iOS线程While-True死循环会发生什么

一、在工作的代码有一段while-True轮训的逻辑，循环中主要的工作是阻塞的IO

　　代码大概如下：

    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        while (YES) {

            NSLog(@"CPU %f", cpu_usage());

            [NSThread sleepForTimeInterval:1];
        }
    });

　　正常的时候，因为io的阻塞关系，线程会在io的方法处等待返回，偶然发现特定情况下，阻塞io方法直接放回错误，这样会不停的进行死循环，因此我想看看死循环会带来什么问题

　　我写了下面的代码

　　

//
//  ViewController.m
//  TestCPU
//
//  Created by lilun on 2019/3/20.
//  Copyright  2019年 lilun. All rights reserved.
//

#import "ViewController.h"
#import <mach/mach.h>
#import <assert.h>

float cpu_usage()
{
    kern_return_t kr;
    task_info_data_t tinfo;
    mach_msg_type_number_t task_info_count;

    task_info_count = TASK_INFO_MAX;
    kr = task_info(mach_task_self(), TASK_BASIC_INFO, (task_info_t)tinfo, &task_info_count);
    if (kr != KERN_SUCCESS) {
        return -1;
    }

    task_basic_info_t      basic_info;
    thread_array_t         thread_list;
    mach_msg_type_number_t thread_count;

    thread_info_data_t     thinfo;
    mach_msg_type_number_t thread_info_count;

    thread_basic_info_t basic_info_th;
    uint32_t stat_thread = 0; // Mach threads

    basic_info = (task_basic_info_t)tinfo;

    // get threads in the task
    kr = task_threads(mach_task_self(), &thread_list, &thread_count);
    if (kr != KERN_SUCCESS) {
        return -1;
    }
    if (thread_count > 0)
        stat_thread += thread_count;

    long tot_sec = 0;
    long tot_usec = 0;
    float tot_cpu = 0;
    int j;

    for (j = 0; j < (int)thread_count; j++)
    {
        thread_info_count = THREAD_INFO_MAX;
        kr = thread_info(thread_list[j], THREAD_BASIC_INFO,
                         (thread_info_t)thinfo, &thread_info_count);
        if (kr != KERN_SUCCESS) {
            return -1;
        }

        basic_info_th = (thread_basic_info_t)thinfo;

        if (!(basic_info_th->flags & TH_FLAGS_IDLE)) {
            tot_sec = tot_sec + basic_info_th->user_time.seconds + basic_info_th->system_time.seconds;
            tot_usec = tot_usec + basic_info_th->user_time.microseconds + basic_info_th->system_time.microseconds;
            tot_cpu = tot_cpu + basic_info_th->cpu_usage / (float)TH_USAGE_SCALE * 100.0;
        }

    } // for each thread

    kr = vm_deallocate(mach_task_self(), (vm_offset_t)thread_list, thread_count * sizeof(thread_t));
    assert(kr == KERN_SUCCESS);

    return tot_cpu;
}

@interface ViewController ()

@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.3

    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        [self runOneThread:1];
        //    [self runOneThread:2];
        //    [self runOneThread:3];
        //    [self runOneThread:4];
    });

    UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(0, 0, self.view.bounds.size.width, 100)];
    label.textColor = [UIColor redColor];
    label.textAlignment = NSTextAlignmentCenter;
    [self.view addSubview:label];
    label.center = self.view.center;
    label.tag = 1234;

    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        while (YES) {

            NSLog(@"CPU %f", cpu_usage());

            dispatch_async(dispatch_get_main_queue(), ^{
                UILabel *label = [self.view viewWithTag:1234];
                label.text = [NSString stringWithFormat:@"CPU %f", cpu_usage()];
            });

            [NSThread sleepForTimeInterval:1];
        }
    });

}

- (void)runOneThread:(NSInteger)i
{

    dispatch_async(dispatch_get_global_queue(0, 0), ^{

        while (YES) {
             NSLog(@"run thread %ld", i);
        }
    });

}

@end

　　上面的代码如果运行在模拟器的时候，大概4核的cpu（400%）会占据40%的CPU，还不是很多，貌似模拟器的CPU占用会限制。

　　　　此时电脑风扇会呼呼转。

　　上面的代码是真机调试的状态下，会出现调试器卡死，Xcode无反应，必须拔掉USB线来中断调试。

　　　　猜测原因是循环输出的log占满了USB通信的带宽，导致调试信息无法传递到Xcode上。或者是调试线程因为工作线程导致无法工作正常

　　如果是非调试模式下面，大概会占用98%（以上的方法测试出来）的CPU，机器会发烫。

　　　　应该极力避免出现死循环的情况，即使是在子线程中出现的死循环。

总结：

　　死循环在子线程也是不能出现的，任何方法一定要限制好频率。

　　NSlog会产生系统中断，频繁的系统中断也会导致性能低下。