Semaphores & threads - what is the point? - multithreading

Question

I've been reading about semaphores and came across this article:
www.csc.villanova.edu/~mdamian/threads/posixsem.html
So, this page states that if there are two threads accessing the same data, things can get ugly. The solution is to allow only one thread to access the data at the same time.
This is clear and I understand the solution, only why would anyone need threads to do this? What is the point? If the threads are blocked so that only one can execute, why use them at all? There is no advantage. (or maybe this is a just a dumb example; in such a case please point me to a sensible one)
Thanks in advance.

Answer 1

Consider this:
void update_shared_variable() {
sem_wait( &g_shared_variable_mutex );
g_shared_variable++;
sem_post( &g_shared_variable_mutex );
}
void thread1() {
do_thing_1a();
do_thing_1b();
do_thing_1c();
update_shared_variable(); // may block
}
void thread2() {
do_thing_2a();
do_thing_2b();
do_thing_2c();
update_shared_variable(); // may block
}
Note that all of the do_thing_xx functions still happen simultaneously. The semaphore only comes into play when the threads need to modify some shared (global) state or use some shared resource. So a thread will only block if another thread is trying to access the shared thing at the same time.
Now, if the only thing your threads are doing is working with one single shared variable/resource, then you are correct - there is no point in having threads at all (it would actually be less efficient than just one thread, due to context switching.)

Answer 2

When you are using multithreading not everycode that runs will be blocking. For example, if you had a queue, and two threads are reading from that queue, you would make sure that no thread reads at the same time from the queue, so that part would be blocking, but that's the part that will probably take the less time. Once you have retrieved the item to process from the queue, all the rest of the code can be run asynchronously.

Answer 3

The idea behind the threads is to allow simultaneous processing. A shared resource must be governed to avoid things like deadlocks or starvation. If something can take a while to process, then why not create multiple instances of those processes to allow them to finish faster? The bottleneck is just what you mentioned, when a process has to wait for I/O.
Being blocked while waiting for the shared resource is small when compared to the processing time, this is when you want to use multiple threads.

Answer 4

This is of course a SSCCE (Short, Self Contained, Correct Example)
Let's say you have 2 worker threads that do a lot of work and write the result to a file.
you only need to lock the file (shared resource) access.

Answer 5

The problem with trivial examples....
If the problem you're trying to solve can be broken down into pieces that can be executed in parallel then threads are a good thing.
A slightly less trivial example - imagine a for loop where the data being processed in each iteration is different every time. In that circumstance you could execute each iteration of the for loop simultaneously in separate threads. And indeed some compilers like Intel's will convert suitable for loops to threads automatically for you. In that particular circumstances no semaphores are needed because of the iterations' data independence.
But say you were wanting to process a stream of data, and that processing had two distinct steps, A and B. The threadless approach would involve reading in some data then doing A then B and then output the data before reading more input. Or you could have a thread reading and doing A, another thread doing B and output. So how do you get the interim result from the first thread to the second?
One way would be to have a memory buffer to contain the interim result. The first thread could write the interim result to a memory buffer and the second could read from it. But with two threads operating independently there's no way for the first thread to know if it's safe to overwrite that buffer, and there's no way for the second to know when to read from it.
That's where you can use semaphores to synchronise the action of the two threads. The first thread takes a semaphore that I'll call empty, fills the buffer, and then posts a semaphore called filled. Meanwhile the second thread will take the filled semaphore, read the buffer, and then post empty. So long as filled is initialised to 0 and empty is initialised to 1 it will work. The second thread will process the data only after the first has written it, and the first won't write it until the second has finished with it.
It's only worth it of course if the amount of time each thread spends processing data outweighs the amount of time spent waiting for semaphores. This limits the extent to which splitting code up into threads yields a benefit. Going beyond that tends to mean that the overall execution is effectively serial.
You can do multithreaded programming without semaphores at all. There's the Actor model or Communicating Sequential Processes (the one I favour). It's well worth looking up JCSP on Wikipedia.
In these programming styles data is shared between threads by sending it down communication channels. So instead of using semaphores to grant another thread access to data it would be sent a copy of that data down something a bit like a network socket, or a pipe. The advantage of CSP (which limits that communication channel to send-finishes-only-if-receiver-has-read) is that it stops you falling into the many many pitfalls that plague multithreaded do programs. It sounds inefficient (copying data is inefficient), but actually it's not so bad with Intel's QPI architecture, AMD's Hypertransport. And it means hat the 'channel' really could be a network connection; scalability built in by design.