Computer Architecture Virtual Memory

What do we want? Physical Logical Memory with infinite capacity

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

Paging A process’s virtual address space is divided into equal sized pages. A virtual address is a pair (p, o).

Paging Physical memory is divided into equal sized frames. size of page = size of frame Physical memory address is a pair (f, o).

Paging

Mapping from a Virtual to a Physical Address

Paging: Virtual Address Translation

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.

Paging: Example

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.

Translation Look-aside Buffer (TLB) Problem - Each (virtual) memory reference requires two memory references! Solution: Translation lookaside buffer.

A Big Picture

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

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

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

Paging: Protection and Sharing Protection is specified per page basis. Sharing Sharing is done by pages in different processes mapped to the same frames. Sharing

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 > 100 + 25,000,000p 10 > 25,000,000p p < 0.0000004 (one fault every 2,500,000 references) Lesson: OS had better do a good job of page replacement!

Replacement Algorithm - LRU (Least Recently Used) Algorithm Replace the page that has not been used for the longest time.

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.

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.

Second-Chance Algorithm valid/invalid bit reference (used) bit frame number

Page Size Small page sizes Large 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.

I/O Interlock Problem - DMA Solutions 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.

Segmentation with Paging

Segmentation with Paging Individual segments are implemented as a paged, virtual address space. A logical address is now a triple (s, p, o)

Segmentation with Paging Address translation

Segmentation with Paging Additional benefits Protection: protection can be specified per segment basis rather than per page basis. Sharing

Typical Memory Hierarchy - The Big Picture

Typical Memory Hierarchy - The Big Picture

Typical Memory Hierarchy - The Big Picture

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