Memory Management Overview

Memory Management
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Overview
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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.
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Approaches
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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;
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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 addresses 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 Second-level page tables
Top-level page table
•  32 bit address with 2 page table fields
•  Two-level page tables
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Page Table Entry
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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
not referenced, modified
referenced, not modified
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
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;
}
•  The first nested loop demonstrates spatial locality
•  The second thrashes.
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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
page table space
internal
fragmentation
•  Where
–  s = average process size in bytes
–  p = page size in bytes
–  e = page entry
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
Process creation
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Create and initialize page table.
Pre-fetch pages.
Context Switch
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Point MMU to page table for new process
TLB flushed
Memory Reference
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Map virtual address to physical address
Determine if page fault
If fault, determine whose fault and resolve
Page Replacement
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Record page access / modifies
Determine page to replace.
Process termination time
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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
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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
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Demand paging of “Clusters”.
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Maintains a “working set” per process, not thread (remember scheduling is per
thread)
–  (code: 8, data: 4). XP does some prepaging after application’s first run.
–  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.
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~ 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
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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 69
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)
Level
Protection on the Pentium
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