PCLSRing
Encyclopedia
PCLSRing is the term used in the ITS operating system
for a consistency principle in the way one process accesses the state of another process.
What should be the visible state of the context of Process A at the time of the access by Process B? In fact, it is in the middle of a system call, but ITS enforces the appearance that system calls are not visible to other processes (or even to the same process).
operating system, where there is restartability, but it is not transparent. Instead, an I/O operation returns the number of bytes actually transferred (or the EINTR error if the operation was interrupted before any bytes were actually transferred), and it is up to the application to check this and manage its own resumption of the operation until all the bytes have been transferred. In the philosophy of UNIX
, this was given by Richard P. Gabriel as an example of the "worse is better
" principle.
operating system, all I/O and other time-consuming operations are inherently asynchronous, which means the semantics of the system call is "start the operation, and perform one or more of these notifications when it completes" after which it returns immediately to the caller. There is a standard set of available notifications (such as set an event flag
, or deliver an asynchronous system trap
), as well as a set of system calls for explicitly suspending the process while waiting for these, which are a) fully restartable in the ITS sense, and b) much smaller in number than the set of actual time-consuming system calls.
OpenVMS provides alternative "start operation and wait for completion" synchronous versions of all time-consuming system calls. These are implemented as "perform the actual asynchronous operation" followed by "wait until the operation sets the event flag". Any access to the process context during this time will see it about to (re)enter the wait-for-event-flag call.
Incompatible Timesharing System
ITS, the Incompatible Timesharing System , was an early, revolutionary, and influential time-sharing operating system from MIT; it was developed principally by the Artificial Intelligence Laboratory at MIT, with some help from Project MAC.In addition to being technically influential ITS, the...
for a consistency principle in the way one process accesses the state of another process.
Problem scenario
This scenario presents particular complications:- Process A makes a time-consuming system call. By "time-consuming", it is meant that the system needs to put Process A into a wait queue and can schedule another process for execution if one is ready-to-run. A common example is an I/OInput/outputIn computing, input/output, or I/O, refers to the communication between an information processing system , and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it...
operation. - While Process A is in this wait state, Process B tries to interact with access Process A, for example, send it a signalSignal (computing)A signal is a limited form of inter-process communication used in Unix, Unix-like, and other POSIX-compliant operating systems. Essentially it is an asynchronous notification sent to a process in order to notify it of an event that occurred. When a signal is sent to a process, the operating system...
.
What should be the visible state of the context of Process A at the time of the access by Process B? In fact, it is in the middle of a system call, but ITS enforces the appearance that system calls are not visible to other processes (or even to the same process).
ITS-solution: transparent restart
If the system call cannot complete before the access, then it must be restartable. This means that the context is backed up to the point of entry to the system call, while the call arguments are updated to reflect whatever portion of the operation has already been completed. For an I/O operation, this means that the buffer start address must be advanced over the data already transferred, while the length of data to be transferred must be decremented accordingly. After the Process B interaction is conplete, Process A can resume execution, and the system call resumes from where it left off.Unix-solution: restart on request
Contrast this with the approach taken in the UNIXUnix
Unix is a multitasking, multi-user computer operating system originally developed in 1969 by a group of AT&T employees at Bell Labs, including Ken Thompson, Dennis Ritchie, Brian Kernighan, Douglas McIlroy, and Joe Ossanna...
operating system, where there is restartability, but it is not transparent. Instead, an I/O operation returns the number of bytes actually transferred (or the EINTR error if the operation was interrupted before any bytes were actually transferred), and it is up to the application to check this and manage its own resumption of the operation until all the bytes have been transferred. In the philosophy of UNIX
Unix philosophy
The Unix philosophy is a set of cultural norms and philosophical approaches to developing software based on the experience of leading developers of the Unix operating system.-McIlroy: A Quarter Century of Unix:...
, this was given by Richard P. Gabriel as an example of the "worse is better
Worse is better
Worse is better, also called the New Jersey style, was conceived by Richard P. Gabriel to describe the dynamics of software acceptance, but it has broader application. The idea is that quality does not necessarily increase with functionality. There is a point where less functionality is a...
" principle.
Asynchronous approaches
A different approach is possible. It is seems in the above that the system call has to be synchronous--that is, the calling process has to wait for the operation to complete. There is no reason for this: in the OpenVMSOpenVMS
OpenVMS , previously known as VAX-11/VMS, VAX/VMS or VMS, is a computer server operating system that runs on VAX, Alpha and Itanium-based families of computers. Contrary to what its name suggests, OpenVMS is not open source software; however, the source listings are available for purchase...
operating system, all I/O and other time-consuming operations are inherently asynchronous, which means the semantics of the system call is "start the operation, and perform one or more of these notifications when it completes" after which it returns immediately to the caller. There is a standard set of available notifications (such as set an event flag
Event flag
An event flag is a process synchronization primitive in the OpenVMS operating system. It has two possible states, set or cleared. The following basic primitive operations are provided:* Set event flag * Clear event flag...
, or deliver an asynchronous system trap
Asynchronous System Trap
Asynchronous system trap refers to a mechanism used in several computer operating systems designed by the former Digital Equipment Corporation of Maynard, Massachusetts....
), as well as a set of system calls for explicitly suspending the process while waiting for these, which are a) fully restartable in the ITS sense, and b) much smaller in number than the set of actual time-consuming system calls.
OpenVMS provides alternative "start operation and wait for completion" synchronous versions of all time-consuming system calls. These are implemented as "perform the actual asynchronous operation" followed by "wait until the operation sets the event flag". Any access to the process context during this time will see it about to (re)enter the wait-for-event-flag call.
External links
- 'PCLSRing: Keeping Process State Modular- (by Alan Bawden; an excellent description of the ITS implementation; from his homepage)