上一篇介绍了因为子表过多，导致innodb crash的情况，但crash的原因是long semaphore waits。
long semaphore waits又为何物？
背景：Innodb使用了mutex和rw_lock来保护内存数据结构，同步的机制要么是互斥，要么是读写阻塞的模式，
Innode认为mutex和rw_lock hold的时间足够短，所以，如果有线程wait mutex或者rw_lock时间过长，那么
很可能是程序有bug，所以就会异常主动crash。
关于innodb的锁：可以参考前面介绍的blog。
1. Innodb如何界定时间过长？
/* The following is the maximum allowed duration of a lock wait. */
UNIV_INTERN ulint srv_fatal_semaphore_wait_threshold = ;
Innodb的认为600s足够长。
2. Innodb如何判断？
/* Create the thread which warns of long semaphore waits */
os_thread_create(&srv_error_monitor_thread, NULL,
thread_ids + + SRV_MAX_N_IO_THREADS);
innodb后台启动了一个线程，专门监控wait队列，
if (sync_array_print_long_waits(&waiter, &sema)
&& sema == old_sema && os_thread_eq(waiter, old_waiter)) {
fatal_cnt++;
if (fatal_cnt > ) {
fprintf(stderr,
"InnoDB: Error: semaphore wait has lasted"
" > %lu seconds\n"
"InnoDB: We intentionally crash the server,"
" because it appears to be hung.\n",
(ulong) srv_fatal_semaphore_wait_threshold); ut_error;
}
判断10次，如果是同一个线程，等待同一个sema，那么就crash掉Innodb。
for (i = ; i < sync_primary_wait_array->n_cells; i++) {
double diff;
void* wait_object;
cell = sync_array_get_nth_cell(sync_primary_wait_array, i);
wait_object = cell->wait_object;
if (wait_object == NULL || !cell->waiting) {
continue;
}
diff = difftime(time(NULL), cell->reservation_time);
if (diff > SYNC_ARRAY_TIMEOUT) {
fputs("InnoDB: Warning: a long semaphore wait:\n",
stderr);
sync_array_cell_print(stderr, cell);
noticed = TRUE;
}
if (diff > fatal_timeout) {
fatal = TRUE;
}
if (diff > longest_diff) {
longest_diff = diff;
*sema = wait_object;
*waiter = cell->thread;
sync_primary_wait_array是一个数组，每一个wait sema的进入队列， 找到等待时间最长的，并且大于600s的，就设置fatal=TRUE。