Question

Is there any mechanism through which I can wake up a thread in another process without going through the kernel? The waiting thread might spin in a loop, no problem (each thread is pegged to a separate core), but in my case the sending thread has to be quick, and can’t afford to go through the kernel to wake up the waiting thread.

Answer 1

No, if the other thread is sleeping (not on CPU). To wake up such thread you need to change its state into “RUNNING” by calling scheduler which is part of the kernel.

Yes, you can syncronize two threads or processes if both are running on different CPUs, and if there is shared memory between them. You should bind all threads to different CPUs. Then you may use spinlock:pthread_spin_lock and pthread_spin_unlock functions from optional part of POSIX’s Pthread ('(ADVANCED REALTIME THREADS)'; [THR SPI]); or any of custom spinlock. Custom spinlock most likely will use some atomic operations and/or memory barriers.

Sending thread will change the value in memory, which is checked in loop by receiver thread.

E.g.

init:

pthread_spinlock_t lock;
pthread_spin_lock(&lock);  // close the "mutex"

then start threads.

waiting thread:

{
pthread_spin_lock(&lock); // wait for event;
work();
}

main thread:

{
do_smth();
pthread_spin_unlock(&lock); // open the mutex; other thread will see this change
 //  in ~150 CPU ticks (checked on Pentium4 and Intel Core2 single socket systems);
 // time of the operation itself is of the same order; didn't measure it.
continue_work();
}