Operating Systems IV. Memory Management - Eurecomsoc.eurecom.fr/OS/docs/CourseOS_IV_MemoryManagement.pdf · Operating Systems IV. Memory Management Ludovic Apvrille [email protected]

Operating Systems

IV. Memory Management

Ludovic [email protected], office 470

http://soc.eurecom.fr/OS/

@OS Eurecom

http://soc.eurecom.fr/OS/

Basics of Memory ManagementMain Mechanisms

What about Windows and Linux?

Hardware ArchitectureMonoprogrammingMultiprogramming

Outline

Basics of Memory ManagementHardware ArchitectureMonoprogrammingMultiprogramming

Main Mechanisms


2/32 Fall 2017 Institut Mines-Telecom Operating Systems - Memory Management




Memory Hierarchy





Memory Access

CPU Memory

Address bus

Data bus

Write

Read





Memory Protection

Goal

Prevent a process to access unauthorized memory

I Memory used by other processesI Memory used by Operating System

I Interrupt vector, interrupt service routines, kernel, services, etc.

Mechanisms

Dual Mode and MMU - Memory Management Unit





Monoprogramming





Multiprogramming: Issues

Process AdmittanceI OS estimates the required memory and allocates it

Dynamic Allocation

I A process may request additional memory space

I A process may release part of its memory space

Process terminationI OS must release all the allocated memory





Memory Allocation

Main issuesI Keep track of memory allocations

I Return requested memory chunks as fast as possible

I Avoid fragmentation

Allocation unitI Smallest amount of continuous memory managed by the OS

I From a few bytes to several KBytesI Impact of this size?

Keeping track of allocation units

I Bitmaps

I Linked Lists





Bitmaps vs. Linked Lists





Allocation Algorithms

First Fit

The system scans the list along until a large enough continuousallocation is found

Next FitI Scanning begins at the last position where a free block has

been found

I Performs slightly worse than First Fit

Best FitI Scans all the list and takes the smallest free block that is

adequateI Performs worse than First Fit!

I Can you guess why?





Allocation Algorithms (Cont.)

Worse FitI Searches for the largest free block

I Not very good either!

Quick FitI Separate lists for most usual sizeI Additional complexity of memory management

I Merging is expensive

I But very quick search!




SwappingVirtual MemorySegmentation and Paging

Outline

Basics of Memory Management

Main MechanismsSwappingVirtual MemorySegmentation and Paging






Swapping: Principle





When to Swap?Swapping is expensive!

I How much time at least is necessary to swap in a 1 GBprocess (e.g., Firefox) with a transfer rate of 500 MB /second (Typical transfer rate for a (very good) ssd)

I May have to perform a swap out before!

Swap out

I When?I Memory occupied over thresholdI A memory allocation request fails

I Which process to swap out?I Recently executed processes (RR

scheduling)I Processes with lower priorities

(Priority-based scheduling)

Swap in

I When a process isready to execute

I I/O completed

I When a largeamount of memoryfreed





Logical vs. Physical Address Space

At compilation time, the exact memory location of a program maynot be known → Virtual Memory

⇒ Two address spaces

I Virtual address space vs. physical address space

I Logical / virtual address: address used at CPU level (i.e.,addresses generated at compilation time)

I Physical address: physical address of the RAM

Address binding (virtual → physical) done at execution time:Memory Management Unit





Memory Management Unit

CPU Memory

Logical addresses

Data bus

Write

Read

MMUPhysical addresses





Segmentation of Memory

I Segment = logical memory unit of variable lengthI Virtual segments mapped to physical memory segmentsI Memory address = segment number + an address within the

segment (= offset)





Segmentation of Processes

A process is a collection of different types of data

I Code, stack, heap, etc.

→ Use of several segments per Process





Data Sharing with Segmentation





Limitations of Segmentation

Fragmentation

Solution: Using algorithms to select segments (e.g., best-fit,first-fit, etc.)

Cost of segment expansion!

If a process allocates more space in a segment and this segmentcannot be expanded, and there is free memory available elsewhere→ memory segment must be moved

1. Process is blocked

2. OS makes a memory copy → segment is moved to anotherlocation

3. Process is unblocked

⇒ Paging!20/32 Fall 2017 Institut Mines-Telecom Operating Systems - Memory Management




Basics of Paging

Paging allows the logical address space of a process to benon contiguous in physical memory

Physical memory

I All physical memory is cut into fixed-size blocks

I Physical memory includes swap partitions

I Logical memory: page

I Physical memory: frame

Virtual memory

I Address divided into two partsI Page number (p)I Page offset (d)





MMU with Paging





Combining Segmentation and Paging

Intel 80386 Address Translation





Segmentation and Page Faults

Memory protection

I Process switching: the OS updates the address table

I MMU detects addresses having no correspondence → trap

Reasons for segment / page faults

I The address is invalid i.e., outside of the process addressspace

I Process is stopped (segmentation fault)

I Segment / page has been swapped outI The OS must make a swap in operation

I A segment / page must first be swapped out if memory is full→ Page Replacement Algorithm





Page Fault Replacement Algorithms

Issue: the page-fault rate should be as low as possible

I FIFO Page Replacement

I Optimal Page Replacement

I LRU Page Replacement

I LRU Approximation Page Replacement

I Counting-Based Page Replacement

I . . . : ongoing research work on this issue





Hardware vs. OS Support

Might be system-dependent!

Mechanism Hardware or OS?

Address translation Hardware

Segment / page allocation OS

MMU configuration (choice ofactive tables, etc.)

OS

Segment / page fault Hardware detection, OSmanagement

Finally:Can you tell what are the main interests of MMUs for OS?





Use of Memory in Programs

…Char * name;…name = (char ) ( malloc ( 20 * sizeof(char) ) );…name[0] = 'h';name[1] = 'i';name[2] = '!';name[3] = '\0'

Process 1

malloc()

page1 page3

Process 1

LibC

OS

Free mem

page2 page4

page1page2page3page4

RAM

brk()

MMU




Outline

Basics of Memory Management

Main Mechanisms





Windows and Linux

Segments / Pages

I Linux and Windows: only pages for user processes

I Many Unix use both techniques

Copy and Write (fork())

I Frame is first shared

I If a write operation is performed, frame is duplicated

Background daemon

Invoked periodically: Page flushing, freeing unused memory

Memory mapped Files

A file can be mapped onto memory




Windows and Linux (Cont.)

Data structures to describe process space

Windows LinuxTree structure, each node iscalled a Virtual Address De-scriptor

Linked lists (just like mostUNIX) of vm area structs

OS vs. users process virtual address spaces (x86, 32-bit mode)

I Higher part: kernel code, and lower part: user code

I Linux: 3GB for the process, 1GB for the kernel

I Windows: 2GB for both

I When switching processes, upper part remains the same





OS vs. users process virtual address spaces (x86, 64-bit mode)

Windows Linux

I Support since March 2005(Windows XPProfessional x64 Edition).

I User processes/OS: 128TB of virtual addressspace (Since Windows8.1)

I Since kernel 2.4 (2001)

I User processes: 128 TBof virtual address space





Windows: Page replacement

Clock algorithm: Circular list of pages in memory, with the”hand” (iterator) pointing to the oldest page in the list

Linux: Page replacement

I Linux 2.2: NRU (Not Recently Used)I OS Scans through memory and evicts every page that wasn’t

accessed since the last scan

I Since kernel 2.4: LRU (Improved in 2.6: ”CLOCK-PRO”)I Counter is increased when the page is referencedI Counter is divided by 2 when it was not referenced

I kswapdI Awakes periodically (e.g., every 1 sec.)I Frees memory if enough is not available


Documents

Operating Systems IV. Memory Management - Eurecomsoc.eurecom.fr/OS/docs/CourseOS_IV_MemoryManagement.pdf · Operating Systems IV. Memory Management Ludovic Apvrille [email protected]