When we launch an app by tapping its icon on an Android device, a series of memory management steps happen behind the scenes. In this post, we’ll break down the process with a practical example.
1. RAM Distribution
Consider a device with 8GB of RAM. Out of this:
- 2GB is reserved for the Android operating system.
- The remaining 6GB is available for all user apps running on the device.
2. Maximum Memory Allocation per App
Each app is allowed a maximum amount of memory, which is determined by the device manufacturer and Android version. For example, let’s say:
- Max memory per app: 192MB (this value can vary by device).
When an app is opened:
- The system allocates up to 192MB from the available 6GB RAM for app.
- This is called
maxMemoryInMB.
3. How Much Memory Does an App Use at Startup?
At launch, the system doesn’t know how much memory app will actually need, so it reserves the maximum allowed (192MB).
- totalMemory: 192MB (dedicated to app, not shared with others)
- usedMemory: 3MB (app actual usage at start)
- freeMemory: 189MB (unused portion of the allocation)
4. largeHeap Flag
- By default, most apps get a “small” heap (e.g., 192MB).
- If we set the
largeHeapflag in our app’s manifest, Android may allow a larger heap (e.g., 512MB), but this depends on the device and manufacturer. - We cannot set this value directly; it’s managed by the system.
5. How Android Dynamically Shrinks Memory
As Our app runs, the system monitors its memory usage. If our app continues to use only a small amount of memory (say, 5MB after a few seconds), Android may decide that our app doesn’t need the full 192MB. The system can then deallocate some of the reserved memory, reducing totalMemory to a smaller value (e.g., 8MB).
6. What Happens When our App Uses More Memory?
If our app starts creating more objects or loading more data, usedMemory increases:
- After some time:
usedMemory= 32MB - Later:
usedMemory= 56MB, 75MB, up to 186MB
The system will allocate more memory as needed, up to the maximum limit.
7. OutOfMemoryError
If the app tries to use more memory than the maximum allowed, it will encounter an OutOfMemoryError.
This happens because Android enforces strict memory limits to ensure system stability and prevent any single app from using too much RAM.
Diagram: Android App Memory Lifecycle
maxMemory limit = 192MB
│
▼
+-------------------------------+
| App Launched (Icon Tap) |
+-------------------------------+
|
v
+-------------------------------+
| System allocates maxMemory |
| (e.g., 192MB) from available |
| RAM for the app |
+-------------------------------+
|
v
+-------------------------------+
| Initial memory usage: |
| totalMemory: 192MB |
| usedMemory: 3MB |
| freeMemory: 189MB |
+-------------------------------+
|
v
+-------------------------------+
| App does a bit more work: |
| totalMemory: 192MB |
| usedMemory: 4MB |
| freeMemory: 188MB |
+-------------------------------+
|
v
+-------------------------------+
| System optimizes for small |
| usage, reduces allocation: |
| totalMemory: 8MB |
| usedMemory: 5MB |
| freeMemory: 3MB |
+-------------------------------+
|
v
+-------------------------------+
| App creates more objects: |
| totalMemory: 32MB |
| usedMemory: 32MB |
| freeMemory: 0MB |
+-------------------------------+
|
v
+-------------------------------+
| App continues to grow: |
| totalMemory: 56MB |
| usedMemory: 56MB |
| freeMemory: 0MB |
+-------------------------------+
|
v
+-------------------------------+
| App continues to grow: |
| totalMemory: 75MB |
| usedMemory: 75MB |
| freeMemory: 0MB |
+-------------------------------+
|
v
+-------------------------------+
| App approaches memory limit: |
| totalMemory: 169MB |
| usedMemory: 169MB |
| freeMemory: 0MB |
+-------------------------------+
|
v
+-------------------------------+
| App nearly at max: |
| totalMemory: 184MB |
| usedMemory: 184MB |
| freeMemory: 0MB |
+-------------------------------+
|
v
+-------------------------------+
| Final stretch before limit: |
| totalMemory: 192MB |
| usedMemory: 192MB |
| freeMemory: 0MB |
+-------------------------------+
|
v
+-------------------------------+
| If usedMemory > maxMemory: |
| OutOfMemoryError! |
+-------------------------------+