iOS線程While-True死循環會發生什么

一、在工作的代碼有一段while-True輪訓的邏輯，循環中主要的工作是阻塞的IO

　　代碼大概如下：

    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        while (YES) {
            
            NSLog(@"CPU %f", cpu_usage());
            
            [NSThread sleepForTimeInterval:1];
        }
    });

　　正常的時候，因為io的阻塞關系，線程會在io的方法處等待返回，偶然發現特定情況下，阻塞io方法直接放回錯誤，這樣會不停的進行死循環，因此我想看看死循環會帶來什么問題

　　我寫了下面的代碼

　　

//
//  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

　　上面的代碼如果運行在模擬器的時候，大概4核的cpu（400%）會占據40%的CPU，還不是很多，貌似模擬器的CPU占用會限制。

　　　　此時電腦風扇會呼呼轉。

 

　　上面的代碼是真機調試的狀態下，會出現調試器卡死，Xcode無反應，必須拔掉USB線來中斷調試。

　　　　猜測原因是循環輸出的log占滿了USB通信的帶寬，導致調試信息無法傳遞到Xcode上。或者是調試線程因為工作線程導致無法工作正常

 

　　如果是非調試模式下面，大概會占用98%（以上的方法測試出來）的CPU，機器會發燙。

　　　　應該極力避免出現死循環的情況，即使是在子線程中出現的死循環。

 

總結：

　　死循環在子線程也是不能出現的，任何方法一定要限制好頻率。

　　NSlog會產生系統中斷，頻繁的系統中斷也會導致性能低下。