Computer Organization Lecture 25

Fall 2006

Lillevik 333f06-l25 1University of Portland School of Engineering

EE 333

Computer OrganizationLecture 25

Virtual memoryPage replacement

Translation lookaside buffer (TLB)

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Memory design goals

• Unlimited memory size– No upper bound on memory addresses– Not practical or possible: cost, implementation

• Infinite memory bandwidth– Zero latency memory accesses– Not practical or possible: cost, laws of physics

• Memory hierarchy can approximate goals

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Virtual memory

• Programmer sees extremely large memory– Memory divided into pages (1K, 4K, 8K, etc.)– Some pages exist in memory, rest on hard drive

• Pages in use cached in memory– Provides good performance-cost tradeoff– Requires table to map memory pages to disk

(cylinder, head, sector)

– Memory access either hit or miss page

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Virtual and physical memory

Page table translates

virtual-to-physical

Memory-resident frames

Disk-resident pages

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OS demand paging

• Bring a page into memory only when it is needed (on first request)– Less I/O needed, less memory needed – Faster response, better performance

• OS (pager) must first check page attributes– Illegal reference error condition– Not-in-memory bring into memory– Memory resident continue

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Page fault steps• First reference to page: traps/interrupts to OS

page fault (miss)

• OS looks at page table– Invalid reference abort, error condition– Page not in memory

• Get empty frame from free frame list• Copy (write) page into frame• Update page table, set valid bit = 1• Restart instruction

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Free frames list

Before allocation After allocation

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What if free frame list is empty?

• Reason– Too many users– Too many programs– Too much data, etc.

• Solution– Find a frame to evict (create space)– May require write-back to disk

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Page Replacement

1. Find the location of the desired page on disk

2. Find a free frame (algorithm)

- If there is a free frame, use it

- If there is no free frame, use a page replacement algorithm to select a victim frame

- Evict victim frame: write to disk if modified (dirty), write-back

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Page Replacement, continued.

3. Read the desired page into the (newly) free frame

- Update page table

- Update frame table

4. Restart the process (instruction)

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Page replacement stepsDMA write

DMA read

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Page replacement algorithm? (Which page to evict)

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Paging memory performance

• Similar to cache performance– Hit: memory page is resident– Miss: memory page on disk only

• Access time

diskmemave

ThThT 1

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Find the access time?Find an expression for the average access time Tave of a memory-hard disk hierarchy? Assume the page rate (hit) is p, memory access time is m, the I/O bus bandwidth is b, the size of a page is s bytes, a page is found dirty (modified) with probability d, and the free page list is always empty (memory full).

diskmemave

TpTpT 1

mTmem

modifiednot modified diskT

read read andback write

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Find the access time? Continued.

busbusdisk

TdTdT 12

b

sd

b

sd 12

b

sd

b

sdpmp 121

diskave TpmpT 1

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Problems with paging

• Page table can become enormous– Several OS solutions– Hashing function, inverted page table

• Each memory access requires page table access (another memory access)– Cache the page table– Translation lookaside buffer (TLB)

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TLB

• Purpose: hardware speed up for page table access

• What it is: small, fast cache of recently used page table entries

• Accesses– Hit: page table value is cached (fast)– Miss: page table value not cached, must read it

from memory

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Pentium 4 execution engine

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Find the effective memory access?TLB = 20 ns, mem = 100 ns, hit = 0.8

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Page replacement algorithm?

• Least recently used (LRU)

• First in, first out (FIFO)• Optimal: lowest page fault rate

• Random

(Which page to evict)

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Find the effective memory access?TLB = 20 ns, mem = 100 ns, hit = 0.8

)(1 mmTLBhmTLBhTave

ns)10010020(8.01100208.0

ns)220(2.01208.0

ns140

Computer Organization Lecture 25

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