We'll hear about many abstractions this semester -- we'll spend a great deal of time discussing various abstractions and how to model them in software. So what is an abstraction?
Again, a very good defintion was provided by a student very early in the discussion:An abstraction is a representation of something that incorporates the essential or relevent properties, while neglecting the irrelevant details.
I think this is a very good defintion. Throughout this semester, we'll often consider something that exists in the real world and then distill it to those properties that areof concern to us. We'll often then take those properties and represent them as data structures and algorithms that that represent the "real world" items within our software systems.
The first abstraction that we'll consider is arguably the most important -- a represention of the work that the system will do on behalf of a user (or, perhpas, itself). I've used a lot of different words to describe this so far: task, job, process, &c. But I've never been very specific about what I've meant -- to be honest, I've been a bit sloppy.
This abstraction is typically called a task. In a slightly different form, it is known as a process. We'll discuss the difference when we discuss threads. The short version of the difference is that a task is an abstraction that represents the instance of a program in execution, whereas a process is a particular type fo task with only one thread of control. But, for now, let's not worry about the difference.
If we say that a task is an instance of a program in execution, what do we mean? What is an instance? What is a program? What do we mean by execution?
A program is a specification. It contains defintions of what type fo data is stored, how it can be accessed, and a set of instructions that tells the computer how to accomplish something useful. If we think of the program as a specification, much like a C++ class, we can think of the task as an instance of that class -- much like an object built from the specification provided by the program.
So, what do we mean by "in execution?" We mean that the task is a real "object" not a "class." Most importantly, the task has state associated with it -- it is in the process of doing something or changing somehow. Hundreds of tasks may be instances of the same program, yet they might behave very differently. This happens because the tasks were exposed to different stimuli and their changed accordingly.
Representing a Task in Software
How do we represent a task within the context of an operating system? We build a data structure, sometimes known as a task_struct or (for processes) a Process Control Block (PCB) that contains all of the information our OS needs about the state of the task. This includes, among many other things:
- content of registers (like the PC)
- content of the stack
- memory pages/segments
- open files
When a context switch occurs, it is this information that needs to be saved and restored to change the executing process.
Just like people, tasks have lifestyles. They aren't always running and they don't live forever. Typical UNIX systems view tasks as existing in one of several states:
- New: Recently created
- Ready: Ready to run, but not yet assigned to a processor
- Running: Actually executing on a processor
- Waiting: Not currently executing. Not currently able to execute. Won't be runnable until some specific external event occurs, such as an I/O operation.
- Terminated: Done executing, won't accomplish anything else useful or again be placed on a CPU.
A task moves from the new state to ready state after it is created. Once this happens, we say that the task is "admitted."
After the scheduler selects a task and assigns it to a processor, we say that the task has been "dispatched."
When a task is done, it "exits." It is then in the terminated state.
If a task is waiting for an event, such as a disk read to complete, it can "block" itself yielding the CPU. It is then in the "wait" state. The system has many different wait queues -- not one universal wait queue -- in fact, there is one wiat queue for each possible reason to wait. This is because it would be very expensive to sift through a long list each time a resource became available or other event occured. It is not a case of needing a list of lists, either -- since each list is associated with the event, it doesn't require any searching -- if we take care of the queue when we handle the event, we're already in the right place.
After the event occurs, the operating system can move it to the "ready" state.
After a task has exhausted its time slice, it can be moved into the ready state to allow another task access to the processor.
Please pay careful attention. The operating system is responsible for creating tasks, dispatching them, readying them after an event, and interrupting them after their time expires. Tasks must exit and block voluntarily.
Creating New Tasks
One of the functions of the operating system is to provide a mechanism for existing tasks to create new tasks. When this happens, we call the original task the parent. The new task is called the child. It is possible for one task to have many children. In fact, even the children can have children.
In UNIX, child tasks can either share resources with the parent or obtain new resources. But existing resources are not partitioned.
In UNIX when a new task is created, the child is a clone of the parent. The new task can either continue to execute with a copy of the parent image, or load another image. Well talk more about this soon, when we talk about the fork() and exec-family() of calls.
After a new task is created, the parent may either wait for the child to finish or continue and execute concurrently (real or imaginary) with the child.
A child may end as the result of the normal completion, it may be terminated by the operating system for "breaking the rules", or it might be killed by the parent. Often times parents will kill their children before they themselves exit, or when their function is no longer required.
In UNIX, when a task terminates, it enters the defunct state. It remains in this state until the parent recognizes the fact that it has ended via the wait-family() of calls. Although a defuct task has given up most of its resources, much of the state information is preserved so that the parent can find out the circumstances of the child's death.
In UNIX children can outlive their parents. When this happens, there is a small complication. The parent is not around to acknowlege the child's death. A dead process is known as a zombie if its parent has already died. The init process waits for all zombies, allowing for them to have a proper burial. Sometimes zobies are known as orphans.
Fork -- A traditional implementation
fork() is the system call that is used to create a new task on UNIX systems. In a traditional implementation, it creates a new task by making a nearly exact copy of the parent. Why nearly exact? Some things don't make sense to be duplicated exactly, the ID number, for example.
The fork() call returns the ID of the child process in the parent and 0 in the child. Other than this type of subtle differences, the two tasks are very much alike. Execution picks up at the same point in both.
If execution picks up at the same point in both, how can fork() return something different in each? The answer is very straightforward. The stack is duplicated and a different value is placed on top of each. (If you don't remeber what the stack is, don't worry, we'll talk about it soon -- just realize that the return value is different).
The difference in the return value of the fork() is very significant. Most programmers check the result of the fork in order to determine whether they are currently the child or parent. Very often the child and parent to very different things.
The Exec-family() of calls
Since the child will often serve a very different purpose that its parent, it is often useful to replace the child's memory space, that was cloned form the parent, with that of another program. By replace, I am referring to the following process:Fork w/copy-on-write
- Deallocate the process' memory space (memory pages, stack, etc).
- Allocate new resources
- Fill these resources with the state of a new process.
- (Some of the parent's state is preserved, the group id, interrupt mask, and a few other items.)
Copying all of the pages of memory associated with a process is a very expensive thing to do. It is even more expensive considering that very often the first act of the child is to deallocate this recently created space.
One alternative to a traditional fork implementation is called copy-on-write. the details of this mechanism won't be completely clear until we study memory management, but we can get the flavor now.
The basic idea is that we mark all of the parent's memory pages as read-only, instead of duplicating them. If either the parent or any child try to write to one of these read-only pages, a page-fault occurs. At this point, a new copy of the page is created for the writing process. This adds some overhead to page accesses, but saves us the cost of unnecessarly copying pages.
Another alternative is also available -- vfork(). vfork is even faster, but can also be dangerous in the worng hands. With vfork(), we do not duplicate or mark the parent's pages, we simply loan them, and the stack frame to the child process. During this time, the parent remains blocked (it can't use the pages). The dangerous part is this: any changes the child makes will be seen by the aprent process.
vfork() is most useful when it is immediately followed by an exec_(). This is because an exec() will create a completely new process-space, anyway. There is no reason to create a new task space for the child, just to have it throw it away as part of an exec(). Instead, we can loan it the parent's space long enough for it to get started (exec'd).
Although there are several (4) different functions in the exec-family, the only difference is the way they are parameterizes; under-the-hood, they all work identically (and are often one).
After a new task is created, the parent will often want to wait for it (and any siblings) to finish. We discussed the defunct and zombie states last class. The wait-family of calls is used for this purpose.