|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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 |
|
|
|
|
- Pages should be large enough to cover
the cost of page fault |
|
|
|
|
4KB to 16KB common |
|
|
|
- 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 expensive |
|
|
|
|
use write-back to store
data |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|