ICOM 5007 - Noack

Memory management

Virtual memoryPaging and segmentation

Demand pagingMemory management hardware

ICOM 5007 - Noack

Virtual memory

Virtual memoryReasons

No program relocation – just remap memory

Security – hardware keeps processes from interfering

Efficiency – only needed memory is loaded

AddressesVirtual address – what the CPU

seesPhysical address – to physical

memoryPage fault interrupt – whenever

virtual address doesn’t map into physical address

Normal operationFirst few lookups cause page faults

and loading of memory blocksAfter a short time, repeated

accesses in same blocks of memory usually work

CPU

Addresstranslation

Physicalmemory

Virtualaddress

Physicaladdress

PageFault

interrupt

ICOM 5007 - Noack

The working set hypothesis

Programs live monotonous livesSmall loops of code or calling same function – these small blocks of code are

used heavilyArrays or structures – program typically works in a few heavily used data areasStack – activity is at the stack top

Common statisticsHit ratio – percent of memory accesses that do not cause page fault interrupts

(or cache hit ratio)Amount of memory needed vs block sizeHit ratio vs number of blocks allowed and block size

Typical tradeoffsPerformance - block size – hit ratio – memory required for

Various program typesNumber of processesAmount of physical memory

ICOM 5007 - Noack

Paging and segmentation

Ideas are familiar to 8086 programmersSegment

Not of fixed size

Access attributes – ownership, access rights (RO, RW, kernel/user)

Few for any given process

Each usually has a purpose

PageFixed size – always a power of 2 – often 4096 bytes

Access attributes – ownership, access rights (RO, RW, kernel/user)

Status and physical page number for each virtual page number

As many as needed

CPU Segmentation Paging Physicalmemory

Virtual address Linear address Physical address

ICOM 5007 - Noack

Demand paging

Basic idea – don’t load it until you need itFeasible because working set hypothesis is valid

Won’t keep working unless some form of page replacement is used to eliminate unused physical pages

Normal operationFirst few lookups cause page faults and loading of memory

blocks

After a short time, repeated accesses in same blocks of memory usually work

Page table formatsDirect – virtualphysical

Inverse - physical virtual

Multilevel - virtual page directory page table physical

ICOM 5007 - Noack

How paging works

Address partsPage number

which pageTranslated by paging

hardware or software

OffsetLength=log2(page size)Not translated by paging

– page is handled as a unit

Translation – directVirtual address used as

index to page table – page table entry says either

Physical page number is…

Or I am on disk at …

Or I am not valid

Page number offset

Address – virtual or physical

offsetPage

number

Directorynumber

Address into multilevel page table

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Multilevel paging and lookaside

Steps Directory base is in a register

Virtual directory number is used to look up page table base

Page table is used to look up page table entry

This gives physical page number or how to find it on disk

ConsequencesThree memory accesses for each real memory transaction

Remedy Transaction lookup buffer in hardware

This remembers last few paging lookups

This is what hit ratios are about

ICOM 5007 - Noack

Page tables and page table entries

Page table entryPhysical page number or device/block location on disk

StatusValid

Modified (also called dirty)

Accessed recently

Privilege level

Page directory entryBase of page table

Privilege level

Size of page table (if not fixed)

ICOM 5007 - Noack

The page frame table

An inverse table for all of physical memoryEach page table entry results in a page frame table entry

Not changed at context switch (why)

Contains accessed and modified bits

Used to find pages to swap out or discardDirty pages are written back

Clean ones are discarded

Used to allocate clean pages and by pagedaemon

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Memory management hardware

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Memory management in Linux

Hardware-independent memory management layerAdapts to 32 and 64-bit address widths

Method is three-level page tablesEntity 64 bit 32 bit

Page directory 10 bits 10 bits

Page middle 10 bits 0 bits

Page table 10 bits 10 bits

Page size 13 bits 12 bits

Note: not all the 64 bits are used 43 bits already gives a 43

Tbyte space

Page middle directory has size 1 for 32-bit architectures

This allows same system calls for 32-bit and 64-bit architectures

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Segmentation in Linux

Almost doesn’t existThe main segments are all 4 GB, spanning the entire address space

Kernel memory is actually in top GB (above 3 GB)

User memory is in bottom 3 GB

Register assignments are CS – either user or kernel code segment