| 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 | |||||||||||||