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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 1.  not referenced, not modified 2.  not referenced, modified 3.  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.

2.  Context Switch -  Point MMU to page table for new process -  TLB flushed

3.  Memory Reference -  Map virtual address to physical address -  Determine if page fault -  If fault, determine whose fault and resolve

4.  Page Replacement –  Record page access / modifies –  Determine page to replace.

5.  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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