Memory Management 1 Overview Basic memory management Address - - PDF document

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Memory Management

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Overview

• Basic memory “management”
• Address Spaces
• Virtual memory
• Page replacement algorithms
• Design issues for paging systems
• Implementation issues
• Segmentation

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Memory Management

• Ideally programmers want memory that is

– large – fast – non volatile

• Memory hierarchy

– small amount of fast, expensive memory – cache – some medium-speed, medium price main memory – gigabytes of slow, cheap disk storage

• Memory manager

– handles the memory hierarchy – Protects processes from each other.

Approaches

• Single Process, Contiguous Memory
• Multiple Processes, Contiguous Memory
• Multiple Processes, “Discontiguous” Memory
• Multiple Processes, Only partially in memory

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Basic Memory Management

Single Process without Swapping or Paging

Three simple ways of organizing memory

• an operating system with one user process

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Binding

• If a program has a line:

int x; When is the address of x determined?

• What are the choices?
• At compile time
• At load time
• At run time

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Base / Limit Registers

• Binding done at run

time.

• Addresses are added to

base value to map to physical address

• Addresses larger than

limit value are an error

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Swapping

Memory allocation changes as

– processes come into memory – leave memory

Shaded regions are unused memory

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Managing Free Memory

• Assume a process being loaded can ask for

any size “chunk” of memory needed.

• We need to be able to find a chunk the right

size.

• How can we keep track of free chunks

efficiently?

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Memory Management with Bit Maps

• Part of memory with 5 processes, 3 holes

– tick marks show allocation units – shaded regions are free

• Corresponding bit map
• Same information as a list

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Memory Management with Linked Lists

Four neighbor combinations for the terminating process X

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Order of Search for Free Memory

• We can search for a large enough free block of free

memory starting from the beginning. That’s called first fit.

• If we already skipped over the first N holes because they

were too small, maybe it’s a waste of time to look there

• again. Try next fit.
• Should we be more selective in our choice? After all

we’re just grabbing the first thing that works…

• How does first (or next) fit effect fragmentation? Could

we do better?

• Can we find a hole that fits faster? What are the

downsides?

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The Contiguous Constraint

• So far a process’s memory has been

contiguous.

• What if it didn’t have to be?
• What problems would that help solve?
• How would the hardware need to change?
• What additional work would the OS have to

do?

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It’s All Gotta Be in Memory (or does it?)

• We have assumed that the entire process has

to be in memory whenever it is running.

• What if it didn’t have to be?
• What problems would that help solve?
• How would the hardware need to change?
• What additional work would the OS have to

do?

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Virtual Memory

The position and function of the MMU

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Paging

The relation between virtual addresses and physical memory addres- ses given by page table

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Page Tables

Internal operation of MMU with 16 4 KB pages

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Page Tables 2-level

• 32 bit address with 2 page table fields
• Two-level page tables

Second-level page tables Top-level page table

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Page Table Entry

• Present/absent is also called Valid
• Modified is also called Dirty
• Referenced is also called Accessed
• Why would caching be disable?

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Pentium PTE

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TLBs – Translation Lookaside Buffers

A TLB to speed up paging

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Inverted Page Tables

Comparison of a traditional page table with an inverted page table

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Page Replacement Algorithms

• Page fault forces choice

– If there are no free page frames, we have to make room for incoming page – Which page should be removed?

• Modified page frame must first be saved before

being evicted

– An unmodified page frame can just overwritten

• Better not to choose an often used page

– Likely to be brought back in again soon

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Optimal Page Replacement Algorithm

• Replace page needed at the farthest point in future

– Optimal but unrealizable

• Estimate by …

– logging page use on previous runs of process – although this is impractical

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Not Recently Used Page Replacement Algorithm

• Each page has Reference bit, Modified bit

– bits are set when page is referenced, modified

• Pages are classified

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not referenced, not modified

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not referenced, modified

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referenced, not modified

4.

referenced, modified

• NRU removes page at random

– from lowest numbered non empty class

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FIFO Page Replacement Algorithm

• Maintain a linked list of all pages

– in order they came into memory

• Page at beginning of list replaced
• Disadvantage

– page in memory the longest may be often used

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Second Chance Page Replacement Algorithm

• Operation of a second chance

– pages sorted in FIFO order – Page list if fault occurs at time 20, A has R bit set (numbers above pages are loading times)

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The Clock Page Replacement Algorithm

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Least Recently Used (LRU)

• Principle: assume a page used recently will be used again

soon

– throw out page that has been unused for longest time

• Implementation

– Keep a linked list of pages

• most recently used at front, least at rear
• update this list every memory reference !!

– Or keep time stamp with each PTE

• choose page with oldest time stamp
• Again, this must be updated with every memory reference.

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LRU

(Another Hardware Solution) LRU using a matrix – pages referenced in order

0,1,2,3,2,1,0,3,2,3

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LRU Approximation?

• How could we approximate LRU?
• We can’t track every time a page is referenced, but we can

sample the data.

• How often? Once per clock tick, perhaps.
• Update a counter for each page that has been referenced in

the last clock tick.

• Take the page with the lowest count

i.e. “Not Frequently Used” (NFU)

• How well does this work?

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Simulating LRU in Software Aging

• The aging algorithm simulates LRU in software
• Note 6 pages for 5 clock ticks, (a) – (e)

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Thrashing

• A program causing page faults every few instructions is

said to be thrashing.

• What causes thrashing?
• If a process keeps accessing random new pages, then it is

hard to anticipate what it will use next.

• Most programs exhibit temporal and spatial locality.

– Temporal locality: if the process accessed a particular address, it is likely to do so again soon. – Spatial locality: if the process accessed a particular address, it is likely to access nearby addresses soon.

• Processes that follow this principle tend not to thrash

unless they have to fight for memory.

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Causing Thrashing within a single process

• The first nested loop demonstrates spatial locality
• The second thrashes.

const int ROWS = 10000; const int COLS = 1024; int arr[ROWS][COLS]; int main() { for (int row = 0; row < ROWS; ++row) for (int col = 0; col < COLS; ++col) arr[row][col] = row * col; for (int col = 0; col < COLS; ++col) for (int row = 0; row < ROWS; ++row) arr[row][col] = row * col; }

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The Working Set Page Replacement Algorithm (1)

• The working set is the set of pages used by the k

most recent memory references

• w(k,t) is the size of the working set at time, t

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The Working Set Page Replacement Algorithm (2) The working set algorithm

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The WSClock Page Replacement Algorithm Operation of the WSClock algorithm

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Review of Page Replacement Algorithms

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Modeling Page Replacement Algorithms Belady's Anomaly

• FIFO with 3 page frames
• FIFO with 4 page frames
• P's show which page references show page faults

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“Stack” Algorithms

State of memory array, M, after each item in reference string is processed

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The Distance String

Probability density functions for two hypothetical distance strings

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The Distance String

• Computation of page fault rate from distance string

– the C vector – the F vector

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Design Issues for Paging Systems

Local versus Global Allocation Policies

• Original configuration
• Local page replacement
• Global page replacement

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Page Fault Rate

• Page fault rate as a function of the number of page frames assigned
• Use to determine if a process should be granted additional pages.

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Load Control

• Despite good designs, system may still thrash
• When PFF algorithm indicates

– some processes need more memory – but no processes need less

• Solution :

Reduce number of processes competing for memory

– swap one or more to disk, divide up frames they held – reconsider degree of multiprogramming

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Page Size (1)

Small page size

• Advantages

– less internal fragmentation – better fit for various data structures, code sections – less unused program in memory

• Disadvantages

– programs need many pages, larger page tables

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Page Size (2)

• Overhead due to page table and internal

fragmentation

• Where

– s = average process size in bytes – p = page size in bytes – e = page entry page table space internal fragmentation

Optimized when

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Separate Instruction and Data Spaces

• One address space
• Separate I and D spaces

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Shared Pages

Two processes sharing same program sharing its page table

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Cleaning Policy

• Need for a background process, paging daemon

– periodically inspects state of memory

• When too few frames are free

– selects pages to “evict” using a replacement algorithm. – Evicted pages are kept in a pool in case they are wanted again.

• Dirty pages are scheduled to be written out.

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Implementation Issues

Operating System vs. Hardware for Paging

1.

Process creation

• Create and initialize page table.
• Pre-fetch pages.

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Context Switch

• Point MMU to page table for new process
• TLB flushed

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Memory Reference

• Map virtual address to physical address
• Determine if page fault
• If fault, determine whose fault and resolve

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Page Replacement

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Record page access / modifies

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Determine page to replace.

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Process termination time

• release page table, pages

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Instruction Backup

An instruction causing a page fault

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Global Flag

• Some pages are used by every process in the

system.

• Which?
• What would we like to have happen special

for those pages during a context switch.

• How can the hardware know?

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Locking Pages in Memory

• Virtual memory and I/O occasionally interact
• Proc issues call for read from device into buffer

– while waiting for I/O, another processes starts up – has a page fault – buffer for the first proc may be chosen to be paged out

• Need to specify some pages locked

– exempted from being target pages

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Backing Store

(a) Paging to static swap area (b) Backing up pages dynamically

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Separation of Policy and Mechanism

Page fault handling with an external pager

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Memory Management

NT & Unix

• Exact details not as easy as in process scheduling.
• Attempt to keep a set of “free pages” using a “page daemon”. (e.g.

Linux: kswapd, NT: Working Set Manger)

• Essentially “demand paging”.

– NT brings in “clusters”, typically 8 pages for code, 4 for data. – Unix’s swapper used to bring in all referenced pages, when swapping a process back in. Now uses demand paging.

• OS present in every process’s virtual address space.

– Reduces changes needed in TLB and cache. – No changes needed for system call.

• Load control: Swap out entire processes “as needed”. This is currently

something that one hopes to avoid, but the feature is still present.

• Some structure to describe where pages are currently, e.g. location in

paging file. Unix: Core Map. NT: Page Frame Database.

• A portion (e.g., 64KB) of the top/bottom of address space unmapped.

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MM in Unix

• Early versions totally swapping
• Current based on paging.
• Page Daemon ¼ sec.
• Refill free list when less then some value.

– BSD: lotsfree = ¼ memory. – SysV: has some min/max. If less than min, fill until max. Goal is to reduce frequency of page daemon, thereby reducing thrashing.

• Originally used Global Clock.
• As memory sizes increased BSD went to 2-handed clock.

– Front clears referenced bit – Back hand picks victim, – Result: a referenced page is really recent.

• SysV instead keeps frequency count of pages not referenced.

– Only boots page out if count greater than some value – Seems they had the opposite problem of the BSD folk.

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MM in Unix

• Load Control.

– If page daemon has to work too often then swap someone out. – Every few seconds look for someone to bring back.

• Decision based on swapped out time, size, niceness, if it was

sleeping…

• Only bring back the “user structure” portion of PCB and the

page tables. (I’m surprised page tables are removed. Must record swapped out page for process somewhere…)

• “Core Map” describes frames, free list, location to

swap to.

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MM in NT

• Demand paging of “Clusters”.

– (code: 8, data: 4). XP does some prepaging after application’s first run.

• Maintains a “working set” per process, not thread (remember scheduling is per

thread)

– WS is based on size, not time window. – Keeps a min/max per process – If we have > max and a page fault then steal process’s page. Otherwise add.

• ~ 1sec. Balance Set Manager, checks for sufficient free pages. Calls Working

Set Manager to free pages.

– Sort processes by “desirability”. Large idle, first. Foreground last. – If < min and “enough page faults” since last check, then leave alone. – Determine #pages to remove from process. Depends on need, size of WS relative to min/max. – Examine all pages in a process. Uses an unreferenced count, like AT&T Unix.

• Pages with “high” unreferenced removed.

– Continue examining additional processes till sufficient pages free. More aggressive passes as needed.

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MM in NT

• Load Control.

– Swapper runs every 4 sec. – If thread asleep for 7 sec. Mark thread’s kernel stack “in transition”. – When all threads in process marked, then swap out.

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MM in NT

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MM in FreeBSD

• 1GB for kernel, unless you want lots of small processes using lots of

kernel services, then configure for 2GB

• First few virtual pages reserved to catch bad pointers.
• Shared libraries placed by default just below default stack limit.
• Memory Lists:

– Wired – Active – Inactive. Dirty (min: 0%, max 4.5%)

• Pageout daemon moves pages from Inactive to “cache”

– attempts to balance i/o load by not starting too many concurrent writes. – Runs as a kernel process. This way it has access to kernel data structures, etc., but can be scheduled to run as a process so it can sleep.

– Cache. Clean. (min: 3%, max 6%) – Free. (min: 0.7%, max 3%)

• One end is zeroed, the other is not.
• Idle process tries to keep 75% of frames on Free list zeroed.

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MM in FreeBSD

• Page coloring. Attempt to avoid cache conflicts.

– Cache holds pages. A 1MB cache with 4KB pages has 256 cache pages. Each physical page on a 1MB boundary maps to the same cache page. Thus each cache page represents as many physical pages as there are MB’s of physical memory. – When allocating frames, color coding attempts to avoid such potential conflicts by making contiguous virtual pages, get frames that map to contiguous cache pages.

• Pure demand paging when swapping back in.

– Back in BSD4.3, count of resident pages was used. DIoF says might return.

• Page Replacement

– Least actively used algorithm. – Page has count of three when first brought in – Incremented each time reference bit found set to a max of 64 – Decremented each time referenced bit found not set.

• At zero page moved from Active to Inactive list.

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Segmentation (1)

• One-dimensional address space with growing tables
• One table may bump into another

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Segmentation (2)

Allows each table to grow or shrink, independently

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Segmentation (3)

Comparison of paging and segmentation

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Implementation of Pure Segmentation

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Segmentation with Paging: MULTICS (1)

• Descriptor segment points to page tables
• Segment descriptor – numbers are field lengths

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Segmentation with Paging: MULTICS (2)

A 34-bit MULTICS virtual address

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Segmentation with Paging: MULTICS (3)

Conversion of a 2-part MULTICS address into a main memory address

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Segmentation with Paging: MULTICS (4)

• Simplified version of the MULTICS TLB
• Existence of 2 page sizes makes actual TLB more complicated

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Segmentation with Paging: Pentium (1)

A Pentium selector

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Segmentation with Paging: Pentium (2)

• Pentium code segment descriptor
• Data segments differ slightly

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Segmentation with Paging: Pentium (3)

Conversion of a (selector, offset) pair to a linear address

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Segmentation with Paging: Pentium (4)

Mapping of a linear address onto a physical address

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Segmentation with Paging: Pentium (5)

Protection on the Pentium

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