chapter 12 virtual memory copyright © 2008. operating systems, by dhananjay dhamdhere copyright ©...

Chapter 12

Virtual MemoryCopyright © 2008

Operating Systems, by Dhananjay Dhamdhere Copyright © 2008 12.2Operating Systems, by Dhananjay Dhamdhere 2

Introduction

• Virtual Memory Basics• Demand Paging• The Virtual Memory Manager• Page Replacement Policies• Controlling Memory Allocation to a Process• Shared Pages• Memory-Mapped Files• Case Studies of Virtual Memory Using Paging• Virtual Memory Using Segmentation

Operating Systems, by Dhananjay Dhamdhere Copyright © 2008 12.3

• MMU translates logical address into physical one• Virtual memory manager is a software component

– Uses demand loading– Exploits locality of reference to improve performance

Operating Systems, by Dhananjay Dhamdhere 3

Virtual Memory Basics


Virtual Memory Basics (continued)


Virtual Memory Using Paging

• MMU performs address translation using page tableEffective memory address of logical address (pi, bi)= start address of the page frame containing page pi + bi


Demand Paging Preliminaries


Demand Paging Preliminaries (continued)

• Memory Management Unit (MMU) raises a page fault interrupt if page containing logical address not in memory



A page fault interrupt is raised because Valid bit of page 3 is 0



• At a page fault, the required page is loaded in a free page frame

• If no page frame is free, virtual memory manager performs a page replacement operation– Page replacement algorithm

– Page-out initiated if page is dirty (modified bit is set)

• Page-in and page-out: page I/O or page traffic• Effective memory access time in demand paging:


• (Empirical) law of locality of reference: logical addresses used by process in a short interval tend to be grouped in certain portions of its logical address space


Page Replacement


• How much memory to allocate to a process


Memory Allocation to a Process


Optimal Page Size

• Size of a page is defined by computer hardware• Page size determines:

– No of bits required to represent byte number in a page

– Memory wastage due to internal fragmentation

– Size of the page table for a process

– Page fault rates when a fixed amount of memory is allocated to a process

• Use of larger page sizes than optimal value implies somewhat higher page fault rates for a process– Tradeoff between HW cost and efficient operation


Paging Hardware

• Page-table-address-register (PTAR) points to the start of a page table


Paging Hardware (continued)


Memory Protection

• Memory protection violation raised if:– Process tries to access a nonexistent page

– Process exceeds its (page) access privileges

• It is implemented through:– Page table size register (PTSR) of MMU

• Kernel records number of pages contained in a process in its PCB

– Loads number from PCB in PTSR when process is scheduled

– Prot info field of the page’s entry in the page table


• Translation look-aside buffer (TLB): small and fast associative memory used to speed up address translation


Address Translation and Page Fault Generation


Address Translation and Page Fault Generation (continued)

• TLBs can be HW or SW managed


Address Translation and Page Fault Generation (continued)

• Some mechanisms used to improve performance:– Wired TLB entries for kernel pages: never replaced

– Superpages

TLB hit ratio


Superpages

• TLB reach is stagnant even though memory sizes increase rapidly as technology advances– TLB reach = page size x no of entries in TLB– It affects performance of virtual memory

• Superpages are used to increase the TLB reach– A superpage is a power of 2 multiple of page size– Its start address (both logical and physical) is aligned on

a multiple of its own size– Max TLB reach = max superpage size x no of entries in

TLB– Size of a superpage is adapted to execution behavior of a

process through promotions and demotions



Support for Page Replacement

• Virtual memory manager needs following information for minimizing page faults and number of page-in and page-out operations:– The time when a page was last used

• Expensive to provide enough bits for this purpose• Solution: use a single reference bit

– Whether a page is dirty• A page is clean if it is not dirty• Solution: modified bit in page table entry


Practical Page Table Organizations

• A process with a large address space requires a large page table, which occupies too much memory

• Solutions:– Inverted page table

• Describes contents of each page frame– Size governed by size of memory

– Independent of number and sizes of processes

– Contains pairs of the form (program id, page #)

• Con: information about a page must be searched

– Multilevel page table• Page table of process is paged


Inverted Page Tables

Use of hash tableSpeeds up search


Multilevel Page Tables

• If size of a table entry is 2e bytes, number of page table entries in one PT page is 2nb/2e

• Logical address (pi , bi) is regrouped into three fields:

– PT page with the number pi

1 contains entry for pi

– pi2 is entry number for pi

in PT page

– bi


I/O Operations in a Paged Environment

• Process makes system call for I/O operations– Parameters include: number of bytes to transfer, logical

address of the data area

• Call activates I/O handler in kernel– I/O subsystem does not contain an MMU, so I/O handler

replaces logical address of data area with physical address, using information from process page table

– I/O fix (bit in misc info field) ensures pages of data area are not paged out

– Scatter/gather feature can deposit parts of I/O operation’s data in noncontiguous memory areas

– Alternatively, data area pages put in contiguous areas


Example: I/O Operations in Virtual Memory


The Virtual Memory Manager


Example: Page Replacement


Overview of Operation of the Virtual Memory Manager

• Virtual memory manager makes two important decisions during its operation:– Upon a page fault, decides which page to replace

– Periodically decides how many page frames should be allocated to a process


Page Replacement Policies

• A page replacement policy should replace a page not likely to be referenced in the immediate future

• Examples:– Optimal page replacement policy

• Minimizes total number of page faults; infeasible in practice

– First-in first-out (FIFO) page replacement policy– Least recently used (LRU) page replacement policy

• Basis: locality of reference

• Page reference strings– Trace of pages accessed by a process during its

operation– We associate a reference time string with each


Example: Page Reference String

• A computer supports instructions that are 4 bytes in length– Uses a page size of 1KB

– Symbols A and B are in pages 2 and 5


Page Replacement Policies (continued)

• To achieve desirable page fault characteristics, faults shouldn’t increase when memory allocation is increased– Policy must have stack (or inclusion) property


FIFO page replacement policy does not exhibit stack property.


Page Replacement Policies (continued)

• Virtual memory manager cannot use FIFO policy – Increasing allocation to a process may increase page

fault frequency of process• Would make it impossible to control thrashing


Practical Page Replacement Policies

• Virtual memory manager has two threads– Free frames manager implements page replacement

policy

– Page I/O manager performs page-in/out operations


Practical Page Replacement Policies (continued)

• LRU replacement is not feasible– Computers do not provide sufficient bits in the ref info

field to store the time of last reference

• Most computers provide a single reference bit– Not recently used (NRU) policies use this bit

• Simplest NRU policy: Replace an unreferenced page and reset all reference bits if all pages have been referenced

• Clock algorithms provide better discrimination between pages by resetting reference bits periodically

– One-handed clock algorithm

– Two-handed clock algorithm

» Resetting pointer (RP) and examining pointer (EP)


Example: Two-Handed Clock Algorithm

• Both pointers are advanced simultaneously• Algorithm properties defined by pointer distance:

– If pointers are close together, only recently used pages will survive in memory

– If pointers are far apart, only pages that have not been used in a long time would be removed


Controlling Memory Allocation to a Process

• Process Pi is allocated alloci number of page frames

• Fixed memory allocation – Fixes alloc statically; uses local page replacement

• Variable memory allocation– Uses local and/or global page replacement

– If local replacement is used, handler periodically determines correct alloc value for a process

• May use working set model

• Sets alloc to size of the working set


Implementation of a Working Set Memory Allocator

• Swap out a process if alloc page frames cannot be allocated

• Expensive to determine WSi(t,∆) and alloci at every time instant t– Solution: Determine working sets periodically

• Sets determined at end of an interval are used to decide values of alloc for use during the next interval


Shared Pages

• Static sharing results from static binding performed by a linker/loader before execution of program

• Dynamic binding conserves memory by binding same copy of a program/data to several processes– Program or data shared retains its identity

– Two conditions should be satisfied:• Shared program should be coded as reentrant

– Can be invoked by many processes at the same time

• Program should be bound to identical logical addresses in every process that shared it


Shared pages should have samepage numbers inall processes


Copy-on-Write

• Feature used to conserve memory when data in shared pages could be modified– Copy-on-write flag in page table entries Memory

allocation decisions are performed statically

A private copy of page k is made

when A modifies it


Memory-Mapped Files

• Memory mapping of a file by a process binds file to a part of the logical address space of the process– Binding is performed when process makes a memory

map system call

– Analogous to dynamic binding of programs and data


Memory-Mapped Files (continued)


Case Studies of Virtual Memory Using Paging

• Unix Virtual Memory• Linux Virtual Memory• Virtual Memory in Solaris• Virtual Memory in Windows


Unix Virtual Memory

• Paging hardware differs in architectures• Pages can be: resident, unaccessed, swapped-out• Allocation of as little swap space as possible• Copy-on-write for fork• Lack reference bit in some HW architectures;

compensated using valid bit in interesting manner• Process can fix some pages in memory• Pageout daemon uses a clock algorithm

– Swaps out a process if all required pages cannot be in memory

– A swap-in priority is used to avoid starvation


Linux Virtual Memory

• Page size of 4 KB• On 64-bit architectures, uses three-level page table• States for page frames: free, active, inactive dirty,

inactive laundered, inactive clean• Page replacement based on a clock algorithm

– Uses two lists called active list and inactive list

• Buddy system allocator for allocating page frames• Several virtual memory regions for a process:

– Zero-filled, file-backed, private memory


Virtual Memory in Solaris

• Supports normal pages and superpages– Superpages:

• Automatically for processes with large address spaces• Can be requested using memcntl system call• Not used for memory-mapped files

• Solaris 6 introduced priority paging to avoid interference between file processing and virtual memory

• Page scanner tries to keep a sufficient number of page frames on cyclic page cache (since Solaris 8)– lotsfree parameter indicates how many page frames

should be free– Uses two-handed clock algorithm on a global basis


Virtual Memory in Windows

• Supports both 32-bit and 64-bit logical addresses• Page size is 4 KB• Process address space is either 2 GB or 3 GB• Two-, three- or four-level page tables and various page

table entry formats– On the X-86 architecture:

• Page frame can be in one of eight states, including: valid, free, zeroed, standby, modified, and bad

• A process must first reserve virtual address space and then commit it for specific entities


Virtual Memory in Windows (continued)

• A section object represents a section of memory that can be shared– A process maps a view to access part of a section

– Copy-on-write feature used for sharing pages

– Prototype PTE is set-up for shared pages

• TLBs are managed by HW (32-bit) or SW (64-bit)• Exploits reference locality: loads a few pages before

and after a page-faulted page into memory• Uses notion of working sets (for memory allocation)

– Clock algorithm

• Page lists: free, zero-initiated, modified, standby


Virtual Memory Using Segmentation

• A segment is a logical entity in a program, such as a function, a data structure, or an object– Or, a module that consists of some or all of these

– Convenient unit for sharing and protection


Example: Effective Address Calculation in Segmentation

• Logical address (si, bi) can be specified as ids– (alpha, beta)

• alpha: name of a segment• beta: id associated with a byte contained in alpha


Management of Memory

• Similarities to paging:– Segment fault indicates segment is not in memory– Segment-in operation is performed to load segment

• Segment-out operations may be needed first

– Can use working set of segment for allocation• Segments can be replaced on NRU basis

• Differences to paging:– Can lead to external fragmentation

• Tackled through compaction or through memory reuse techniques (first fit, best fit, etc.)

– Segments can dynamically grow or shrink in size


Sharing and Protection

• Two important issues in protection and sharing of segments are:– Static and dynamic sharing of segments

– Detecting use of invalid addresses• Protection exception if bi exceeds size of si


• External fragmentation exists in a virtual memory using segmentation– Solution: segmentation with paging


Segmentation with Paging


Summary

• Basic actions in virtual memory using paging: address translation and demand loading of pages– Implemented jointly by

• Memory Management Unit (MMU): Hardware• Virtual memory manager: Software

• Memory is divided into page frames • Virtual memory manager maintains a page table

– Inverted and multilevel page tables use less memory but are less efficient

– A fast TLB is used to speed up address translation


Summary (continued)

• Which page should VM manager remove from memory to make space for a new page?– Page replacement algorithms exploit locality of reference

• LRU has stack property, but is expensive• NRU algorithms are used in practice

– E.g., clock algorithms

• How much memory should manager allocate?– Use working set model to avoid thrashing

• Copy-on-write can be used for shared pages• Memory mapping of files speeds up access to data

chapter 12 virtual memory copyright © 2008. operating systems, by dhananjay dhamdhere copyright ©...

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