Книга: Distributed operating systems

8.2.1. Processes

8.2.1. Processes

A process in Mach consists primarily of an address space and a collection of threads that execute in that address space. Processes are passive. Execution is associated with the threads. Processes are used for collecting all the resources related to a group of cooperating threads into convenient containers.

Figure 8-2 illustrates a Mach process. In addition to an address space and threads, it has some ports and other properties. The ports shown in the figure all have special functions. The process port is used to communicate with the kernel. Many of the kernel services that a process can request are done by sending a message to the process port, rather than making a system call. This mechanism is used throughout Mach to reduce the actual system calls to a bare minimum. A small number of them will be discussed in this chapter, to give an idea of what they are like.

In general, the programmer is not even aware of whether or not a service requires a system call. All services, including both those accessed by system calls and those accessed by message passing, have stub procedures in the library. It is these procedures that are described in the manuals and called by application programs. The procedures are generated from a service definition by the MIG (Mach Interface Generator) compiler.

The bootstrap port is used for initialization when a process starts up. The very first process reads the bootstrap port to learn the names of kernel ports that provide essential services. UNIX processes also use it to communicate with the UNIX emulator.

Fig. 8-2. A Mach process.

The exception port is used to report exceptions caused by the process. Typical exceptions are division by zero and illegal instruction executed. The port tells the system where the exception message should be sent. Debuggers also use the exception port.

The registered ports are normally used to provide a way for the process to communicate with standard system servers. For example, the name server makes it possible to present a string and get back the corresponding port for certain basic servers.

Processes also have other properties. A process can be runnable or blocked, independent of the state of its threads. If a process is runnable, those threads that are also runnable can be scheduled and run. If a process is blocked, its threads may not run, no matter what state they are in.

The per-process items also include scheduling parameters. These include the ability to specify which processors the process' threads can run on. This feature is most useful on a multiprocessor system. For example, the process can use this power to force each thread to run on a different processor, or to force them all to run on the same processor, or anything in between. In addition, each process has a default priority that is settable. When a thread is created, the new thread is given this priority. It is also possible to change the priority of all the existing threads.

Emulation addresses can be set to tell the kernel where in the process' address space system call handlers are located. The kernel needs to know these addresses to handle UNIX system calls that need to be emulated. These are set once when the UNIX emulator is started up and are inherited by all of the emulator's children (i.e., all the UNIX processes).

Finally, every process has statistics associated with it, including the amount of memory consumed, the run times of the threads, and so on. A process that is interested in this information can acquire it by sending a message to the process port.

It is also worth mentioning what a Mach process does not have. A process does not have a uid, gid, signal mask, root directory, working directory, or file descriptor array, all of which UNIX processes do have. All of this information is managed by the emulation package, so the kernel knows nothing at all about it.

Mach provides a small number of primitives for managing processes. Most of these are done by sending messages to the kernel via the process port, rather than actual system calls. The most important of these calls are shown in Fig. 8-3. These, like all calls in Mach, have prefixes indicating the group they belong to, but we have omitted these here (and in subsequent tables) for the sake of brevity.

Fig. 8-3. Selected process management calls in Mach.

The first two calls in Fig. 8-3 are for creating and destroying processes, respectively. The process creation call specifies a prototype process, not necessarily the caller. The child is a copy of the prototype, except that the call has a parameter that tells whether or not the child is to inherit the parent's address space. If it does not inherit the parent's address space, objects (e.g., text, initialized data, and a stack) can be mapped in later. Initially, the child has no threads. It does, however, automatically get a process port, a bootstrap port, and an exception port. Other ports are not inherited automatically since each port may have only one reader.

Processes can be suspended and resumed under program control. Each process has a counter, incremented by the suspend call and decremented by the resume call, that can block or unblock it. When the counter is 0, the process is able to run. When it is positive, it is suspended. Having a counter is more general than just having a bit, and helps avoid race conditions.

The priority and assign calls give the programmer control over how and where its threads run on multiprocessor systems. CPU scheduling is done using priorities, so the programmer has fine-grain control over which threads are most important and which are least important. The assign call makes it possible to control which thread runs on which CPU or group of CPUs.

The last two calls of Fig. 8-3 return information about the process. The former gives statistical information and the latter returns a list of all the threads.

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