PiersBull
asked on
Virtual Memory
Hi,
I think I pretty much understand virtual memory, but I need an example of a program which would run very efficiently on a virtual memory system, and one which definitely wouldn't. Also a quick explanation of why would be appreciated.
Cheers.
I think I pretty much understand virtual memory, but I need an example of a program which would run very efficiently on a virtual memory system, and one which definitely wouldn't. Also a quick explanation of why would be appreciated.
Cheers.
Any program which makes use of a memory set which cannot be held in real memory will suffer performance degradation. The issue is that virtual memory is orders of magnitude slower than real memory. Once your program starts needing it, execution slows to a crawl. Most computers today can access a word of memory in under 100nS (that's 100 X 10^-9 seconds or 1 x 10^-7 seconds). Virtual memory is stored on the disk and most disk systems provide access times on the order of 10mS (that's 10 x 10^-3 seconds or 1 x 10^-2 seconds). If we compare the two, we are talking about 5 orders of magnitude. In other words, RAM access is 100,000 times faster than disk access.
ASKER
jhance,
Cool. I understand the massive loss in performance, but it seems that some programs work better with virtual memory than others. I need to know what type of application will utilise virtual memory better than another. I need to know as I'm preparing for final exams in C.S. at Uni, and the question has come up several times in the past.
Cheers.
Cool. I understand the massive loss in performance, but it seems that some programs work better with virtual memory than others. I need to know what type of application will utilise virtual memory better than another. I need to know as I'm preparing for final exams in C.S. at Uni, and the question has come up several times in the past.
Cheers.
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