Chapter 8: Virtual Memory

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Chapter 8 Virtual Memory

• CS 472 Operating Systems
• Indiana University Purdue University Fort Wayne

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Virtual memory

• Load on demand paging and/or segmentation
• Allows more processes to be active at a time,
  since only part of each process is in memory at a
  time
• One process may be larger than all of main memory
• The resident set is the set of pages or segments
  currently loaded in memory
• Virtual memory is possible because of the
  principle of locality

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Principle of locality
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Principle of locality

• References to program instructions and data
  within a process tend to cluster
• Only a few pages of a process are needed over a
  short period of time
• It is possible to make intelligent guesses about
  which pages will be needed in the future
• This suggests that virtual memory may work
  efficiently

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Virtual memory with paging

• A valid bit must be added to each PTE to indicate
  if the page has been loaded into a page frame
• A page fault is an internal interrupt caused when
  there is a reference to a program page that has
  not been loaded into a page frame
• A page fault causes the OS to load the needed
  page into an available page frame

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Thrashing

• Thrashing is when there is excessive page fault
  activity
• . . . particularly when pages are frequently
  replaced just before they are needed
• The processor spends most of its time swapping
  pages rather than executing user instructions

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Page fault activity

• The page fault handler first moves the running
  process to the blocked state
• It then identifies a page frame to hold the
  needed page
• Typically, a loaded page needs to be overwritten
• The fault handler then directs the DMA to load
  the needed page from disk into the available
  frame
• When the DMA finishes, the interrupt handler
  moves the process that caused the fault from the
  blocked state to the ready queue

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Hardware support for virtual memory

• Additional hardware support is needed in addition
  to that for simple paging
• The valid bit in each PTE mentioned earlier
• A modify bit in each PTE to indicate if the page
  has been altered since it was last loaded into
  main memory
• If no change has been made, the page does not
  need to be written back to disk before it is
  overwritten by a new page

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VAX system PTE
31 30 27 26 25
21 20

0
V m
frame number
Holds 26-bit disk address if page not loaded
protection (each page)

• Valid bit (v) If the page is not loaded,
  generate a page fault and get disk address from
  bits 0-25
• Modify bit (m) Set the bit with the first write
  to the page in memory
• Avoids write-back of clean page to disk

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VAX system PTE

• Virtual memory size 232 bytes 4 GB
• Page size frame size 512 bytes (29 bytes)
• Real memory with up to 221 frames is possible
• 221 frames x 29 bytes/frame 230 bytes 1 GB

user processes
0 230
231
232
user space stack space system
space unused
shared
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• Note that each logical memory reference now
  requires two physical references.

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Page tables real or virtual?

• The entire page table may take up too much main
  memory
• VAX example for each process
• Potentially, 221 PTEs (4 bytes each) 8 MB
• Therefore, page tables are also stored in virtual
  memory
• When a process is running, only part of its page
  table is in main memory

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Page tables real or virtual?

• The system root page table for the process page
  tables must be locked in real memory
• The root page table may never swapped out
• Address translation now becomes a 2-step lookup
• Get the PTE for the process page table
• Get the PTE for the reference page
• Get the instruction or data
• Note that each logical memory reference now
  requires three physical references
• A hardware solution is needed to keep things from
  running to slow

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Translation Lookaside Buffer (TLB)

• A high-speed cache for page table entries
• Contains PTEs that have been most recently used
• Hardware algorithm
• Given a virtual address, processor examines the
  TLB
• If PTE is present (TLB hit), retrieve frame
  number and form real address
• If PTE is not found (TLB miss), do the 2-step
  lookup and update the TLB with the missing PTE
  for next time
• Note on step 1 of the 2-step lookup
• This uses the TLB again for the process page
  table lookup
• This might result in an additional page fault on
  the process page table page (independent of using
  a TLB). If so, block waiting for the process
  page table page and then resume

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Page s PTEs
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Inverted page table

• Found on PowerPC, UltraSPARC, and IA-64
  architecture
• Page number portion of a virtual address is
  hashed
• Hash value is an index into the inverted page
  table
• There is one PTE per available frame, in contrast
  to one for each virtual page (many fewer PTEs)
• There is only one table for all processes
• Inverted page table is of fixed size
• If hash succeeds, use frame , else generate page
  fault

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Other issues

• Cache memory
• PTE may be in TLB cache, real memory, or disk
• Instructions and data may be in real memory
  cache, real memory, or disk
• Page size
• Large
• more internal fragmentation on the last page
• resident set less able to adapt to principle of
  locality
• Small
• larger page tables
• more page faults

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Example page sizes
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Segmented virtual memory

• Segments may be of unequal, dynamic size
• Simplifies handling of growing data structures
• Allows a segment to be altered, recompiled, and
  relinked without linking and loading all segments
• Lends itself to sharing data among processes
• Lends itself to protection

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Segmented virtual memory tables

• Segment entry for each segment
• Each entry contains the length of the segment
  along with the base address
• A valid bit is needed to determine if segment
  is already in main memory
• A modify bit is needed to determine if the
  segment has been modified since it was loaded

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Combined paging/segmentation

• Paging is transparent to the programmer
• Segmentation is visible to the programmer
• Each segment is broken into fixed-size pages

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OS policies for virtual memory
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Fetch policy

• Determines when a page should be brought into
  memory
• Demand paging brings a page into main memory when
  a reference is made to a location on the page
• This results in many page faults when process
  first started
• Prepaging anticipates pages will be needed
• It is efficient to bring in pages that reside
  contiguously on the disk
• Pointless if the pages are never referenced
• Prepaging is usually not warranted

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Placement and replacement policy

• A placement policy determines where in real
  memory a process piece is to reside
• This is important in a segmentation system
• A replacement policy determines which page is
  replaced
• The page removed should be the page least likely
  to be referenced in the near future
• Most policies predict the future behavior on the
  basis of past behavior

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Replacement policy

• Consider only the set of pages available to be
  swapped out
• Ignore locked frames
• Locked frames contain . . .
• Kernel of the operating system
• Control structures
• System root page tables, for example
• I/O buffers
• Associate a lock bit with each frame

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Replacement policy algorithms

• Optimal replacement policy
• Selects for replacement that page for which the
  time to the next reference is the longest (if
  ever)
• Not practical
• Impossible to have perfect knowledge of future
  events
• So, use the principle of locality to improve the
  chances that the replaced page is unlikely to be
  referenced soon

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Replacement policy algorithms

• Least Recently Used (LRU)
• Replaces the page that has not been referenced
  for the longest time
• By the principle of locality, this should be the
  page least likely to be referenced in the near
  future
• Inefficient to implement
• Each frame would need to be tagged with the time
  of last reference (much overhead)

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Replacement policy algorithms

• First-in, first-out (FIFO)
• Logically treat page frames allocated to a
  process as arranged in a circular buffer
• Pages are replaced in round-robin style
• Simplest replacement policy to implement
• Page that has been in memory the longest is
  replaced
• But, these pages may be needed again very soon
• Imagine a grocery store eliminating bread and
  milk just because they had been stocked a long
  time

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Replacement policy algorithms

• Clock
• Requires hardware use bit for each frame
• When a page is first loaded, the use bit is set
  to 1
• When the page is referenced, the use bit is set
  to 1
• When it is time to replace a page, the OS does a
  round-robin scan, choosing the first frame
  encountered which still has a clear use bit
• During the search for replacement, the OS clears
  each use bit it encounters

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Replacement policy algorithms

• Clock (continued)
• The frame has to earn the right to stay the next
  time around
• This is the most effective practical method
• It can be enhanced by using the modify bit in
  the PTE
• Unmodified pages are preferred for replacement

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Comparison of Placement Algorithms
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Page buffering

• A replaced page is added to a pool of free frames
  which have been replaced but not yet
  overwritten
• The pool is separated into two lists
• Free page list of unmodified (clean) free pages
• Modified page list of pages that need to be
  written back to disk
• Modified pages can be cleaned in batches and
  moved to the free page list
• This decouples cleaning from replacement
• Some page faults can be handled by simple
  bookkeeping
• Page buffering is best used in conjunction with
  the FIFO page replacement policy
• Used by the VAX/VMS operating system

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Resident set size

• Fixed-allocation for each process
• Gives a process a fixed number of page frames
  within which to execute
• When a page fault occurs, a frame is reclaimed
  from the current process
• Difficult to allocate the optimal number of frames

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Resident set size

• Variable-allocation for each process
• Size of the resident set of a process varies and
  adapts over the lifetime of the process
• Involves OS overhead
• Global scope a page fault gives the process a
  new frame taken from any process
• Local scope frames are reclaimed from the
  current process, but the allocation size is
  adjusted periodically based on the fault rate

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Cleaning policy

• Demand cleaning
• A page is written out only when it has been
  selected for replacement
• Precleaning
• Pages are written out in batches (cleaned)
• It works best to combine precleaning with page
  buffering

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Working set

• Definition The working set W(t,?) of a process is
  the set of process pages at virtual time t that
  have been referenced in the previous ? virtual
  time units
• The page fault rate is low if
• resident set gt W(t,?)
• Goal Allow as many processes as possible
  consistent with a low fault rate

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Load control (LS Criterion)

• Adjust the number of processes so that
• mean time between faults mean time needed to
  process a fault
• Determines the optimal multiprogramming level
• the number of processes resident in main memory
• Processor use is maximized at this point
• If too few processes, all processes may be
  blocked from time to time
• If too many processes, thrashing will occur
• To reduce the number or processes, some could be
  suspended

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Multiprogramming level
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Process Suspension choose the . . .

• Lowest priority process
• Faulting process - does not have much of its
  working set in main memory so it will be blocked
  anyway
• Last process activated - unlikely to have its
  working set resident
• Process with smallest resident set - requires the
  least future effort to reload
• Largest process yields the most free frames