CS 152 Computer Architecture and

Engineering

Lecture 11 - Virtual Memory and Caches

Krste AsanovicElectrical Engineering and Computer Sciences

University of California at Berkeley

http://www.eecs.berkeley.edu/~krstehttp://inst.eecs.berkeley.edu/~cs152

3/6/2008 CS152-Spring’08 2

Last time in Lectures 10

• Modern page-based virtual memory systems provide:– Translation

» to avoid memory fragmentation and provide each user with own virtual address space

– Protection

» to protect users from each other, and to protect operating system from users

– Virtual memory

» to allow main memory to act as a cache of a larger disk memory, equivalent

• Translation and protection information stored in page tables, held in main memory

• Translation and protection information cached in “translation lookaside buffer” (TLB) to provide single cycle translation+protection check in common case

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Today is a review

• Many of you had questions about virtual memory and interaction with caches

• Going to go back over core of last two lectures with some more interactive explanation

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Simple Base and Bound Translation

Load X

ProgramAddressSpace

BoundRegister

BoundsViolation?

Main

Mem

ory

currentsegment

BaseRegister

+

PhysicalAddressEffective

Address

Base and bounds registers are visible/accessible only when processor is running in the supervisor mode

Base Physical Address

Segment Length

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Separate Areas for Program and Data

What is an advantage of this separation?(Scheme used on all Cray vector supercomputers prior to X1, 2002)

Load X

ProgramAddressSpace

Main

Mem

ory

datasegment

Data BoundRegister

Effective AddrRegister

Data BaseRegister

+

BoundsViolation?

Program BoundRegister

ProgramCounter

Program BaseRegister

+

BoundsViolation?

programsegment

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Memory Fragmentation

As users come and go, the storage is “fragmented”. Therefore, at some stage programs have to be moved around to compact the storage.

OSSpace

16K

24K

24K

32K

24K

user 1

user 2

user 3

OSSpace

16K

24K

16K

32K

24K

user 1

user 2

user 3

user 5

user 4

8K

Users 4 & 5 arrive

Users 2 & 5leave

OSSpace

16K

24K

16K

32K

24K

user 1

user 48K

user 3

free

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• Processor generated address can be interpreted as a pair <page number, offset>

• A page table contains the physical address of the base of each page

Paged Memory Systems

Page tables make it possible to store the pages of a program non-contiguously.

0123

01

23

Address Spaceof User-1

Page Table of User-1

10

2

3

page number offset

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Private Address Space per User

• Each user has a page table • Page table contains an entry for each user page

VA1User 1

Page Table

VA1User 2

Page Table

VA1User 3

Page Table

Physi

cal

Mem

ory

free

OSpages

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Page Tables in Physical Memory

VA1

User 1

PT User 1

PT User 2

VA1

User 2

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Linear Page Table

VPN Offset

Virtual addressPT Base Register

VPN

Data word

Data Pages

Offset

PPNPPN

DPNPPN

PPNPPN

Page Table

DPN

PPN

DPNDPN

DPNPPN

• Page Table Entry (PTE) contains:– A bit to indicate if a page exists

– PPN (physical page number) for a memory-resident page

– DPN (disk page number) for a page on the disk

– Status bits for protection and usage

• OS sets the Page Table Base Register whenever active user process changes

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Size of Linear Page Table

With 32-bit addresses, 4-KB pages & 4-byte PTEs: 220 PTEs, i.e, 4 MB page table per user 4 GB of swap needed to back up full virtual address

space

Larger pages?• Internal fragmentation (Not all memory in a page is used)

• Larger page fault penalty (more time to read from disk)

What about 64-bit virtual address space???• Even 1MB pages would require 244 8-byte PTEs (35 TB!)

What is the “saving grace” ?

sparsity of virtual address usage

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Hierarchical Page Table

Level 1 Page Table

Level 2Page Tables

Data Pages

page in primary memory page in secondary memory

Root of the CurrentPage Table

p1

offset

p2

Virtual Address

(ProcessorRegister)

PTE of a nonexistent page

p1 p2 offset01112212231

10-bitL1 index

10-bit L2 index

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Address Translation & Protection

• Every instruction and data access needs address translation and protection checks

A good VM design needs to be fast (~ one cycle) and space efficient

Physical Address

Virtual Address

AddressTranslation

Virtual Page No. (VPN) offset

Physical Page No. (PPN) offset

ProtectionCheck

Exception?

Kernel/User Mode

Read/Write

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Translation Lookaside BuffersAddress translation is very expensive!

In a two-level page table, each reference becomes several memory accesses

Solution: Cache translations in TLBTLB hit Single Cycle Translation

TLB miss Page Table Walk to refill

VPN offset

V R W D tag PPN

physical address PPN offset

virtual address

hit?

(VPN = virtual page number)

(PPN = physical page number)

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TLB Designs

• Typically 32-128 entries, usually fully associative– Each entry maps a large page, hence less spatial locality across pages

more likely that two entries conflict

– Sometimes larger TLBs (256-512 entries) are 4-8 way set-associative

• Random or FIFO replacement policy

• TLB Reach: Size of largest virtual address space that can be simultaneously mapped by TLB

Example: 64 TLB entries, 4KB pages, one page per entry

TLB Reach = _____________________________________________?64 entries * 4 KB = 256 KB (if contiguous)

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Address Translation in CPU Pipeline

• Software handlers need restartable exception on TLB fault

• Handling a TLB miss needs a hardware or software mechanism to refill TLB

• Need mechanisms to cope with the additional latency of a TLB:

– slow down the clock

– pipeline the TLB and cache access

– virtual address caches

– parallel TLB/cache access

PCInst TLB

Inst. Cache D Decode E M

Data TLB

Data Cache W+

TLB miss? Page Fault?Protection violation?

TLB miss? Page Fault?Protection violation?

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Handling a TLB Miss

Software (MIPS, Alpha)TLB miss causes an exception and the operating system walks the page tables and reloads TLB. A privileged “untranslated” addressing mode used for walk

Hardware (SPARC v8, x86, PowerPC)A memory management unit (MMU) walks the page tables and reloads the TLB

If a missing (data or PT) page is encountered during the TLB reloading, MMU gives up and signals an exception for the original instruction

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Virtual Memory

• More than just translation and protection

• Use disk to extend apparent size of main memory

• Treat DRAM as cache of disk contents

• Only need to hold active working set of processes in DRAM, rest of memory image can be swapped to disk

• Inactive processes can be completely swapped to disk (except usually the root of the page table)

• Combination of hardware and software used to implement this feature

• (ATLAS was first implementation of this idea)

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Page Fault Handler

• When the referenced page is not in DRAM:– The missing page is located (or created)

– It is brought in from disk, and page table is updated Another job may be run on the CPU while the first job waits

for the requested page to be read from disk

– If no free pages are left, a page is swapped out Pseudo-LRU replacement policy

• Since it takes a long time to transfer a page (msecs), page faults are handled completely in software by the OS

– Untranslated addressing mode is essential to allow kernel to access page tables

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Caching vs. Demand Paging

CPU cacheprimarymemory

secondarymemory

Caching Demand pagingcache entry page framecache block (~32 bytes) page (~4K bytes)cache miss rate (1% to 20%) page miss rate (<0.001%)cache hit (~1 cycle) page hit (~100 cycles)cache miss (~100 cycles) page miss (~5M cycles)a miss is handled a miss is handled in hardware mostly in software

primarymemory

CPU

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Address Translation:putting it all together

Virtual Address

TLBLookup

Page TableWalk

Update TLBPage Fault(OS loads page)

ProtectionCheck

PhysicalAddress(to cache)

miss hit

the page is memory memory denied permitted

ProtectionFault

hardwarehardware or softwaresoftware

SEGFAULT

Restart instruction

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Protection and Translation

• These have been combined in a modern virtual memory system, but are really separate functions

• Question, does translation itself provide enough protection?

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CS152 Administrivia

• Tuesday Mar 18, Quiz 3– Virtual memory hierarchy lectures Lab 8-10

3/6/2008 CS152-Spring’08 24

Address Translation in CPU Pipeline

• Software handlers need restartable exception on TLB fault

• Handling a TLB miss needs a hardware or software mechanism to refill TLB

• Need mechanisms to cope with the additional latency of a TLB:

– slow down the clock

– pipeline the TLB and cache access

– virtual address caches

– parallel TLB/cache access

PCInst TLB

Inst. Cache D Decode E M

Data TLB

Data Cache W+

TLB miss? Page Fault?Protection violation?

TLB miss? Page Fault?Protection violation?

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Virtual Address Caches

• one-step process in case of a hit (+)• cache needs to be flushed on a context switch unless address

space identifiers (ASIDs) included in tags (-)• aliasing problems due to the sharing of pages (-)• maintaining cache coherence (-) (see later in course)

CPU PhysicalCache

TLB PrimaryMemory

VAPA

Alternative: place the cache before the TLB

CPU

VA

(StrongARM)VirtualCache

PATLB

PrimaryMemory

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Aliasing in Virtual-Address Caches

VA1

VA2

Page Table

Data Pages

PA

VA1

VA2

1st Copy of Data at PA

2nd Copy of Data at PA

Tag Data

Two virtual pages share one physical page

Virtual cache can have two copies of same physical data. Writes to one copy not visible

to reads of other!

General Solution: Disallow aliases to coexist in cache

Software (i.e., OS) solution for direct-mapped cache

VAs of shared pages must agree in cache index bits; this ensures all VAs accessing same PA will conflict in direct-mapped cache (early SPARCs)

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Concurrent Access to TLB & Cache

Index L is available without consulting the TLBcache and TLB accesses can begin simultaneously

Tag comparison is made after both accesses are completed

Cases: L + b = k L + b < k L + b > k

VPN L b

TLB Direct-map Cache 2L

blocks2b-byte block

PPN Page Offset

=hit?

DataPhysical Tag

Tag

VA

PA

VirtualIndex

k

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Virtual-Index Physical-Tag Caches: Associative Organization

Is this scheme realistic?

VPN a L = k-b b

TLBDirect-map

2L blocks

PPN Page Offset

=hit?

Data

Phy.Tag

Tag

VA

PA

VirtualIndex

kDirect-map

2L blocks

2a

=2a

After the PPN is known, 2a physical tags are compared

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Concurrent Access to TLB & Large L1The problem with L1 > Page size

Can VA1 and VA2 both map to PA ?

VPN a Page Offset b

TLB

PPN Page Offset b

Tag

VA

PA

Virtual Index

L1 PA cacheDirect-map

= hit?

PPNa Data

PPNa Data

VA1

VA2

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A solution via Second Level Cache

Usually a common L2 cache backs up both Instruction and Data L1 caches

L2 is “inclusive” of both Instruction and Data caches

CPU

L1 Data Cache

L1 Instruction

CacheUnified L2

Cache

RF Memory

Memory

Memory

Memory

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Anti-Aliasing Using L2: MIPS R10000

VPN a Page Offset b

TLB

PPN Page Offset b

Tag

VA

PA

Virtual Index L1 PA cacheDirect-map

= hit?

PPNa Data

PPNa Data

VA1

VA2

Direct-Mapped L2

PA a1 Data

PPN

into L2 tag

• Suppose VA1 and VA2 both map to PA and VA1 is already in L1, L2 (VA1 VA2)

• After VA2 is resolved to PA, a collision will be detected in L2.

• VA1 will be purged from L1 and L2, and VA2 will be loaded no aliasing !

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Virtually-Addressed L1:Anti-Aliasing using L2

VPN Page Offset b

TLB

PPN Page Offset b

Tag

VA

PA

VirtualIndex & Tag

PhysicalIndex & Tag

L1 VA Cache

L2 PA Cache L2 “contains” L1

PA VA1 Data

VA1 Data

VA2 Data

“VirtualTag”

Physically-addressed L2 can also be used to avoid aliases in virtually-addressed L1

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Variable-Sized Page Support

Level 1 Page Table

Level 2Page Tables

Data Pages

page in primary memorylarge page in primary memory page in secondary memoryPTE of a nonexistent page

Root of the CurrentPage Table

p1

offset

p2

Virtual Address

(ProcessorRegister)

p1 p2 offset01112212231

10-bitL1 index

10-bit L2 index

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Variable-Size Page TLBSome systems support multiple page sizes.

VPN offset

physical address PPN offset

virtual address

hit?

V RWD Tag PPN L

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Atlas Revisited

• One PAR for each physical page

• PAR’s contain the VPN’s of the pages resident in primary memory

• Advantage: The size is proportional to the size of the primary memory

• What is the disadvantage ?

VPN

PAR’s

PPN

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Hashed Page Table:Approximating Associative Addressing

hashOffset

Base of Table

+PA of PTE

PrimaryMemory

VPN PID PPN

Page Table

VPN d Virtual Address

VPN PID DPN

VPN PID

PID

• Hashed Page Table is typically 2 to 3 times larger than the number of PPN’s to reduce collision probability

• It can also contain DPN’s for some non-resident pages (not common)

• If a translation cannot be resolved in this table then the software consults a data structure that has an entry for every existing page

3/6/2008 CS152-Spring’08 37

Acknowledgements

• These slides contain material developed and copyright by:

– Arvind (MIT)

– Krste Asanovic (MIT/UCB)

– Joel Emer (Intel/MIT)

– James Hoe (CMU)

– John Kubiatowicz (UCB)

– David Patterson (UCB)

• MIT material derived from course 6.823

• UCB material derived from course CS252