Virtual Memory

A page fault incurs slowdown in the activity of the OS. We can find the new average memory access time that takes such a slowdown into account. To calculate it, we need to know (1) the normal average memory access time (which we calculated on slide 26), (2) the time it takes to handle a page fault, and (3) the page fault rate on a scale from $0$ to $1$ (the fraction of memory accesses in which page faults happen).

Examples:

• If the normal average memory access time is $200$ ns, the page fault handling time is $8000000$ ns, and the page fault rate is $0.001$, the updated average memory access time would be $0.001 \cdot 8000000 + (1 - 0.001) \cdot 200 = 8000 + 199.8 $$= 8199.8$ ns (about $8.2$ microseconds), which is about $40$ times greater than $200$ ns!
• If the normal average memory access time is $50$ ns, the page fault handling time is $8000000$ ns, and the page fault rate is $0.002$, the updated average memory access time would be $0.002 \cdot 8000000 + (1 - 0.002) \cdot 50 = 16,000 + 49.9 $$= 16,049.9$ ns (about $16$ microseconds), which is about $320$ times greater than $50$ ns!