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。