Fabián Baptista @fbaptista

Android Memory Management

Fabián Baptista @fbaptista

Android Memory Management

Talk about…Android ArchitectureConcepts

How it works

Tools and logs

Under the hood…• Since Android is built on top of a Linux

Kernel, we should know a little bit about memory management on Linux …

• Other key topics that helps a lot:– Virtual Memory– Android Activity Lifecycle– Garbage Collection–Memory Heap

Architecture

Linux Kernel• Linux 2.6 + Google architecture changes.• Android core functionality is based on

top of Linux– Hardware interface & drivers–Memory Management– Process Management– Networking– Security– Etc.

Architecture

Android Runtime• Dalvik Virtual Machine– Like a special Java VM – Optimized for mobile world– Not .class -> .dex (bulit from a .class)

–Multiple instances of the VM running

• Core Libraries–Most of the functionalities of the Java SE

libraries

Architecture

Virtual Memory

• Maps process memory addresses (virtual) into physical addresses.

• Hardware: Address translation inside CPU (called MMU)

• Software: Extend physical memory size

Virtual Memory - Key Benefits• Avoid process to handle a shared

space.• More security & isolation• More memory available• Easier programming / hidden

fragmentation

Concepts• Swap Space– Extend physical (full) memory with HD

• Paging– Uses HD to store & retrieve “pages” of memory.– Allow non-contiguous address space for process – One of the most important part of typical Virtual

Memory implementation• Shared Memory• Heap

Garbage Collection

Garbage Collection

Android Way• Memory:– No Swap space, Paging–Memory mapping

• Shared Memory– Framework code & resources– Intercommunication (processes)– Intensively used

Android Way• New drivers:– ASHMEM (re-implemented)– Low Memory Killer (handle OOM situations)

• Empty Process–When a process is terminated, not all

memory is released.– Reduces response time when user access

same process.

Garbage Collection1. Pre Gingerbread:– GC “stops the world”– Full heap collection– Pause time often > 100ms

2. Gingerbread and beyond:– Concurrent (mostly)– Partial Collections– Pause time usually < 5ms

Garbage Collection - MsgTypes

• GC_CONCURRENT: Occurs when heap is growing (to avoid enlarge heap in time)

• GC_EXPLICIT: Triggered when an application calls System.gc(). Don’t use it!

• GC_FOR_MALLOC: Triggered when the heap is full and the application needs more memory. (stops app)

• GC_HPROF_DUMP_HEAP: Triggered when an HPROF file is created for memory analysis

• GC_BEFORE_OOM: (Undocumented) It’s supposed to be triggered when dalvik heap usage is over 90%

Android App Memory• Dalvik heap– Defines logical heap size–Memory can grow to a defined limit

(defined by the system)– One single virtual memory range– Physically non-contiguous (analyzed on GC)

• Logical size is different than Physical size• Hard to know exactly how much RAM

your process are using

Memory Numbers…

Proportional Set Size (PSS)• PSS is considered by Android as your

physical footprint

• Example of Dalvik Heap usage:– 20MB of (Private)– 12MB of Shared (between 4 process)– PSS = 20 + (12/4) = 23

Memory Numbers…

Private RAM• Private (Dirty and Clean) is memory

used by the App.• All heap allocations (Dalvik + Native)• Memory that will be available if your

process is destroyed.• Can’t be paged into storage

Memory Numbers…

Low Memory Killer (Linux)• Look for “bad” process (heuristics):– High memory consuming is bad– Long running process are good– Process with a lot of child process are

bad– Root processes are good (3% off)

Low Memory Killer (Android)

5 - Empty Process

4 - Background Process

3 - Started Services Process

2 - Visible Process

1 - Active Process Critical Priority

High Priority

Low Priority

Low Memory Killer (Android)

Threshold varies based on RAM size. (Loaded during booting time from init.rc file)

Memory Pages(4KB each)

3 Memory Tips 4 Apps1. Use Services in the right way– Only one process should take care of UI– Take care about dependencies

2. Release memory when– Your UI becomes hidden– Memory becomes tight

(ComponentCallbacks)

3. Use the right Bitmaps for each screen resolution

Services• Application component without UI.• Can be used to perform long-running

operations in background.• Inter-process communication• Provide a client-server model to bind

with

Services State

Started• startService()

Bound• bindService()

Services Started> startService();

• Can run indefinitely• Even if the activity who started it is destroyed

• Once it is finished, the service should stop itself

Services Bound> bindService();

• Client / Server interface to:• Request service• Get results• IPC

• Bind by one• Is destroyed only when all of them unbind

Release Memory On…• interface hidden:– implement onTrimMemory() callback

(Activity classes)• memory becomes tight:– onTrimMemory() (added in API level 14)– onLowMemory() (for older versions)

Bitmaps• Load the bitmap that your app

needs!– Scale down image to your resolution– Or you need more than 1 resolution per

image

• API_level <= 10:

Bitmaps• Load the bitmap that your app

needs!– Scale down image to your resolution– Or you need more than 1 resolution per

image

• API_level >= 11: (Honeycomb & higher)

Tools & logs– Logs:

• Dalvik Log Messages: All Garbage Collection logs• ART: Logged when GC are deemed slow:

– GC Pauses > 5ms– GC duration > 100ms– Explicit GCs

– Android Device Monitor for tracking Allocations– MAT: Eclipse Memory Analyzer Tool– Monkop: Memory Footprints on real devices

• GC behavior• RAM footprint• Threads• CPU / Network and others

References• http://www.android-app-market.com/android-

architecture.html• https://developer.android.com/training/articles/

memory.html• Android Memory Management

https://gist.github.com/blixtra/ca6e1f289462fe2679af• Google I/O 2011: Memory management for Android

Appshttps://www.youtube.com/watch?v=_CruQY55HOk

• Low Ram – Android 4.4 Optimizationshttp://source.android.com/devices/tech/low-ram.html

Thank you