The difference between process and thread

Hi LQ,

I am a sys admin. I can't totally understand system calls and other programming stuff. However, I am good at using top, ps, kill, mpstat, vmstat.... and other linux commands.
I want to understand the difference between process and thread using the above commands which I am good at.

I would be really happy if you guys have any ideas!!

Thanks.

Originally Posted by visu_kvg Hi LQ, I am a sys admin. I can't totally understand system calls and other programming stuff. However, I am good at using top, ps, kill, mpstat, vmstat.... and other linux commands. I want to understand the difference between process and thread using the above commands which I am good at.

Using those commands is pointless, unless you understand what they give you.
http://en.wikipedia.org/wiki/Thread_...from_processes

Each process has its own address space but threads of a same process have same address space. Address space is memory area in which instructions are executed and data is stored. Each process has a large set of metadata such as PC(program counter which stores next instruction to be executed) and to move to next process we have to move out current process while it has not completed its task.When we switch from a one process to another process then before switching to other process we have to store some data related to that process so that when we switch back to that process we can start from where we left. So process switching is costly in terms of memory & processing. If some metadata is common for all executing instance then that executing instance is thread which is actually a process but shares more data with other threads which are related to this thread.So we say that threads which are related is classified in a single process(We commonly say that threads are a part of single process). Now we have the benefit that when we switch from one thread to other much less overhead for data storage & context switching is there. When we encounters two tasks having some strong relation with each other we put them into one process as threads.

This is just a overview ,other things you can google.
Regards ,
Tarun

Originally Posted by tarunchawla Each process has its own address space but threads of a same process have same address space.

Good start. But I think your description got both less accurate and less understandable after that. I would advise the OP to stop reading your post after that first sentence.

... When we encounters two tasks having some strong relation with each other we put them into one process as threads.

I'm having trouble guessing what you might mean by "we encounters". It is especially hard to find a meaning that makes that sentence correct.

From the point of view of selecting one or the other in adding parallelism to an application design, the big difference between threads and processes is that multiple threads within a process share that process's address space.

Two processes can share memory that is mapped into each of their address spaces. So even that big difference is not an absolute design constraint in choosing multiple threads vs. multiple processes.

There is a big overhead to creating, maintaining, and eventually deleting and address space. So having multiple processes has more overhead than multiple threads within a process.

But the design decision between the two usually comes down to shared memory.

With multiple threads in one process, it takes extra programming effort every time you want to not share some memory between threads. For example, there are often static or global variables that would cause conflicts if freely shared and don't really need to be shared at all: Things would be simpler if each thread just had its own copy. It is possible to set up such thread local replacements for static or global variables, but it is significant extra work. Often it is easier to use locking to control sharing even though there is no underlying benefit to having that sharing (vs. private copies).

With multiple processes, memory by default is not shared. Static and global variable are private to each process and you need extra effort everywhere you do want to share memory.