Assess Energy Usage

Metrics that relate to energy use and can be viewed in the Energy pane of Activity Monitor (see Figure 16-1). Use the View > Columns menu to enable or customize the metrics displayed.

Figure 16-1Energy columns in Activity Monitor

As shown in Figure 16-2, graphs denoting overall energy impact and battery statistics are displayed at the bottom of the Energy pane.

Figure 16-2Energy statistic graphs in Activity Monitor

The Energy pane in Activity Monitor displays the following columns:

• Preventing Sleep. Indicates when an app is preventing the system from sleeping, perhaps because it has initiated activity that has disabled sleep (see Determine When Your App Is Preventing Sleep).

• Energy Impact. Assigns an energy impact score to your app, based on how efficiently it is operating. A variety of factors are taken into account when this score is assigned, including CPU usage, network activity, disk I/O, and more.

• Avg Energy Impact. Indicates the average energy impact score over the past 8 hours, or since the time the Mac was restarted.

• App Nap. Indicates when an app has been placed in App Nap (see Extend App Nap). You can use this column to monitor your running app and make sure App Nap activates when it’s placed in the background and hidden or fully obscured by another app.

Assess CPU Usage

High CPU usage is a major contributing factor to high energy consumption, and Activity Monitor implements a variety of features that can help assess the amount of CPU your app is utilizes. The CPU pane shows the percentage of CPU your app is using at any given time, in relation to the other active processes on the system, as shown in Figure 16-3. When your app isn’t active, the number in this column should be 0.0. High values here, especially consistent ones, indicate that your app may not be as energy efficient as it could be. You should look carefully at your app for ways to reduce its CPU requirements.

Figure 16-3CPU columns in Activity Monitor

Graphs and statistics related to overall system CPU usage are displayed at the bottom of the CPU pane. See Figure 16-4.

Figure 16-4CPU statistic graphs in Activity Monitor

Activity Monitor also includes provides graphical displays of historical and current state CPU core utilization.

• CPU Usage. Represents system-wide usage across all CPU cores. To display this window, choose Window > CPU Usage. See Figure 16-5.

  Figure 16-5The CPU Usage dialog in Activity Monitor

• CPU History. Represents system-wide usage across all CPU cores over a period of time. It begins to capture usage once the window is displayed, and updates as usage fluctuates. To display this window, choose Window > CPU History. See Figure 16-5.

  Figure 16-6The CPU History dialog in Activity Monitor

Debugging Gauges

The debug navigator in Xcode (choose View > Navigators > Show Debug Navigator) provides a series of gauges that allow you to analyze the energy impact of your app while testing it within Xcode. These gauges are displayed when your app has been launched by Xcode and is actively running or paused.

• Energy impact gauge. Displays a report of your app’s current energy impact and a bar graph of its recent impact. When users interact with your app, it should have a low energy impact. When users aren’t interacting with your app, it should have zero energy impact.

  Figure 16-7Monitoring energy impact in Xcode

• CPU gauge. Monitors your app and reports on its current and historical CPU usage. Spikes when your app is supposed to have low CPU activity or when your app is idle may indicate problem areas where optimizations can be made.

  Figure 16-8Monitoring CPU usage in Xcode

• Disk usage gauge. Alerts you to disk read and write activity and files your app has opened. Use this gauge to identify unexpected or recurring small I/O activity.

  Figure 16-9Monitoring disk I/O in Xcode

• Network usage gauge. Accounts for all inbound and outbound network traffic. Look for discretionary activity that your app performs directly, and consider updating it to be performed by the system at more energy optimal times.

  Figure 16-10Monitoring network activity in Xcode

Debugging

Knowing how to test and debug your app can go a long way toward helping you identify potential energy problems and areas for improvement. Debug Your App in Xcode Overview and Testing with Xcode describe common debugging techniques. If your app performs tasks asynchronously using GCD, NSOperationQueue, or XPC, debugging can be a little harder. There are, however, techniques that can help you debug your app even in these complex situations.

• Queue debugging. If your app uses GCD dispatch queues to perform operations, use queue debugging techniques to ensure that work is being performed as expected. To learn about GCD, dispatch queues, and queue debugging, watch the session WWDC 2013: Debugging with Xcode, and review the Concurrency Programming Guide and Grand Central Dispatch (GCD) Reference.

• Activity tracing. This technology makes finding bugs faster and easier by logging trace messages as your app runs. When a problem occurs, follow these messages to identify the code that produced the failure. For information on activity tracing, see the session WWDC 2014: Fix Bugs Faster Using Activity Tracing.