mangoe@umcp-cs.UUCP (Charley Wingate) (12/11/85)
In article <34595@lanl.ARPA> jlg@a.UUCP (Jim Giles) writes: >It should also be noted that most things like bounds >checks, shared memory segments, and the like can exist without any paging. Quite true, and I don't know why I ever said otherwise. >>There are lots of programs, however, which do not consist of a huge table >>with a little bit of code hanging off the side. Consider an event-driven >>simulation program. A significant fragment of the code is going to be >>simply looking for the next event. A relatively small area of the data is >>going to be exercised disproportionately. THis even more true of data >>retrieval systems. These programs tend to have data swapping built into >>them (since they tend to allow for unlimited expansion through the use of >>files), so why should there be an objection to using paging? >All that you are saying is quite true. I have no objection to paging >schemes for the types of applications you describe. In fact, if paging >didn't slow down the types of codes we run here, I would have no objection >to it at all. Hardware should be selected for those features which best >run the type of workload that you have. Paging is just another of those >features with strengths and weaknesses like any other - but some people on >this network don't seem to realize that virtual memory has weaknesses. No doubt. Even if there is enough memory to where page faults almost never outrun the paging device's ability to handle them (and on a machine like a Cray, this is a LOT of memory-- maybe billions of pages?), there's still the overhead of the necessary context switches. Maybe our disagreement was only an illusion. >By the way, virtual memory does not allow for 'unlimited expansion'. I >know people with jobs that routinely access sets of files that hold more >than the entire virtual memory space of most byte addressable machines >(VAXen and the like). For them, virtual memory is just another form of >offline store with a set of limitations similar to any other. I'm sorry if I was unclear; my point was that these systems act somewhat like paging systems with very limited memory spaces. They are essentially paging the immense virtual memory of their files into their programs. Charley Wingate