Virtual memory: design issues

Design issues for VM are related to HUGE cost of a miss (page fault)

Accessing disk may take MILLIONS of clock cycles

SRAM: 5-25ns

DRAM: 60-120ns

Disk: 10-20 million ns

- Pages should be large enough to cover the cost of page fault

transfer time is much less than access time

4KB to 16KB common

newer systems: 32KB - 64KB

- Reducing page fault rate has high priority

fully-associative page placement

- Page faults can be handled in software

overhead is small compared to cost of disk access

use clever algorithms to minimize page faults

- Write-through is too slow

use write-back to store modified data