong> perform other optimizations to prevent
thermal throttling.</li>
<li>Implement the new hint <code>POWER_HINT_SUSTAINED_PERFORMANCE</code> in
power HAL.</li>
<li>Declare support by returning TRUE through the
<code>isSustainedPerformanceModeSupported()</code> API.</li>
<li>Implement <code>Window.setSustainedPerformanceMode</code>.</li>
</ul>

<p>In the Nexus reference implementation, the power hint caps the
maximum frequencies of the CPU and GPU at the highest sustainable levels. Keep
in mind that lowering the MAX bar in CPU/GPU frequency will lower the frame
rate, but this lower rate is preferred in this mode due to its sustainability.
For example, a device using normal max clocks might be able to render at 60
FPS for a few minutes, but after the device heats up,  it may throttle to 30 FPS
by the end of 30 minutes. When using sustained mode, the device can, for
example, render consistently at 45 FPS for the entire 30 minutes. The goal is a
frame rate when using the mode that is as high (or higher) than the frame rate
when not using the mode, and consistent over time so that developers don't have
to chase a moving target.</p>
<p>We strongly recommend implementing sustained mode such that the device
achieves the highest possible sustained performance—not just the minimum values
required to pass the test (e.g. choose the highest possible MAX frequency caps
that do not cause the device to thermally throttle over time).</p>

<p class="note"><strong>Note</strong>: Capping MAX clock rates is not required
to implement sustained mode.</p>

<h3 id=validation>Validation</h3>
<p>OEMs can use a new Android 7.0 CTS test to verify their implementation of the
sustained performance API. The test runs a workload for approximately 30 minutes
and benchmarks the performance with and without sustained mode enabled:</p>
<ul>
<li>With sustained mode enabled, the frame rate must remain relatively constant
(test measures the percentage of change in frame rate over time and requires a
&lt;5% change).</li>
<li>With sustained mode enabled, the frame rate must not be lower than the frame
rate at the end of 30 minutes with the mode disabled.</li>
</ul>
<p>In addition, you can manually test your implementation with several CPU- and
GPU-intensive workloads to ensure the device does not thermally throttle after
30 minutes of use. In internal testing, we used sample workloads including
games and benchmarking apps (e.g.
<a href="https://gfxbench.com/result.jsp">gfxbench</a>).</p>

<h2 id=exclusive_core>Exclusive cores</h2>
<p>For CPU-intensive, time-sensitive workloads, getting preempted by another
thread can be the difference between making frame deadlines or not. For apps
that have strict latency and frame rate requirements (such as audio or virtual
reality apps), having an exclusive CPU core can guarantee an acceptable level of
performance.</p>
<p>Devices running Android 7.0 can now reserve one core explicitly for the top
foreground application, improving performance for all foreground apps and giving
apps with high intensity workloads more control over how their work is allocated
across CPU cores.</p>
<p>To support an exclusive core on a device:</p>
<ul>
<li>Enable <code>cpusets</code> and configure a <code>cpuset</code> that
contains only the top foreground application.</li>
<li>Ensure one core (this is the exclusive core) is reserved for threads from
this <code>cpuset</code>.</li>
<li>Implement the getExclusiveCores API to return the core number of the
exclusive core.</li>
</ul>
<p>To determine which processes are scheduled on which cores, use
<code>systrace</code> while running any workload and verify no userspace threads
from applications other than the top foreground application are scheduled on the
exclusive core.</p>
<p>To view a reference implementation for the Nexus 6P, refer to
<code>android//device/huawei/angler/power/power.c</code>.</p>