🚀 go-pugleaf

The issue of virtual memory usage on large memories keeps coming up,
so I felt compelled to propose the following as the reasons for
using virtual memory in a computer system:

1) The program doesn't fit in the memory, or the job mix doesn't fit
but the working set does.  This reason may go away as memory sizes
increase, unless applications sizes increase at a faster rate, or cpu
powers increase such that more jobs or tasks need to be in memory
to keep the cpu busy.

2) Memory management hardware usually provides memory protection
and automatic relocation (which may improve load time) but these 
features are available without virtual memory.

3) The real reason for virtual memory (and the one which won't
away when memories get big) is that I can quickly load the
working set of a large program, execute it, and go on to the next
program, while a real memory system might be still loading its program.

In fact, I can start executing when the first page gets in, on
a properly implemented system where everything is already
in paging format.

This assumes that the working set is smaller than the program,
and that the programmer didn't do some work to only load those
pieces of program and data which were needed this time.

This kind of programmer work is probably non-productive
especially since hardware (and software LRU algorithms)
can do it better. 

Another case peculiar to Unix systems is VFORK. Why duplicate
the whole address space when the most likely thing to do next
is EXEC, which will clobber the space you just copied?

In summary, my guess is that we will continue to pay
the cost of virtual memory overhead for a long time
yet, for the majority of applications.


Jan Stubbs    ....sdcsvax!ncr-sd!stubbs
619 485-3052
NCR Corporation
16550 W. Bernardo Drive MS4010
San Diego, CA. 92127