Memory Management

What if pgm mem > main mem ?

Memory Management

What if pgm mem > main mem ?

Overlays – program controlled

Memory Management

What if pgm mem > main mem ?

Virtual Memory – OS controlled (with architecture help)

Memory Management

Separate physical, logical address space

Page faults

Demand paging

Memory Access in VM

Is access legal ? (seg fault) If page in physical memory, return mem Else

Find free page Schedule disk operation Perform disk operation Update page table Restart program at offending address

Memory Access Time

Assume machine characteristics 200ns clock rate Main Mem access of 5 cycles 1000ns 25 milisecond page service time

Computing Average Access Time

MAT = (1 – p) * 1000 + p * 25,000,000

e.g. assume page fault rate of .001

MAT = .999 * 1000 + .001 * 25,000,000 = 999 + 25,000 26x penalty

Your Turn

Those were 1998 numbers Assume 2x speedup in clock speed every two

years, 2x speedup in disk access time every 4 years

What penalty would .001 page fault rate lead to in 2010?

What page fault rate would allow MAT of 2x main memory access time.

Replacement Policy (Page)

FIFO Random Optimal LRU LRU approximation

Sample References

7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1

FIFO – 4 pages

7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1

2

1 4

3 72104

2 0 1 2

1 20

3

3 4 0 1

7 0 1 2 3 4 0

Optimal – 4 pages

7 0 1 2 0 3 0 4 2 3 0 3 2 1 0 1 7 0 1

2

1 4

2 112

1 7

0 00

4

0 0

7 3 3 3 3

LRU – 4 pages

7 0 1 2 0 3 0 4 2 3 0 3 2 1 0 1 7 0 1

2

1 1

2 222

1 1

0 00

4

0 0

7 3 3 3 7

Your Turn

7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1

Show FIFO, LRU, Optimal for references above but with 3 pages.