Using Multi-threading and/or Multi-process

Hi,
Today I read some code which creates multiple threads. For each of them, a child process is created to call a stand-alone executable with communication in input to and output from it. I asked myself why not simple using multi-process without multi-thread.

So here I would like to know in principle how to choose between multiple threading and multiple processing and when to combine them together with multi-process in each thread or multi-thread in each process, i.e. in what scenarios which way is better in terms of speed and difficulty in communication and implementation, etc?

If I have multiple tasks on a multiple core/cpu server, which way is better(faster): call them as multi-process in a code file or open several terminals each of which calls the executable of a task?

Thanks and regards!

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Today I read some code which creates multiple threads. For each of them, a child process is created to call a stand-alone executable with communication in input to and output from it. I asked myself why not simple using multi-process without multi-thread.

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My best guess is that the threads act as the interface between the main thread and the sub-processes. The "process threads" do I/O with the sub-process, and communicate with the main thread in some way internal to the program.

The main difference between processes and threads is that threads within the same process share the same address space. Different processes of course don't. That means, creating processes are much more expensive than creating threads (not sure if that's still true with systems implementing copy-on-write). Also, inter-process communication (IPC) is more difficult. For threads, you would just have both threads read/write the same places in memory (with proper protections, of course).

EDIT: and for when to use what, use processes if the tasks are relatively independent from each other, and communication between processes is not needed much (since IPC is usually expensive) and/or you want each task to be better protected (one process crash won't take down other processes, a thread crashing will take down other threads in the same process). Use threads if you need speed (creating and destroying a lot of threads) and/or a lot of data sharing and/or to save memory (each process has their own addressing space and file descriptors and other things. Especially important if you are creating many processes).

Thanks!
A few questions on multi-process:
If I have several tasks(executables) and I write a program, where I create a sub-process to execute each task without waiting it to finish, will the runing of several tasks be parallel on a server with multiple core/cpus by the OS automatically? If not, how to do it in my code?
Is this way a fast way to execute several independent tasks? Is it the same as open a terminal for each task to execute it?

Yes, they will run parallel.

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That means, creating processes are much more expensive than creating threads

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This is dependent on the OS and the threading implementation. On Linux for instance, threads are implemented as processes. In other words, there is little difference between calling pthread_create(), and fork(). They both just call clone() internally with different parameters.

Although, threads are usually seen as different parts of a program. You can easily suspend and resume threads, for example.
Aside from that, you may also take into account that if one thread crashes, it brings down the entire process, whereas if you make them all processes, they can't bring down each other (but you may need to use different techniques to communicate between them).