1  ---
  2  id: ENERGY_GUIDE_DEV
  3  title:
  4  sidebar_label: Energy Guide
  5  ---
  6  
  7  ##  What impacts battery performance?
  8  
  9  ### 1. CPU
 10  That one is obvious, if the app consumes too much CPU, it won't be energy efficient.
 11  
 12  Is's important to test this data *on a device*.
 13  
 14  1. Desktop CPUs have different architecture and some algorithms that are hardware accelerated on a desktop-class CPU might cause bottlenecks in a mobile processor.
 15  
 16  2. Mobile operating systems might have different syscalls implemented with different efficency.
 17  
 18  Hence, everything should be profiled and optimized in an environment as close to what an end user will have.
 19  
 20  **Instruments to triage CPU bottlenecks**
 21  iOS: Instruments (Time Profiler)
 22  
 23  ### 2. Networking
 24  
 25  One important trick that allows devices to live that long from a battery is that they try to switch to a low-power mode as soon as it is possible.
 26  
 27  That relates not only to CPU but also to the networking stack. Radios that are used in the wireless connections are very resource-hungry. To workaround that limitation, system tries to turn them on fully only when it absolutely has to.  That is usually some timeout after the network packets were sent or received.
 28  
 29  One important consequence of that fact is that if the packets are all sent and
 30  received randomly, the radio will never have a chance to switch into a low
 31  power mode.
 32  
 33  To avoid that, networking should happen in "bursts", when during a short period of time a lots of packets are sent/received.
 34  
 35  ![](https://developer.apple.com/library/content/documentation/Performance/Conceptual/EnergyGuide-iOS/Art/new_chart_2x.png)
 36  *source: [Apple: Energy & Networking](https://developer.apple.com/library/content/documentation/Performance/Conceptual/EnergyGuide-iOS/EnergyandNetworking.html)*
 37  
 38  #### 2.1. Networking, Energy and open sockets.
 39  If socket connection is open, but isn't active (no reads or writes happening) that doesn't prevent radios to go into a low-power mode.
 40  Even if there is a read operation from a socket, if a server doesn't write anything, it still helps with energy preservation.
 41  That way, by grouping socket messages, we can improve the battery performance.
 42  
 43  *The way how energy efficient networking can be controlled by a server*
 44  
 45  Client code (no specific optimizations)
 46  ```go
 47  for {
 48         tcpAddr, _ := net.ResolveTCPAddr("tcp4", address)
 49         conn, _ := net.DialTCP("tcp", nil, tcpAddr)
 50         for {
 51                 result := make([]byte, 10)
 52                 io.ReadFull(conn, result)
 53                 fmt.Println(string(result))
 54         }
 55  }
 56  
 57  ```
 58  
 59  Server code (energy efficient).
 60  
 61  ```python
 62  import socket
 63  import time
 64  import sys
 65  
 66  TCP_IP = '0.0.0.0'
 67  TCP_PORT = 5008
 68  MESSAGE = "1234567890"
 69  
 70  BUFFER_SIZE = 20  # Normally 1024, but we want fast response
 71  
 72  s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
 73  s.bind((TCP_IP, TCP_PORT))
 74  s.listen(1)
 75  
 76  conn, addr = s.accept()
 77  print 'Connection address:', addr
 78  try:
 79      while 1:
 80          print 'Sending', MESSAGE
 81          for i in range(500):
 82              conn.send(MESSAGE)
 83          print "sleeping"
 84          for i in range(30):
 85              print i,
 86              sys.stdout.flush()
 87              time.sleep(1) # seconds
 88  finally:
 89      conn.close()
 90      print 'Closed'
 91  ```
 92  
 93  #### 2.2 The Tradeoff
 94  
 95  There is a tradeoff between having energy efficient networking and real-time communications. The time that is needed for the radio to go to the low-power mode is quite substantal (>= 10 seconds). That can easily be seen in "overhead" graph in Xcode.
 96  That rate means that any app that requires real-time communication (a messsenger) will not be energy efficient in foreground.
 97  
 98  What is possible to do, is to reduce energy consumption is background. There, we can sync data more rarely, or only by a signal of a silent push notification.
 99  
100  
101  
102  
103  **Instruments to triage networking issues**
104  
105  iOS: Xcode (Energy Efficiency, Network), Instruments (Network)
106  
107  Android: TBD
108  
109  
110  
111  
112  ## Profiling Process: iOS
113  
114  **‼️** Instrument's "Energy Usage Log" [**seems to be broken** on iOS 10/11](https://forums.developer.apple.com/thread/70540) **‼️**
115  
116  1. Profiling should happen using Wi-Fi debugging feature.
117  Otherwise, the device will be charging during the measuring process and that will skew the data.
118  
119  2. Profiling should happen only on a device.
120  
121  3. Profiling should happen on a **release** build configuration.
122  Different debugging mechanisms can add much additional overhead, hiding the
123  actual reasons on why something isn't efficient.
124  
125  #### Tools
126  **Xcode: Debug Navigator**
127  ![](./_assets/xcode-debug-nav.png)
128  
129  What is "Overhead"? That is energy impact of "other system resources".
130  An example of that is a radio that is in high-power mode after networking data exchange.
131  
132  **Instruments: Time Profiler, Network**
133  
134  
135  
136  
137  
138  
139  ## Profiling Process: Android
140  
141  TBD
142  
143  
144  
145  
146  # Read further
147  
148  ## iOS
149  
150  
151  - [Energy Efficiency Guide for iOS Apps](https://developer.apple.com/library/content/documentation/Performance/Conceptual/EnergyGuide-iOS/index.html#//apple_ref/doc/uid/TP40015243-CH3-SW1)
152  
153  - WWDC sessions
154      - 2017: 238
155      - 2015: 708
156      - 2014: 710, 712