I have a computer with 1 cpu, 4 cores and 2 threads per core that can run. So I have effiency with maximum 8 running threads.

When I write a program in C and create threads using pthred_create function, how many threads is recommended to be created: 7 or 8? Do I have to substract the main thread, thus create 7, or main thread should not be counted and I can effiently crete 8? I know that in theory you can create much more, like thousands, but I want to be effiently planned, according with my computer architecture.

CodePudding user response：

Which thread started which is not much relevant. A program's initial thread is a thread: while it is scheduled on an execution unit, no other thread can use that execution unit. You cannot have more threads executing concurrently than you have execution units, and if you have more than that eligible to run at any given time then you will pay the cost of extra context switches without receiving any offsetting gain from additional concurrency.

To a first approximation, then, yes, you must count the initial thread. But read the above carefully. The relevant metric is not how many threads exist at any given time, but rather how many are contending for execution resources. Threads that are currently blocked (on I/O, on acquiring a mutex, on pthread_join(), etc.) do not contend for execution resources.

More precisely, then, it depends on your threads' behavior. For example, if the initial thread follows the pattern of launching a bunch of other threads and then joining them all without itself performing any other work, then no, you do not count that thread, because it does not contend for CPU to any significant degree while the other threads are doing so.