W 0300 codingfor_life-batterylifethatis

Coding for Life--Battery
Life, That Is
Jeff Sharkey
May 27, 2009

Post your questions for this talk on Google Moderator:
code.google.com/events/io/questions

Why does this matter?
Phones primarily run on battery power, and each device
has a "battery budget"
When it's gone, it's gone
Apps need to work together to be good citizens of that
shared resource
Current measured in mA, battery capacity in mAh

HTC Dream: 1150mAh
HTC Magic: 1350mAh
Samsung I7500: 1500mAh
Asus Eee PC: 5800mAh

Where does it all go?

Source: Values measured using an industrial power
monitor at 5kHz sampling rate, and taking average
power with lowest standard deviation.

Where does it all go?

How do these numbers add up in real life?
Watching YouTube: 340mA = 3.4 hours
Browsing 3G web: 225mA = 5 hours
Typical usage: 42mA average = 32 hours
EDGE completely idle: 5mA = 9.5 days
Airplane mode idle: 2mA = 24 days

What costs the most?

Waking up in the background when the phone would
otherwise be sleeping
App wakes up every 10 minutes to update
Takes about 8 seconds to update, 350mA
Cost during a given hour:
3600 seconds * 5mA = 5mAh resting
6 times * 8 sec * 350 mA = 4.6mAh updating
Just one app waking up can trigger cascade

What costs the most?

Bulk data transfer such as a 6MB song:
EDGE (90kbps): 300mA * 9.1 min = 45 mAh
3G (300kbps): 210mA * 2.7 min = 9.5 mAh
WiFi (1Mbps): 330mA * 48 sec = 4.4 mAh
Moving between cells/networks
Radio ramps up to associate with new cell
BroadcastIntents fired across system
Parsing textual data, regex without JIT

How can we do better?
Networking

Networking

Check network connection, wait for 3G or WiFi

ConnectivityManager mConnectivity;
TelephonyManager mTelephony;

// Skip if no connection, or background data disabled
NetworkInfo info = mConnectivity.getActiveNetworkInfo();
if (info == null ||
!mConnectivity.getBackgroundDataSetting()) {
return false;
}

Networking

Check network connection, wait for 3G or WiFi

// Only update if WiFi or 3G is connected and not roaming
int netType = info.getType();
int netSubtype = info.getSubtype();

if (netType == ConnectivityManager.TYPE_WIFI) {
return info.isConnected();
} else if (netType == ConnectivityManager.TYPE_MOBILE
&& netSubtype == TelephonyManager.NETWORK_TYPE_UMTS
&& !mTelephony.isNetworkRoaming()) {
return info.isConnected();
} else {
return false;
}

Networking

Use an efficient data format and parser

Source: Timings obtained by downloading and
parsing a 6-item RSS feed repeatedly for 60
seconds and averaging results.

Networking

Use an efficient data format and parser
Use "stream" parsers instead of tree parsers
Consider binary formats that can easily mix binary and
text data into a single request
Fewer round-trips to server for faster UX

Networking

Use GZIP for text data whenever possible
Framework GZIP libs go directly to native code , and are
perfect for streams
import java.util.zip.GZIPInputStream;

HttpGet request =
new HttpGet("http://example.com/gzipcontent");
HttpResponse resp =
new DefaultHttpClient().execute(request);
HttpEntity entity = response.getEntity();
InputStream compressed = entity.getContent();
InputStream rawData = new GZIPInputStream(compressed);

Networking

Use GZIP for text data whenever possible

Source: Timings averaged over multiple trials of
downloading 1800-item RSS feed of textual data.

Foreground apps

Foreground apps

Wakelocks are costly if forgotten
Pick the lowest level possible, and use specific
timeouts to work around unforseen bugs
Consider using android:keepScreenOn to ensure
correctness

<LinearLayout
android:orientation="vertical"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:keepScreenOn="true">

Foreground apps

Recycle Java objects, especially complex objects
Yes, we have a GC, but usually better to just create
less garbage that it has to clean up
XmlPullParserFactory and BitmapFactory
Matcher.reset(newString) for regex
StringBuilder.setLength(0)
Watch for synchronization issues, but can be safe when
driven by UI thread
Recycling strategies are used heavily in ListView

Foreground apps

Use coarse network location, it's much cheaper
GPS: 25 seconds * 140mA = 1mAh
Network: 2 seconds * 180mA = 0.1mAh
1.5 uses AGPS when network available
GPS time-to-fix varies wildly based on environment, and
desired accuracy, and might outright fail
Just like wake-locks, location updates can continue
after onPause(), so make sure to unregister
If all apps unregister correctly, user can leave GPS
enabled in Settings

Foreground apps

Floating point math is expensive
Using microdegrees when doing bulk geographic math
// GeoPoint returns value 37392778, -122041944
double lat = GeoPoint.getLatitudeE6() / 1E6;
double lon = GeoPoint.getLongitudeE6() / 1E6;

Caching values when doing DPI work with
DisplayMetrics
float density =
getResources().getDisplayMetrics().density;
int actualWidth =
(int)(bitmap.getWidth() * density);

Foreground apps

Accelerometer/magnetic sensors
Normal: 10mA (used for orientation detection)
UI: 15mA (about 1 per second)
Game: 80mA
Fastest: 90mA
Same cost for accelerometer, magnetic, orientation
sensors on HTC Dream

Background apps

Background apps

Services should be short-lived; these aren't daemons
Each process costs 2MB and risks being
killed/restarted as foreground apps need memory
Otherwise, keep memory usage low so you're not the
first target
Trigger wake-up through AlarmManager or with <receiver>
manifest elements
stopSelf() when finished

Background apps

Start service using AlarmManager
Use the _WAKEUP flags with caution
App that updates every 30 minutes, but only when
device is already awake
AlarmManager am = (AlarmManager)
context.getSystemService(Context.ALARM_SERVICE);

Intent intent = new Intent(context, MyService.class);
PendingIntent pendingIntent =
PendingIntent.getService(context, 0, intent, 0);

long interval = DateUtils.MINUTE_IN_MILLIS * 30;
long firstWake = System.currentTimeMillis() + interval;

am.setRepeating(AlarmManager.RTC,
firstWake, interval, pendingIntent);

Background apps

Use setInexactRepeating() so the system can bin your
update together with others

Background apps

Start your service using <receiver> in manifest
Intent.ACTION_TIMEZONE_CHANGED
ConnectivityManager.CONNECTIVITY_ACTION
Intent.ACTION_DEVICE_STORAGE_LOW
Intent.ACTION_BATTERY_LOW
Intent.ACTION_MEDIA_MOUNTED
<receiver android:name=".ConnectivityReceiver">
<intent-filter>
<action android:name=
"android.net.conn.CONNECTIVITY_CHANGE" />
</intent-filter>
</receiver>

Background apps

Dynamically enabling/disabling <receiver> components in
manifest, especially when no-ops
<receiver android:name=".ConnectivityReceiver"
android:enabled="false">
...
</receiver>

ComponentName receiver = new ComponentName(context,
ConnectivityReceiver.class);
PackageManager pm = context.getPackageManager();
pm.setComponentEnabledSetting(receiver,
PackageManager.COMPONENT_ENABLED_STATE_ENABLED,
PackageManager.DONT_KILL_APP);

Background apps

Checking current battery and network state before running
a full update
public void onCreate() {
// Register for sticky broadcast and send default
registerReceiver(mReceiver, mFilter);
mHandler.sendEmptyMessageDelayed(MSG_BATT, 1000);
}

IntentFilter mFilter =
new IntentFilter(Intent.ACTION_BATTERY_CHANGED);

BroadcastReceiver mReceiver = new BroadcastReceiver() {
public void onReceive(Context context, Intent intent) {
// Found sticky broadcast, so trigger update
unregisterReceiver(mReceiver);
mHandler.removeMessages(MSG_BATT);
mHandler.obtainMessage(MSG_BATT, intent).sendToTarget();
}
};

Users will be watching!

SpareParts has "Battery history"
1.5 is already keeping stats on
which apps are using CPU,
network, wakelocks
Simplified version coming in
future, and users will uninstall
apps that abuse battery
Consider giving users options for
battery usage, like update intervals,
and check the "no background data"
flag

Takeaways

Use an efficient parser and GZIP to make best use of
network and CPU resources
Services that sleep or poll are bad, use <receiver> and
AlarmManager instead
Disable manifest elements when no-op
Wake up along with everyone else (inexact alarms)
Wait for better network/battery for bulk transfers
Give users choices about background behavior

Q&A
Post your questions for this talk on Google Moderator:
code.google.com/events/io/questions

W 0300 codingfor_life-batterylifethatis

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