Relationship to operating system:
If one tries to execute such an application on top of a general-purpose operating system it quickly becomes clear that many necessary functions are absent from the operating system. Traditionally two approaches have been engaged to this problem:
1) Write a new, simpler and ‘vastly superior’ operating system.2) Extend the basic OS to have the desired function.
The first approach was extremely popular in the mid-sixties and is having a renaissance with the advent of minicomputers. The early cost of a data management system is so low that almost any large customer can justify ‘rolling his own’. The performance of such modified systems is often ten times better than one based on a general purpose system. One should trade this off against the problems of maintaining the system as it grows to meet new needs and applications. Group’s that followed this path now search themselves maintaining a rather large operating system which must be modified to support new devices (tape archives, faster disks,...) and new protocols (example networks and displays.) Slowly these systems have grown to include all the functions of a general-purpose operating system. Possibly the most successful approach to this has been to implement a hypervisor that runs both the data management operating system and some non-standard operating system. The ‘standard’ OS runs when the data manager is idle. The hypervisor is merely an interrupt handler which dispatches one or another system.
The second approach of expanding the basic operating system is plagued with a different set of difficulties. The principal difficulty is the performance penalty of a general-purpose operating system. Very little systems are designed to deal with very large files or with networks of thousands of nodes. To take a specific instance considers the process structure of a general-purpose system: The allocation and de-allocation of a process should be extremely fast (500 instructions for the pair is expensive) because we want to do it 100 times per second. The storage engaged by the process descriptor must also be small (less than 1000 bytes.) finally pre-emptive scheduling of processes makes no sense since they aren’t CPU bound (they do a lot of I/O). A usual system uses 16000 bytes to represent a process and requires 200000 instructions to allocate as well as de-allocate this structure (systems without protection do it cheaper.)
One more problem is that the general-purpose systems have been designed for batch and time-sharing operation. They haven’t paid sufficient attention to issues such as continuous operation - keeping the system up for weeks at a time and gracefully degrading in case of some hardware or software error.
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