iOS检测卡顿/OOM

卡顿的发生通常有以下几个原因：

• UI过于复杂，图文混排的绘制量过大；
• 在主线程上进行同步的网络请求；
• 在主线程上进行大量的IO操作；
• 函数运算量过大，持续占用较高的CPU；
• 死锁和主子线程抢锁；

dump线程

iOS 的线程技术与Mac OS X类似，也是基于 Mach 线程技术实现的，在 Mach 层中thread_basic_info 结构体封装了单个线程的基本信息：

struct thread_basic_info {
    time_value_t  user_time;      /* user run time */
    time_value_t  system_time;    /* system run time */
    integer_t    cpu_usage;       /* scaled cpu usage percentage */
    policy_t     policy;          /* scheduling policy in effect */
    integer_t    run_state;       /* run state (see below) */
    integer_t    flags;           /* various flags (see below) */
    integer_t    suspend_count;   /* suspend count for thread */
    integer_t    sleep_time;      /* number of seconds that thread  has been sleeping */
}

一个Mach Task包含它的线程列表。内核提供了task_threads API 调用获取指定 task 的线程列表，然后可以通过thread_info API调用来查询指定线程的信息，在 thread_act.h 中有相关定义。

task_threads 将target_task 任务中的所有线程保存在act_list数组中，act_listCnt表示线程个数：

kern_return_t task_threads
(
    task_t target_task,
    thread_act_array_t *act_list,
    mach_msg_type_number_t *act_listCnt
);

thread_info结构如下：

kern_return_t thread_info
(
    thread_act_t target_act,
    thread_flavor_t flavor,  // 传入不同的宏定义获取不同的线程信息
    thread_info_t thread_info_out,  // 查询到的线程信息
    mach_msg_type_number_t *thread_info_outCnt  // 信息的大小
);

所以我们如下来获取CPU的占有率：

#import "LSLCpuUsage.h"
#import <mach/task.h>
#import <mach/vm_map.h>
#import <mach/mach_init.h>
#import <mach/thread_act.h>
#import <mach/thread_info.h>

@implementation LSLCpuUsage

+ (double)getCpuUsage {
    kern_return_t           kr;
    thread_array_t          threadList;         // 保存当前Mach task的线程列表
    mach_msg_type_number_t  threadCount;        // 保存当前Mach task的线程个数
    thread_info_data_t      threadInfo;         // 保存单个线程的信息列表
    mach_msg_type_number_t  threadInfoCount;    // 保存当前线程的信息列表大小
    thread_basic_info_t     threadBasicInfo;    // 线程的基本信息
    
    // 通过“task_threads”API调用获取指定 task 的线程列表
    //  mach_task_self_，表示获取当前的 Mach task
    kr = task_threads(mach_task_self(), &threadList, &threadCount);
    if (kr != KERN_SUCCESS) {
        return -1;
    }
    double cpuUsage = 0;
    for (int i = 0; i < threadCount; i++) {
        threadInfoCount = THREAD_INFO_MAX;
        // 通过“thread_info”API调用来查询指定线程的信息
        //  flavor参数传的是THREAD_BASIC_INFO，使用这个类型会返回线程的基本信息，
        //  定义在 thread_basic_info_t 结构体，包含了用户和系统的运行时间、运行状态和调度优先级等
        kr = thread_info(threadList[i], THREAD_BASIC_INFO, (thread_info_t)threadInfo, &threadInfoCount);
        if (kr != KERN_SUCCESS) {
            return -1;
        }
        
        threadBasicInfo = (thread_basic_info_t)threadInfo;
        if (!(threadBasicInfo->flags & TH_FLAGS_IDLE)) {
            cpuUsage += threadBasicInfo->cpu_usage;
        }
    }
    
    // 回收内存，防止内存泄漏
    vm_deallocate(mach_task_self(), (vm_offset_t)threadList, threadCount * sizeof(thread_t));

    return cpuUsage / (double)TH_USAGE_SCALE * 100.0;
}
@end

博主文中提到：关于 phys_footprint 的定义可以在 XNU 源码中，找到 osfmk/kern/task.c 里对于 phys_footprint 的注释，博主认为注释里提到的公式计算的应该才是应用实际使用的物理内存。

/*
 * phys_footprint
 *   Physical footprint: This is the sum of:
 *     + (internal - alternate_accounting)
 *     + (internal_compressed - alternate_accounting_compressed)
 *     + iokit_mapped
 *     + purgeable_nonvolatile
 *     + purgeable_nonvolatile_compressed
 *     + page_table
 *
 * internal
 *   The task's anonymous memory, which on iOS is always resident.
 *
 * internal_compressed
 *   Amount of this task's internal memory which is held by the compressor.
 *   Such memory is no longer actually resident for the task [i.e., resident in its pmap],
 *   and could be either decompressed back into memory, or paged out to storage, depending
 *   on our implementation.
 *
 * iokit_mapped
 *   IOKit mappings: The total size of all IOKit mappings in this task, regardless of
     clean/dirty or internal/external state].
 *
 * alternate_accounting
 *   The number of internal dirty pages which are part of IOKit mappings. By definition, these pages
 *   are counted in both internal *and* iokit_mapped, so we must subtract them from the total to avoid
 *   double counting.
 */

如何判断发生了OOM

解决

• oom instsrument测试工具
• 开发过程检测泄露
• 大对象， 图片等资源的缓存
• hook alloc dealloc

参考

https://engineering.fb.com/2015/08/24/ios/reducing-fooms-in-the-facebook-ios-app/