首先，我在stackoverflow.com上搜索过，也在google过，但是没有找到有效的结果。
我的问题是：为什么 pthread_cond_wait 消耗这么多 CPU？我认为这不正常。
我的程序遇到过这种情况：%CPU 会间歇性地变高，并且会持续十多秒。当 %CPU 稳定在低位时，它在 1 左右。当它变高时，它在 50 到 300 之间。
我使用 top -H -p 来查找当进程的 CPU 百分比变高时消耗最多 CPU 的单个线程，然后使用 strace -T -r -c -p 来查找更多信息：
strace -T -r -c -p 1701
% time seconds usecs/call calls errors syscall
------ ----------- ----------- --------- --------- ----------------
88.54 0.482646 43 11157 3020 futex
9.85 0.053682 0 131052 read
1.50 0.008192 38 213 nanosleep
0.04 0.000214 1 239 write
0.03 0.000154 1 213 open
0.02 0.000111 1 213 munmap
0.02 0.000085 0 239 stat
0.01 0.000044 0 213 mmap
0.00 0.000018 0 213 close
0.00 0.000000 0 213 fstat
0.00 0.000000 0 213 lseek
------ ----------- ----------- --------- --------- ----------------
100.00 0.545146 144178 3020 total
该线程的堆栈：
线程 6 (线程 0x7f1404f41700 (LWP 1701))：
#0 0x0000003d6f60b63c in pthread_cond_wait@@GLIBC_2.3.2 () from /lib64/libpthread.so.0
#1 0x0000000000406045 in foo(void*) ()
#2 0x0000003d6f607a51 in start_thread () from /lib64/libpthread.so.0
#3 0x0000003d6eee893d in clone () from /lib64/libc.so.6
以及相关代码片段：
static std::deque<std::string> conveyor;
static pthread_mutex_t conveyor_mtx = PTHREAD_MUTEX_INITIALIZER;
static pthread_cond_t conveyor_cond = PTHREAD_COND_INITIALIZER;
#define POP_NUM 4
static void *foo(void *arg)
{
write_log(LOG_INFO, "thread foo created");
int ret = pthread_detach(pthread_self());
if (ret != 0) {
write_log(LOG_ERR, "pthread_detach[foo] failed with errno[%d]", ret);
write_log(LOG_ERR, "thread foo exiting");
return (void *)-1;
}
std::string paths[POP_NUM], topic;
int n;
do {
if ((ret = pthread_mutex_lock(&conveyor_mtx)) != 0) {
write_log(LOG_WARNING, "pthread_mutex_lock[conveyor_mtx] failed"
" with errno[%d]", ret);
sleep(1);
continue;
}
while (conveyor.empty()) {
write_log(LOG_INFO, "conveyor empty");
pthread_cond_wait(&conveyor_cond, &conveyor_mtx);
}
for (n = 0; n < POP_NUM; n++) {
paths[n].assign(conveyor.front());
conveyor.pop_front();
if (conveyor.empty()) break;
}
if ((ret = pthread_mutex_unlock(&conveyor_mtx)) != 0) {
write_log(LOG_WARNING, "pthread_mutex_unlock[conveyor_mtx] failed"
" with errno[%d]", ret);
}
for (int i = 0; i < n; i++) {
if (!extract_topic_from_path(paths[i], topic)) continue;
produce_msgs_and_save_offset(topics[topic],
const_cast<char *>(paths[i].c_str()));
}
} while (true);
write_log(LOG_ERR, "thread foo exiting");
return (void *)0;
}
static void *bar(void *arg)
{
write_log(LOG_INFO, "thread bar created");
int inot_fd = (int)(intptr_t)arg, n, ret;
struct pollfd pfd = { inot_fd, POLLIN | POLLPRI, 0 };
do {
//n = poll(&pfd, 1, -1);
//n = poll(&pfd, 1, 300000);
n = poll(&pfd, 1, 120000);
if (n == -1) {
if (errno == EINTR) {
write_log(LOG_WARNING, "poll interrupted by a signal");
continue;
}
write_log(LOG_ERR, "poll failed with errno[%d]", errno);
write_log(LOG_ERR, "thread bar exiting");
return (void *)-1;
} else if (n == 0) {
write_log(LOG_WARNING, "poll timed out after 120 seconds");
sleep(60);
}
int i;
for (i = 0; i < 3; i++) {
if ((ret = pthread_mutex_lock(&conveyor_mtx)) != 0) {
write_log(LOG_WARNING, "pthread_mutex_lock[conveyor_mtx] failed"
"[%d] with errno[%d]", i, ret);
continue;
} else {
break;
}
}
if (i == 3) {
write_log(LOG_ERR, "thread bar exiting");
return (void *)-1;
}
if ((n = baz(inot_fd)) > 0) {
pthread_mutex_unlock(&conveyor_mtx);
pthread_cond_broadcast(&conveyor_cond);
} else if (n == 0) {
pthread_mutex_unlock(&conveyor_mtx);
} else {
pthread_mutex_unlock(&conveyor_mtx);
pthread_cond_broadcast(&conveyor_cond);
write_log(LOG_ERR, "thread bar exiting");
return (void *)-1;
}
if ((n = poll_producer(producer, 1000, 2)) > 0) {
write_log(LOG_INFO, "rdkafka poll events[%d] of producer"
" for possible big outq size", n);
}
} while (true);
write_log(LOG_ERR, "thread bar exiting");
return (void *)0;
}
而且，如果我不使用 pthread_cond_wait/pthread_cond_broadcast，而是将上面代码片段中的“pthread_cond_wait”替换为“sleep”，strace 将显示成本最高的系统调用是 nanosleep。
uname -a Linux d144122 2.6.32-358.el6.x86_64 #1 SMP 星期五 2 月 22 日 00:31:26 UTC 2013 x86_64 x86_64 x86_64 GNU/Linux
答案1
我在内核中遇到了这个问题，因为它不支持 futex 系统调用。
相关的内核选项是 CONFIG_FUTEX。确保您的内核是使用该选项构建的（通常都是这样）。
事实上，你的 strace 输出显示了如此多的 futex 错误，这让我强烈怀疑这就是问题所在。
（我知道这个问题已经很老了，但这是一个令人沮丧的问题，我想为其他可怜的迷失的灵魂记录下解决方案）
答案2
我遇到了同样的问题。就我而言，这是由于使用 引起的#pragma pack，例如在此线程中：https://stackoverflow.com/questions/22166474/pthread-cond-wait-doesnt-make-cpu-sleep
删除包解决了我的问题...