Threads: some questions

Question

Hi, I have couple of questions on threads. Could you please clarify.

1. Suppose process with one or multiple threads. If the process is prempted/suspended, does the threads also get preempted or does the threads continue to run?

2. When the suspended process rescheduled, does the process threads also gets scheduled? If the process has process has multiple threads, which threads will be rescheduled and on what basis?

3. if the thread in the process is running and recieves a signal(say Cntrl-C) and the default action of the signal is to terminate a process, does the running thread terminates or the parent process will also terminate? What happens to the threads if the running process terminates because of some signal?

4. If the thread does fork fallowed exec, does the exece'd program overlays the address space of parent process or the running thread? If it overlays the parent process what happens to threads, their data, locks they are holding and how they get scheduled once the exec'd process terminates.

5. Suppose process has multiple threads, how does the threads get scheduled. If one of the thread blocks on some I/O, how other threads gets scheduled. Does the threads scheduled with the parent process is running?

6. While the thread is running what the current kernel variable points(parent process task_stuct or threads stack_struct?

7. If the process with the thread is running, when the thread starts does the parent process gets preempted and how each threads gets scheduled?

8. If the process running on CPU creates multiple threads, does the threads created by the parent process schedule on another CPU on multiprocessor system?

Thanks, Ganesh

Answer 1

Assuming your questions are about POSIX threads, then

1a. A process that's preempted by the O/S will have all its threads preempted.

1b. The O/S will suspend all the threads of a process that is sent a SIGSTOP.

1. The O/S will resume all thread of a suspended process that is sent a SIGCONT.

2. By default, a SIGINT will terminate all the threads in a process.

3. If a thread calls fork(), then all its threads are duplicated. If it then call one of the exec() functions, then all the duplicated threads disappear.

4. POSIX allows for user-selection of the thread scheduling algorithm.

5. I don't understand the question.

6. I don't understand the question.

7. How threads are mapped to CPU-s is implementation-dependent. Many implementations will try to distribute threads amongst the available CPU-s to improve performance.

Answer 2

1. Depends. If a thread is preempted because the OS scheduler decides to give CPU time to some other thread, then other threads in the process will continue running. If the process is suspended (i.e. it gets the SIGSTP signal) then AFAIK all the threads will be suspended.

2. When a suspended process is woken up, all the threads are marked as waiting or blocked (if they are waiting e.g. on a mutex). Then the scheduler at some points run them. There is no guarantee about any specific order the threads are run after waking up the process.

3. The process will terminate, and with it the threads as well.

4. When you fork you get a new address space, so there is no "overlay". Note that fork() and the exec() family affect the entire process, not only the thread from which they where called. When you call fork() in a multi-threaded process, the child gets a copy of that process, but with only the calling thread. Then if you call exec() in one or both of the processes (presumably only in the child process, but that's up to you), then the process which calls exec() (and with it, all its threads) is replaced by the exec()'ed program.

5. The thread scheduling order is decided by the OS scheduler, there is no guarantee given about any particular order.

6. From the kernel perspective a process is an address space with one or more threads (and some other gunk). There is no concept of threads that somehow exist without a process.

7. There is no such thing as a process without a single thread. A "plain process" is just a process with a single thread.

8. Probably yes. This is determined by the OS scheduler. Note that there are API's and tools (numactl) that one can use to force some thread(s) to run on a specific CPU core.

Answer 3

The Linux kernel doesn't distinguish between threads and processes. As far as kernel is concerned, a thread is simply another process which happens to share address space with other processes. (You would call the set of "processes" (i.e. threads) which share a single address space a "process".)

So POSIX threads are scheduled exactly as full-blown processes would be. There is no difference in scheduling whether you have one process with five threads, or five separate processes.

There are kernel calls that provide fine grained control over what is shared between processes. The POSIX threads API wraps over them.

Answer 4

First, I should clear up some terminology that you appear to be confused about. In POSIX, a "process" is a single address space plus at least one thread of control, identified by a process ID (PID). A thread is an individually-scheduled execution context within a process.

All processes start life with just one thread, and all processes have at least one thread. Now, onto the questions:

1. Suppose process with one or multiple threads. If the process is prempted/suspended, does the threads also get preempted or does the threads continue to run?

Threads are scheduled independently. If a thread blocks on a function like connect(), then other threads within the process can still be scheduled.

It is also possible to request that every thread in a process be suspended, for example by sending SIGSTOP to the process.

1. When the suspended process rescheduled, does the process threads also gets scheduled? If the process has process has multiple threads, which threads will be rescheduled and on what basis?

This only makes sense in the context that an explicit request was made to stop the entire process. If you send the process SIGCONT to restart the process, then any of the threads which are not blocked can run. If more threads are runnable than there are processors available to run them, then it is unspecified which one(s) run first.

1. If the thread in the process is running and recieves a signal(say Cntrl-C) and the default action of the signal is to terminate a process, does the running thread terminates or the parent process will also terminate? What happens to the threads if the running process terminates because of some signal?

If a thread recieves a signal like SIGINT or SIGSEGV whose action is to terminate the process, then the entire process is terminated. This means that every thread in the process is unceremoniously killed.

1. If the thread does fork followed by exec, does the exece'd program overlays the address space of parent process or the running thread? If it overlays the parent process what happens to threads, their data, locks they are holding and how they get scheduled once the exec'd process terminates.

The fork() call creates a new process by duplicating the address space of the original process, and duplicating just the single thread that called fork() within that new address space.

If that thread in the new process calls execve(), it will replace the new, duplicated address space with the exec'd program. The original process, and all its threads, continue running normally.

1. Suppose process has multiple threads, how does the threads get scheduled. If one of the thread blocks on some I/O, how other threads gets scheduled. Does the threads scheduled with the parent process is running?

The threads are scheduled independently. Any of the threads that are not blocked can run.

1. While the thread is running what the current kernel variable points(parent process task_stuct or threads stack_struct?

Each thread has its own task_struct within the kernel. What userspace calls a "thread" is called a "process" in kernel space. Thus current always points at the task_struct corresponding to the currently executing thread (in the userspace sense of the word).

1. If the process with [a second] thread is running, when the thread starts does the parent process gets preempted and how each threads gets scheduled?

Presumably you mean "the process's main thread" rather than "parent process" here. As before, the threads are scheduled independently. It's unspecified whether one runs before the other - and if you have multiple CPUs, both might run simultaneously.

1. If the process running on CPU creates multiple threads, does the threads created by the parent process schedule on another CPU on multiprocessor system?

That's really up to the kernel, but the threads are certainly allowed to execute on other CPUs.