mutex实现与性能初探

Posted 2022-02-24 东北码农

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大家好，我是东北码农。
今天讲讲工作中的事吧，领导要求调研一个事，mutex性能如何？mutex无竞争时和有竞争时效率如何？

0x1、结论

我和领导汇报时，都习惯先说结论，领导可以很容易抓住重点，如果感兴趣就继续看。

结论如下：

没有争抢时，不进入内核态。性能与自旋锁类似。
有争抢时，没抢到，进入内核态等待唤醒。

0x2、mutex实现

mutex实现时，分用户态部分和内核态部分，分别承担不同的功能：

用户态：使用test and set来测试是否可以抢到锁。
内核态：使用futex系统调用来等待和唤醒。

0x3、实验过程

使用我们的老朋友strace命令来观察抢锁时的系统调用情况。

strace使用可以参考前面的文章：
如何使用strace
动手实现一个strace

0x31、场景一：单线程加锁，无争抢

测试代码：

std::mutex g_lock;
void test_no_race()

    for(int i = 0;i < 10;++i)
    
        g_lock.lock();
        usleep(100*1000);
        g_lock.unlock();

strace看一下，lock unlock无系统调用（只有sleep时有系统调用）。

***@xxx:~/code/case/case30_c11_fence$ strace -f ./a.out
。。。
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
。。。

0x31、场景二：多线程加锁，有争抢

测试代码：

std::mutex g_lock;

void test_no_race()

    for(int i = 0;i < 10;++i)
    
        g_lock.lock();
        usleep(100*1000);
        g_lock.unlock();
    


void test_race()

    std::thread t1(test_no_race);
    std::thread t2(test_no_race);
    
    t1.join();
    t2.join();

用strace看一下，此时会大量调用futex 系统调用，来等待（FUTEX_WAIT_PRIVATE）和唤醒（FUTEX_WAKE_PRIVATE）。

[pid   552] futex(0x7f61977959d0, FUTEX_WAIT, 553, NULL <unfinished ...>
[pid   554] <... set_robust_list resumed>) = 0
[pid   554] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   553] <... clock_nanosleep resumed>NULL) = 0
[pid   553] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
[pid   554] <... futex resumed>)        = 0
[pid   553] <... futex resumed>)        = 1
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] <... futex resumed>)        = 0
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   554] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   553] <... clock_nanosleep resumed>NULL) = 0
[pid   553] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   554] <... futex resumed>)        = 0
[pid   554] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
[pid   553] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1 <unfinished ...>
[pid   554] <... futex resumed>)        = 0
[pid   553] <... futex resumed>)        = 1
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   554] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   553] futex(0x560716857060, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
[pid   554] <... clock_nanosleep resumed>NULL) = 0
[pid   554] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 1
[pid   553] <... futex resumed>)        = 0
[pid   554] mmap(NULL, 134217728, PROT_NONE, MAP_PRIVATE|MAP_ANONYMOUS|MAP_NORESERVE, -1, 0 <unfinished ...>
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000,  <unfinished ...>
[pid   554] <... mmap resumed>)         = 0x7f618e794000
[pid   554] munmap(0x7f618e794000, 25608192) = 0
[pid   554] munmap(0x7f6194000000, 41500672) = 0
[pid   554] mprotect(0x7f6190000000, 135168, PROT_READ|PROT_WRITE) = 0
[pid   554] madvise(0x7f6196794000, 8368128, MADV_DONTNEED) = 0
[pid   554] exit(0)                     = ?
[pid   554] +++ exited with 0 +++
[pid   553] <... clock_nanosleep resumed>NULL) = 0
[pid   553] futex(0x560716857060, FUTEX_WAKE_PRIVATE, 1) = 0
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0
[pid   553] clock_nanosleep(CLOCK_REALTIME, 0, tv_sec=0, tv_nsec=100000000, NULL) = 0

搞定！

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