Skip to content

memory_management.md

Latest commit

 

History

History
58 lines (38 loc) · 3.83 KB

memory_management.md

File metadata and controls

58 lines (38 loc) · 3.83 KB

Memory Management

Stack vs Heap in memory

All stored in RAM. But thread have it own stack but share heap memory.

Stack Heap
Auto delete memory when variable go out of scope Remain until collect by program or GC triggerd
Fixed size -> cause StackOverflow Can get more memory from OS
Faster, only need to move pointer to allocate Slower, synchronize to keep thread safe, complex flow to allocate
Attach to thread, when thread stop -> reclaim Attach to app process, process exit -> reclaim
Store local data, return data, params address, primitive variables Store block of data, all object instance, static variable (Perm Gen)
Fixed size vary by Android API Max 64Mb default in Java, max Android heap size vary by device's screen size
Cause StackOverflow Cause memory fragmentation, memory leaks, OutOfMemoryError

Android heap

Android heap contain Java heap, native heap and ashmem.

Note that memory usage on modern operating systems like Linux is an extremely complicated and difficult to understand area.

Android limit heap size per process. Max heap size is vary by screen resolution. As higher-resolution screens tend to want to manipulate larger bitmaps -> need more heap -> Google makes heap size recommendations and hope device manufacturers will abide by.

OutOfMemory error mostly cause from bitmaps, leak memory that GC cannot free. Enable hardware accelerate (allow draw canvas using GPU) will consume more RAM memory.

  • getRuntime().maxMemory() is total bytes of heap that our app is allowed to use.
  • ActivityManager.getMemoryClass() is total MB of heap that our app should use to respect limit of device.
  • Read heapstartsize, heapgrowlimit, heapsize information inside system/build.prop file.
  • Put android:largeHeap="true" in AndroidManifest.xml let the heap grow from heapgrowlimit to heapsize (Android API >= 11). But no guarantee.
  • Request native heap using NDK JNI.

Java heap

Eden S0 S1 Old Gen Perm Gen
Major GC

Native heap

The native heap used by the C++ new operator. There is much more memory available here. The app is limited only by the physical memory available on the device. There is no garbage collection and nothing to slow things down. However, C++ programs are responsible for freeing every byte of memory they allocate, or they will leak memory and eventually crash.

The native heap is managed by dlmalloc(), which uses a combination of mmap() and standard calls like sbrk() to allocate memory. The managed ("Dalvik") heap is (mostly) one large chunk allocated with mmap(). It's all running on top of the Linux kernel.

Ashmem (ashmem.c)

  • short term for Android Shared Memory.
  • This operates much like the native heap but has additional system calls.
  • Share memory between process using memory maps by name.
  • Android can "unpin" the memory rather than freeing it. This is a lazy free; the memory is freed only if the system actually needs more memory. Then "pin" later on when device have enough memory back.
  • Better support low-mem devices because it can discard shared mem unit under memory pressure.
  • Removed in Android 5.0 and higher.
  • Using C.