Operating Systems - garryowen.csisdmz.ul.iegarryowen.csisdmz.ul.ie/~cs5212/resources/lect19.pdf · Spring 2009-2010 CS5212. Virtual Memory – Silberschatz: §9 Outline 1 Virtual

Virtual Memory – Silberschatz: §9

Operating Systems

Spring 2009-2010

CS5212

Virtual Memory – Silberschatz: §9

Outline

1 Virtual Memory – Silberschatz: §9IntroductionDemand Paging

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Outline

1 Virtual Memory – Silberschatz: §9IntroductionDemand Paging

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Background

Virtual memory: separation of user logical memory fromphysical memory

Only part of the program / program’s data structures needsto be in memory for executionLogical address space can therefore be much larger thanphysical address spaceAllows address spaces to be shared by several processesAllows for more efficient process creation

Virtual memory can be implemented via:Demand pagingDemand segmentation

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Virtual Memory That is Larger Than Physical Memory

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Virtual Address Space

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Shared Library Using Virtual Memory

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Outline

1 Virtual Memory – Silberschatz: §9IntroductionDemand Paging

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Demand Paging Basics

Bring a page into memory only when it is neededLess I/O neededLess memory neededFaster responseMore users

Page is needed because a reference was made to itif invalid reference then abortif not-in-memory then bring in to memory

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Valid-Invalid Bit

With each page table entry a valid-invalid bit is associated(v means in-memory, i not-in-memory)Initially valid-invalid bit is set to i on all entriesExample of a page table snapshot:

Frame # V/Ivvi

...ii

Page TableDuring address translation, if valid-invalid bit in page tableentry is i then this triggers a page fault

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Page Table When Some Pages Are Not in MainMemory

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Page Faulting

If there is a reference to a page, first reference to that pagewill trap to operating system: page faultOperating system looks at another table to decide whetherit is:

an invalid reference; if so then abortjust not in memory

Get empty frameMove page into frame (performed by the pager)Update tablesSet valid-invalid bit = vRestart the instruction that caused the page fault

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Steps in Handling a Page Fault

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Performance of Demand Paging

Page Fault Rate 0 ≤ ρ ≤ 1.0if ρ = 0, no page faultsif ρ = 1, every reference is a fault

Effective Access Time (EAT):

EAT = (1− ρ)×memory access+ρ× (page fault overhead

+paging out+paging in

+restart overhead)

CS5212

Virtual Memory – Silberschatz: §9IntroductionDemand Paging

Example of Demand Paging

Memory access time = 200 nanosecondsAverage page-fault service time = 8 millisecondsEffective Access Time

EAT = (1− ρ)× 200ns + ρ(8ms)

= (1− ρ)× 200ns + ρ(8,000,000)

= 200ns + ρ7,999,800

If one access out of 1,000 causes a page fault, then

EAT = 8.2ms

This is a slowdown by a factor of 40!!

CS5212