How much overhead is there when creating a thread?

Solution 1:

To resurrect this old thread, I just did some simple test code:

#include <thread>

int main(int argc, char** argv)
{
  for (volatile int i = 0; i < 500000; i++)
    std::thread([](){}).detach();
  return 0;
}

I compiled it with g++ test.cpp -std=c++11 -lpthread -O3 -o test. I then ran it three times in a row on an old (kernel 2.6.18) heavily loaded (doing a database rebuild) slow laptop (Intel core i5-2540M). Results from three consecutive runs: 5.647s, 5.515s, and 5.561s. So we're looking at a tad over 10 microseconds per thread on this machine, probably much less on yours.

That's not much overhead at all, given that serial ports max out at around 1 bit per 10 microseconds. Now, of course there's various additional thread losses one can get involving passed/captured arguments (although function calls themselves can impose some), cache slowdowns between cores (if multiple threads on different cores are battling over the same memory at the same time), etc. But in general I highly doubt the use case you presented will adversely impact performance at all (and could provide benefits, depending), despite having you already preemptively labeled the concept "really terrible code" without even knowing how much time it takes to launch a thread.

Whether it's a good idea or not depends a lot on the details of your situation. What else is the calling thread responsible for? What precisely is involved in preparing and writing out the packets? How frequently are they written out (with what sort of distribution? uniform, clustered, etc...?) and what's their structure like? How many cores does the system have? Etc. Depending on the details, the optimal solution could be anywhere from "no threads at all" to "shared thread pool" to "thread for each packet".

Note that thread pools aren't magic and can in some cases be a slowdown versus unique threads, since one of the biggest slowdowns with threads is synchronizing cached memory used by multiple threads at the same time, and thread pools by their very nature of having to look for and process updates from a different thread have to do this. So either your primary thread or child processing thread can get stuck having to wait if the processor isn't sure whether the other process has altered a section of memory. By contrast, in an ideal situation, a unique processing thread for a given task only has to share memory with its calling task once (when it's launched) and then they never interfere with each other again.

Solution 2:

I have always been told that thread creation is cheap, especially when compared to the alternative of creating a process. If the program you are talking about does not have a lot of operations that need to run concurrently then threading might not be necessary, and judging by what you wrote this might well be the case. Some literature to back me up:

http://www.personal.kent.edu/~rmuhamma/OpSystems/Myos/threads.htm

Threads are cheap in the sense that

1. They only need a stack and storage for registers therefore, threads are cheap to create.

2. Threads use very little resources of an operating system in which they are working. That is, threads do not need new address space, global data, program code or operating system resources.

3. Context switching are fast when working with threads. The reason is that we only have to save and/or restore PC, SP and registers.

More of the same here.

In Operating System Concepts 8th Edition (page 155) the authors write about the benefits of threading:

Allocating memory and resources for process creation is costly. Because threads share the resource of the process to which they belong, it is more economical to create and context-switch threads. Empirically gauging the difference in overhead can be difficult, but in general it is much more time consuming to create and manage processes than threads. In Solaris, for example, creating a process is about thirty times slower than is creating a thread, and context switching is about five times slower.

Solution 3:

...sends Messages on a serial port ... for every message a pthread is created, bits are properly set up, then the thread terminates. ...how much overhead is there when actually creating a thread?

This is highly system specific. For example, last time I used VMS threading was nightmarishly slow (been years, but from memory one thread could create something like 10 more per second (and if you kept that up for a few seconds without threads exiting you'd core)), whereas on Linux you can probably create thousands. If you want to know exactly, benchmark it on your system. But, it's not much use just knowing that without knowing more about the messages: whether they average 5 bytes or 100k, whether they're sent contiguously or the line idles in between, and what the latency requirements for the app are are all as relevant to the appropriateness of the code's thread use as any absolute measurement of thread creation overhead. And performance may not have needed to be the dominant design consideration.

Solution 4:

There is some overhead in thread creation, but comparing it with usually slow baud rates of the serial port (19200 bits/sec being the most common), it just doesn't matter.