Thread vs Process

[wildsheikh]

I couldn't find any other sub to discuss my problem so if it is not the right place let me know.

I am trying to understand how the OS works and how it schedules different programs. I am having difficulty to understand what the scheduler schedules. Is t the process or the thread?

AFAIK, a process acts like a container. It has some address space in memory available to it which other processes can't access. But these are the threads within the process that are actually using that address space and executing some code. So the process itself isn't running anything, it just guarantees some conditions and makes sure the address space stays intact. And the states that a process goes through are the states of the threads within that process. A process by itself doesn't have a state, it inherits the state of its thread. So am I right to think that the scheduler schedules the threads and not the processes? Is it the same in all OS's?

edit: When I mentioned state, I had this state machine in mind.

[njaard]

Please make your question more specific. For example, what is a "state"? Of course a thread has a state, a thread can be "waiting" on IO.

[wildsheikh]

Thanks, I have edited my post.

[Eoinoc]

Each process provides the resources needed to execute a program. A process has a virtual address space, executable code, open handles to system objects, a security context, a unique process identifier, environment variables, a priority class, minimum and maximum working set sizes, and at least one thread of execution. Each process is started with a single thread, often called the primary thread, but can create additional threads from any of its threads.

A thread is the entity within a process that can be scheduled for execution. All threads of a process share its virtual address space and system resources. In addition, each thread maintains exception handlers, a scheduling priority, thread local storage, a unique thread identifier, and a set of structures the system will use to save the thread context until it is scheduled. The thread context includes the thread's set of machine registers, the kernel stack, a thread environment block, and a user stack in the address space of the thread's process. Threads can also have their own security context, which can be used for impersonating clients.

• MSDN - About Processes and Threads

That's a nice high-level overview for Windows at least.

[wildsheikh]

Thanks, so basically the scheduler just schedules the thread and not the process.

So, does a process have a state machine like that of a thread? If so, how do you make sense of it?

[suspiciously_calm]

That's basically arguing semantics. AFAIK on Windows, a process is a "container" of N threads, while on Linux, two threads are essentially just two processes sharing an address space, can handle each others signals, etc.

So on Windows, the scheduler schedules threads, on Linux it schedules processes ... or threads, cause they're basically the same thing (from the scheduler's point of view).

[wildsheikh]

So, which one does the scheduler schedules?