在Android中，即使当用户退出应用程序之后，应用程序的进程也还是存在于系统中，这样是为了方便程序的再次启动，但是这样的话，随着打开的程序数量的增加，系统的内存会变得不足，就需要杀掉一部分进程以释放内存空间。至于是否需要杀死一些进程和哪些进程需要被杀死，是通过Low Memory Killer机制来进行判定的。
Android的Low Memory Killer基于Linux的OOM机制，在Linux中，内存是以页面为单位分配的，当申请页面分配时如果内存不足会通过以下流程选择bad进程来杀掉从而释放内存：
alloc_pages -> out_of_memory() -> select_bad_process() -> badness()
在Low Memory Killer中通过进程的oom_adj与占用内存的大小决定要杀死的进程，oom_adj越小越不容易被杀死。
Low Memory Killer Driver在用户空间指定了一组内存临界值及与之一一对应的一组oom_adj值，当系统剩余内存位于内存临界值中的一个范围内时，如果一个进程的oom_adj值大于或等于这个临界值对应的oom_adj值就会被杀掉。
可以通过修改/sys/module/lowmemorykiller/parameters/minfree与/sys/module/lowmemorykiller/parameters/adj来改变内存临界值及与之对应的oom_adj值。minfree中数值的单位是内存中的页面数量，一般情况下一个页面是4KB。
比如如果向/sys/module/lowmemorykiller/parameters/adj写入0,8，向/sys/module/lowmemorykiller/parameters/minfree中写入1024,4096，假设一个页面大小为4KB，这样当系统空闲内存位于1024*4~4096*4KB之间时oom_adj大于等于8的进程就会被杀掉。
在lowmemorykiller.c中定义了阈值表的默认值，可以通过init.rc自定义：
static int lowmem_adj[6] = { 0, 1, 6, 12, }; static int lowmem_adj_size = 4; static size_t lowmem_minfree[6] = { 3 * 512, /* 6MB */ 2 * 1024, /* 8MB */ 4 * 1024, /* 16MB */ 16 * 1024, /* 64MB */ };
static int lowmem_minfree_size = 4;
在init.rc中定义了init进程的oom_adj为-16，不可能会被杀死(init的PID是1)：
on early-init # Set init and its forked children's oom_adj. write /proc/1/oom_adj -16
在Linux中有一个kswapd的内核线程，当linux回收内存分页的时候，kswapd线程将会遍历一张shrinker链表，并执行回调，定义如下：
/* * A callback you can register to apply pressure to ageable caches. * * 'shrink' is passed a count 'nr_to_scan' and a 'gfpmask'. It should * look through the least-recently-used 'nr_to_scan' entries and * attempt to free them up. It should return the number of objects * which remain in the cache. If it returns -1, it means it cannot do * any scanning at this time (eg. there is a risk of deadlock). * * The 'gfpmask' refers to the allocation we are currently trying to * fulfil. * * Note that 'shrink' will be passed nr_to_scan == 0 when the VM is * querying the cache size, so a fastpath for that case is appropriate. */
struct shrinker { int (*shrink)(int nr_to_scan, gfp_t gfp_mask); int seeks; /* seeks to recreate an obj */ /* These are for internal use */ struct list_head list; long nr; /* objs pending delete */ }; #define DEFAULT_SEEKS 2 /* A good number if you don't know better. */ extern void register_shrinker(struct shrinker *); extern void unregister_shrinker(struct shrinker *);
通过register_shrinker与unregister_shrinker向shrinker链表中添加或移除回调。当注册Shrinker后就可以在回收内存分页时按自己定义的规则释放内存。
Android Low Memory Killer的代码在drivers/staging/android/lowmemorykiller.c中，通过以下代码在模块初始化时注册Shrinker：
static int lowmem_shrink(int nr_to_scan, gfp_t gfp_mask); static struct shrinker lowmem_shrinker = { .shrink = lowmem_shrink, .seeks = DEFAULT_SEEKS * 16 }; static int __init lowmem_init(void) { register_shrinker(&lowmem_shrinker); return 0; } static void __exit lowmem_exit(void) { unregister_shrinker(&lowmem_shrinker); } module_init(lowmem_init); module_exit(lowmem_exit);
这样就可以在回收内存分页时调用lowmem_shrink函数。
Low Memory Killer的实现
lowmem_shrink的定义如下：
static int lowmem_shrink(int nr_to_scan, gfp_t gfp_mask) { struct task_struct *p; struct task_struct *selected = NULL; int rem = 0; int tasksize; int i; int min_adj = OOM_ADJUST_MAX + 1; int selected_tasksize = 0; int selected_oom_adj; int array_size = ARRAY_SIZE(lowmem_adj); int other_free = global_page_state(NR_FREE_PAGES); int other_file = global_page_state(NR_FILE_PAGES); if (lowmem_adj_size < array_size) array_size = lowmem_adj_size; if (lowmem_minfree_size < array_size) array_size = lowmem_minfree_size; for (i = 0; i < array_size; i++) { if (other_free < lowmem_minfree[i] && other_file < lowmem_minfree[i]) { min_adj = lowmem_adj[i]; break; } } if (nr_to_scan > 0) lowmem_print(3, "lowmem_shrink %d, %x, ofree %d %d, ma %d\n", nr_to_scan, gfp_mask, other_free, other_file, min_adj); rem = global_page_state(NR_ACTIVE_ANON) + global_page_state(NR_ACTIVE_FILE) + global_page_state(NR_INACTIVE_ANON) + global_page_state(NR_INACTIVE_FILE); if (nr_to_scan <= 0 || min_adj == OOM_ADJUST_MAX + 1) { lowmem_print(5, "lowmem_shrink %d, %x, return %d\n", nr_to_scan, gfp_mask, rem); return rem; } selected_oom_adj = min_adj; read_lock(&tasklist_lock); for_each_process(p) { struct mm_struct *mm; int oom_adj; task_lock(p); mm = p->mm; if (!mm) { task_unlock(p); continue; } oom_adj = mm->oom_adj; if (oom_adj < min_adj) { task_unlock(p); continue; } tasksize = get_mm_rss(mm); task_unlock(p); if (tasksize <= 0) continue; if (selected) { if (oom_adj < selected_oom_adj) continue; if (oom_adj == selected_oom_adj && tasksize <= selected_tasksize) continue; } selected = p; selected_tasksize = tasksize; selected_oom_adj = oom_adj; lowmem_print(2, "select %d (%s), adj %d, size %d, to kill\n", p->pid, p->comm, oom_adj, tasksize); } if (selected) { lowmem_print(1, "send sigkill to %d (%s), adj %d, size %d\n", selected->pid, selected->comm, selected_oom_adj, selected_tasksize); force_sig(SIGKILL, selected); rem -= selected_tasksize; } lowmem_print(4, "lowmem_shrink %d, %x, return %d\n", nr_to_scan, gfp_mask, rem); read_unlock(&tasklist_lock); return rem; }