Understanding the Garbage Collection Logs in Android Applications

Memory utilisation on the mobile app has a significant impact on the customer experience. If your app creates a lot of objects, then Android Run Time (ART) environment will trigger Garbage Collection (GC) frequently. Android Garbage Collection is an automatic process which removes unused objects from memory. However, frequent Garbage Collection consumes a lot of CPU + it will also pause the app. Frequent pauses can jank the app (i.e. stuttering, juddering, or halting).

Thus, you need to understand how many objects your app is creating, how frequently Garbage collection is triggered, how much time it takes to complete, how much memory is reclaimed after every event. All this information is present in the runtime log messages. Whenever a GC event runs, a log line is printed in the runtime log messages. You can view those log lines through logcat.

This is how a typical ART GC log line will look:

07-01 16:00:44.690: I/art(801): Explicit concurrent mark sweep GC freed 65595(3MB) 
AllocSpace objects, 9(4MB) LOS objects, 34% free, 38MB/58MB, paused 1.195ms total 87.219ms

This one single log line has multiple fields. Let’s review them:

#	Field	Value	Description
1	Timestamp	07-01 16:00:44.690	Timestamp at which this Garbage Collection event ran
2	GC Reason	Explicit	Reason why Garbage Collection event was triggered. ‘Explicit’ indicates that garbage collection was explicitly requested by an app, for example, by calling gc() or gc().
3	GC Name	concurrent mark sweep	ART has various different GCs which can get run. In this case, ‘Concurrent Mark Sweep’ indicates – a whole heap collector which frees collects all spaces other than the image space.
4	Objects freed	65595(3MB)	Amount of objects garbage collected from non-large objects space. Totally 65595 unique objects, whose cumulative size of 3mb is garbage collected (i.e. freed) in this event.
5	Large objects freed	9(4MB)	Amount of objects garbage collected from Large objects space. Totally 9 unique objects, whose cumulative size of 4mb is garbage collected (i.e. freed)
6	Heap usage	38MB/58MB	38mb of objects is alive after this particular GC event. Total allocated heap size for this application is 58mb.
7	GC Pause Time	1.195ms	During certain phases of GC event, the application is paused. In this GC event, pause time is 1.195ms. During pause time, application freezes. One should target for low pause time.
8	GC Total Time	87.219ms	Amount of time this GC event took to complete. It includes the GC Pause time as well.

Tools

<img class="wp-image-2240 size-full" src="https://bloggceasy.files.wordpress.com/2017/05/art-1.png?w=736" alt="ART-1" width="736" height="367" /> Fig 1: Android memory usage – report generated by GCeasy.io<strong><em> </em></strong> <img class="wp-image-2241 size-full" src="https://bloggceasy.files.wordpress.com/2017/05/art-2.png?w=736" alt="ART-2" width="736" height="383" /> Fig 2: Android GC Statistics – report generated by GCeasy.io

As it might get tedious to analyse every single GC log line, you can also consider using free online tools such as GCeasy.io – a universal Garbage collection log analyser, to analyses Android GC logs. You can just upload the Android GC Logs to this tool. Tool parses GC log and generates report instantly. The report contains interactive graphs and insightful metrics. A Few excerpts from this report were given above for your reference.