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,因爲io的阻塞關係,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麼問題
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%)會佔據40%的CPU,還不是很多,貌似模擬器的CPU佔用會限制。
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佔滿了USB通信的帶寬,導致調試信息無法傳遞到Xcode上。或者是調試線程因爲工作線程導致無法工作正常
如果是非調試模式下面,大概會佔用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。