Memory/Storage Architecture Lab Computer Architecture Virtual Memory

2 Memory/Storage Architecture Lab 2 What do we want? Physical Logical Memory with infinite capacity

3 Memory/Storage Architecture Lab 3 Virtual Memory Concept  Hide all physical aspects of memory from users. Memory is a logically unbounded virtual (logical) address space of 2 n bytes. Only portions of virtual address space are in physical memory at any one time.

4 Memory/Storage Architecture Lab 4 Paging  A process’s virtual address space is divided into equal sized pages.  A virtual address is a pair (p, o).

5 Memory/Storage Architecture Lab 5 Paging  Physical memory is divided into equal sized frames. size of page = size of frame  Physical memory address is a pair (f, o).

6 Memory/Storage Architecture Lab 6 Paging

7 Memory/Storage Architecture Lab 7 Mapping from a Virtual to a Physical Address

8 Memory/Storage Architecture Lab 8 Paging: Virtual Address Translation

9 Memory/Storage Architecture Lab 9 Paging: Page Table Structure  One table for each process - part of process’s state.  Contents Flags: valid/invalid (also called resident) bit, dirty bit, reference (also called clock or used) bit. Page frame number.

10 Memory/Storage Architecture Lab 10 Paging: Example

11 Memory/Storage Architecture Lab 11 Demand Paging  Bring a page into physical memory (i.e., map a page to a frame) only when it is needed.  Advantages: Program size is no longer constrained by the physical memory size. Less memory needed  more processes. Less I/O needed  faster response. Advantages from paging − Contiguous allocation is no longer needed  no external fragmentation problem. − Arbitrary relocation is possible. − Variable-sized I/O is no longer needed.

12 Memory/Storage Architecture Lab 12 Translation Look-aside Buffer (TLB)  Problem - Each (virtual) memory reference requires two memory references!  Solution: Translation lookaside buffer.

13 Memory/Storage Architecture Lab 13 A Big Picture

14 Memory/Storage Architecture Lab 14 On TLB misses  If page is in memory Load the PTE (page table entry) from memory and retry Could be handled in hardware − Can get complex for more complicated page table structures Or in software − Raise a special exception, with optimized handler  If page is not in memory (page fault) OS handles fetching the page and updating the page table Then restart the faulting instruction

15 Memory/Storage Architecture Lab 15 TLB Miss Handler  TLB miss indicates Page present, but PTE not in TLB Page not preset  Must recognize TLB miss before destination register overwritten Raise exception  Handler copies PTE from memory to TLB Then restarts instruction If page not present, page fault will occur

16 Memory/Storage Architecture Lab 16 Page Fault Handler  Use faulting virtual address to find PTE  Locate page on disk  Choose page to replace If dirty, write to disk first  Read page into memory and update page table  Make process runnable again Restart from faulting instruction

17 Memory/Storage Architecture Lab 17 Paging: Protection and Sharing  Protection Protection is specified per page basis.  Sharing Sharing is done by pages in different processes mapped to the same frames. Sharing

18 Memory/Storage Architecture Lab 18 Virtual Memory Performance  Example Memory access time: 100 ns Disk access time: 25 ms Effective access time − Let p = the probability of a page fault − Effective access time = 100(1-p) + 25,000,000p − If we want only 10% degradation 110 > ,000,000p 10 > 25,000,000p p < (one fault every 2,500,000 references)  Lesson: OS had better do a good job of page replacement!

19 Memory/Storage Architecture Lab 19 Replacement Algorithm - LRU (Least Recently Used) Algorithm  Replace the page that has not been used for the longest time.

20 Memory/Storage Architecture Lab 20 LRU Algorithm - Implementation  Maintain a stack of recently used pages according to the recency of their uses. Top: Most recently used (MRU) page. Bottom: Least recently used (LRU) page.  Always replace the bottom (LRU) page.

21 Memory/Storage Architecture Lab 21 LRU Approximation - Second-Chance Algorithm  Also called the clock algorithm.  A variation used in UNIX.  Maintain a circular list of pages resident in memory. At each reference, the reference (also called used or clock) bit is simply set by hardware. At a page fault, clock sweeps over pages looking for one with reference bit = 0. − Replace a page that has not been referenced for one complete revolution of the clock.

22 Memory/Storage Architecture Lab 22 Second-Chance Algorithm valid/invalid bit reference (used) bit frame number

23 Memory/Storage Architecture Lab 23 Page Size  Small page sizes + less internal fragmentation, better memory utilization. - large page table, high page fault handling overheads.  Large page sizes + small page table, small page fault handling overheads. - more internal fragmentation, worse memory utilization.

24 Memory/Storage Architecture Lab 24 I/O Interlock  Problem - DMA Assume global page replacement. A process blocked on an I/O operation appears to be an ideal candidate for replacement. If replaced, however, I/O operation can corrupt the system.  Solutions 1. Lock pages in physical memory using lock bits, or 2. Perform all I/O into and out of OS space.

25 Memory/Storage Architecture Lab 25 Segmentation with Paging

26 Memory/Storage Architecture Lab 26 Segmentation with Paging  Individual segments are implemented as a paged, virtual address space. A logical address is now a triple (s, p, o)

27 Memory/Storage Architecture Lab 27 Segmentation with Paging  Address translation

28 Memory/Storage Architecture Lab 28 Segmentation with Paging  Additional benefits Protection: protection can be specified per segment basis rather than per page basis. Sharing

29 Memory/Storage Architecture Lab 29 Typical Memory Hierarchy - The Big Picture

30 Memory/Storage Architecture Lab 30 Typical Memory Hierarchy - The Big Picture

31 Memory/Storage Architecture Lab 31 Typical Memory Hierarchy - The Big Picture

32 Memory/Storage Architecture Lab 32 A Common Framework for Memory Hierarchies  Question 1: Where can a Block be Placed? One place (direct- mapped), a few places (set associative), or any place (fully associative)  Question 2: How is a Block Found? Indexing (direct-mapped), limited search (set associative), full search (fully associative)  Question 3: Which Block is Replaced on a Miss? Typically LRU or random  Question 4: How are Writes Handled? Write-through or write- back