Memory Management
Another significant advantage was realized for virtual memory. In the B5000 design, if a data block were rolled out, all descriptors referencing that block needed to be found in order to update the presence bit and address. With the master descriptor, only the presence bit in the master descriptor needs changing. Also the MCP can move blocks around in memory for compaction and only needs to change the address in the master descriptor.
A difference between the B5000 and most other systems is that other systems mainly used paged virtual memory, that is pages are swapped out in fixed-sized chunks regardless of the structure of the information in them. B5000 virtual memory works with varying-size segments as described by the descriptors.
When the memory is filled to a certain capacity, an OS process called the 'Working Set Sheriff' is invoked to either compact memory or start moving segments out of memory. It chooses code segments first, since these cannot change and can be reloaded from the original in the code file, so do not need writing out, and then data segments which are written out to the virtual memory file.
P-bit interrupts are also useful to measure system performance. For first-time allocations, 'init p-bits' indicate a potential performance problem in a program, for example if a procedure allocating an array is continually called. Reloading blocks from virtual memory on disk can significantly degrade system performance and is not the fault of any specific task. This is why many of today's computers may gain increased system performance by adding memory. On B5000 machines, 'other p-bits' indicate a system problem, which can be solved either by better balancing the computing load across the day, or by adding more memory.
Thus the Burroughs large systems architecture helps optimization of both individual tasks and the system as a whole.
Read more about this topic: Burroughs Large Systems Descriptors
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