Bottleneck in Threads C++

Question

So I am just trying to verify my understanding and hope that you guys will be able to clear up any misunderstandings. So essentially I have two threads which use the same lock and perform calculations when they hold the lock but the interesting thing is that within the lock I will cause the thread to sleep for a short time. For both threads, this sleep time will be slightly different for either thread. Because of the way locks work, wont the faster thread be bottlenecked by the slower thread as it will have to wait for it to complete?

For example:

Thread1() {

   lock();
   usleep(10)
   lock();

}

-

Thread2() {

   lock();
   sleep(100)
   lock();

}

Now because Thread2 holds onto the lock longer, this will cause a bottleneck. And just to be sure, this system should have a back and forth happens on who gets the lock, right?

It should be:

Thread1 gets lock
Thread1 releases lock
Thread2 gets lock
Thread2 releases lock
Thread1 gets lock
Thread1 releases lock
Thread2 gets lock
Thread2 releases lock

and so on, right? Thread1 should never be able to acquire the lock right after it releases it, can it?

Answer 1

Thread1 should never be able to acquire the lock right after it releases it, can it?

No, Thread1 could reacquire the lock, right after it releases it, because Thread2 could still be suspended (sleeps because of the scheduler)

Also sleep only guarantees that the thread will sleep at least the wanted amount, it can and will often be more.

In practice you would not hold a lock while calculating a value, you would get the lock, get the needed values for calculation, unlock, calculate it, and then get the lock again, check if the old values for the calculation are still valid/wanted, and then store/return your calculated results. For this purpose, the std::future and atomic data types were invented.

...this system should have a back and forth happens on who gets the lock, right?

Mostly The most of the time it will be a back and forth but some times there could/will be two lock/unlock cycles by Thread1. It depends on your scheduler and any execution and cycle will probably vary.

Answer 2

Depending on what you are trying to achieve there are several possibilities.

If you want your threads to run in a particular order then have a look here. There are basically 2 options:
- one is to use events where a thread is signaling the next one it has done his job and so the next one could start.
- the other one is to have a scheduler thread that handle the ordering with events or semaphores.

If you want your threads to run independently but have a lock mechanism where the order of attempting to get the lock is preserved you can have a look here. The last part of the answer uses a queue of one condition variable per thread seem good.

And as it was said in previous answers and comments, using sleep for scheduling is a bad idea. Also lock is just a mutual exclusion mechanism and has no guarentee on the execution order. A lock is usually intended for preventing concurrent access on a critical resource so it should just do that. The smaller the critical section is, the better.
Finally yes trying to order threads is making "bottlenecks". In this particular case if all calculations are made in the locked sections the threads won't do anything in parallel so you can question the utility of using threads.

Edit :
Just on more warning: be careful, with threads it's not because is worked (was scheduled as you wanted to) 10 times on your machine that it always will, especially if you change any of the context (machine, workload...). You have to be sure of it by design.

Answer 3

Absolutely nothing prevents either thread from immediately reacquiring the lock after releasing it. I have no idea what you think prevents this from happening, but nothing does.

In fact, in many implementations, a thread that is already running has an advantage in acquiring a lock over threads that have to be made ready-to-run. This is a sensible optimization to minimize context switches.

If you're using a sleep as a way to simulate work and think this represents some real world issue with lock fairness, you are wrong. Threads that sleep are voluntarily yielding the remainder of their timeslice and are treated very differently from threads that exhaust their timeslice doing work. If these threads were actually doing work, eventually one thread would exhaust its timeslice.