Android Memory Management

Transcription

1 Android Memory Management Rajesh prodduturi ( ) Under the Guidance of Prof. D.B.phatak Department of Computer Science and Engineering, Indian Institute Of Technology, Bombay August 10, 2012 (Rajesh prodduturi) Android Memory Management August 10, / 15

2 INTRODUCTION Outline This presentation gives an overview of the following: OOM in linux Low Memory Killer in Android ashmem pmem (Rajesh prodduturi) Android Memory Management August 10, / 15

3 Out Of Memory (OOM) in Linux If applicaitons running in a system have memory leaks. Then in worst case the virtual memory of all the process is > (physical memory + swap) To avoid the above situations we need OOM. In low memory situations, if page frame reclaiming algorithm fails to free free pages > some threshold, then it calls OOM. Order of system calls alloc pages() > try to free pages(), in low memory situations. if try to free pages() function fails to free pages then it calls out of memory (Rajesh prodduturi) Android Memory Management August 10, / 15

4 OOM source[ulk] (Rajesh prodduturi) Android Memory Management August 10, / 15

5 OOM in Linux How Does OOM Kills Process: Calculate score for each process using badness(), and remove the process which have highest scores. out of memory() > select bad process() > badness() some of the heuristics are given below: 1 high memory occupied process gets high score 2 long runing process gets low score 3 process which have large no of child process, to be killed. 4 do not kill super users process (Rajesh prodduturi) Android Memory Management August 10, / 15

6 Low Memory Killer in Android How it differ from OOM Low Memory Killer of the Android behaves a bit different against OOM killer. It clasifies process into different groups. In low memory situation it starts killing the process from low priority groups. List of different groups: Foreground(active), user interacting applicaitons. visible, process which are not in foreground but responding to user events. Started Service Processes, Background Processes Empty processes which were already closed. But the memory allocated to that process was still in memory. (Rajesh prodduturi) Android Memory Management August 10, / 15

7 Low Memory Killer in Android Order of killing process source[ (Rajesh prodduturi) Android Memory Management August 10, / 15

8 Low Memory Killer in Android How Does Low Memory Killer Works: It stores oom adj values for each group of processes. oom adj value ranges from -17 to 16. Foreground 0 visible 1 Background Processes 2 Empty processes 15 It also stores memory thresholds for each group of processes at which these process class will be killed. All these values are in in number of pages(4kb). Foreground 1536 visible 2048 Background Processes 4096 Empty processes 6144 Memory thersholds are varies based on RAM size. (Rajesh prodduturi) Android Memory Management August 10, / 15

9 ashmem - Android shared memory Implemented in mm/ashmem.c It better-supports low memory devices, because it can discard shared memory units under memory pressure. shmem(linux) vs ashmem(android) Ashmem classifies shared memory pages into two types. One is pinned pages. another one is unpinned pages. In low memory situations kernel can evict unpinned pages. Pinned pages sholud be removed only after unpinned pages. (Rajesh prodduturi) Android Memory Management August 10, / 15

10 ashmem - Android shared memory Creating a shred memory region: fd = ashmem c reate r egion( my s hm r egion,size); if(fd 0) return -1; data = mmap(null, size, PROT READ PROT WRITE, MAP S HARED,fd,0); if(data == MAP FAILED) goto out; If another processes wants to access shared memory, it does not access through file name. Because of security issues. Process share the fd of shared memory through IPC binder. (Rajesh prodduturi) Android Memory Management August 10, / 15

11 PMEM (process memory allocator) pmem is used to manage large (1-16+MB) physically contiguous regions of memory shared between userspace and kernel drivers (dsp, gpu,etc). Due to some hardware limitations in devices like MSM7201A, they need continous memory. ashmem and pmem are very similar. Both are used for sharing memory between processes. ashmem uses virtual memory, whereas pmem uses physically contiguous memory. (Rajesh prodduturi) Android Memory Management August 10, / 15

12 PMEM (process memory allocator) Difference Between ashmem and pmem: ashmem maintains ref-count for each shared region. ref-count represents that how many process are currently accessing shared memory. If ref-count is zero, then no process access that shared memory. Now low memory killer can remove this region. pmem does not work in above manner.because it needs to maintain a physical to virtual mapping. The process which creates shared memory through pmem should hold file descriptor untill all the refernces are closed. Application can Choose ashmem or pmem depending upon the size of physically shared contiguous memory. (Rajesh prodduturi) Android Memory Management August 10, / 15

13 No Swap space in Android Reason: Due to wear-levling limitation of flash memory. Solution: Compressed cache (Compcache) Compcache project implements swap space inside physical memory. It compress the pages and swaps into Compcache. Whenever the application needs it decompress the page and returns back. Limitation: Reduce in performance Cpu needs to spend some time for compression and decompression. (Rajesh prodduturi) Android Memory Management August 10, / 15

14 REFERENCES I Killer Kernel Features (Rajesh prodduturi) Android Memory Management August 10, / 15

15 Thank You (Rajesh prodduturi) Android Memory Management August 10, / 15