Memory Management in Representative Operating Systems

1
Memory Management in Representative Operating
Systems

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Memory Management and Virtual Memory

• Memory management requirements
• Memory partitioning
• Paging
• Segmentation
• Virtual memory with
• Paging
• Segmentation
• Combined paging and segmentation
• Virtual memory management policies and strategies

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UNIX and Solaris Memory Management

• Evolution
• Early implementations of UNIX used variable
  partitioning and no virtual memory
• Current implementations SVR4 and Solaris use
  paged virtual memory
• Memory management schemes in SVR4 and Solaris
• Paging system
• Allocates page frames in main memory to processes
• Allocates page frames to disk block buffers
• Kernel memory allocator

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UNIX and Solaris Memory Management (cont.)

• Paging system
• Data structures
• Page table
• One page table per process
• One entry for each page in virtual memory for
  that process
• Disk block descriptor
• Has an entry associated with each page of a
  process
• Each entry describes the disk copy of the virtual
  page
• Page frame data table
• Describes each frame of real memory
• Indexed by frame number
• Swap-use table
• One table for each swap device
• One entry for each page on the device

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UNIX and Solaris Memory Management (cont.)

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UNIX and Solaris Memory Management (cont.)
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UNIX and Solaris Memory Management (cont.)

• Paging system
• Page replacement
• Page frame data table used for page replacement
• Lists are created within table using pointers
  (e.g., list of free frames available when number
  of frames on this table drops below a threshold,
  the kernel will steal a number of pages to
  compensate)
• Page replacement algorithm in SVR4 is a modified
  clock policy algorithm, the two-handed clock
  algorithm
• It uses the reference bit in the page table entry
  for each page in memory that is eligible (not
  locked) to be swapped out
• Bit is set to 0 when page is first brought in and
  set to 1 when page is referenced for read or
  write
• The front-hand of the algorithm sweeps through
  the eligible pages and sets reference bits to 1
• The backhand, later, sweeps same list and checks
  the referenced bit if 0, page is placed on the
  list to be paged out
• Parameters Scanrate and Handspead

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UNIX and Solaris Memory Management (cont.)

• Kernel memory allocator
• Requirement
• Kernel generates and destroys frequently small
  tables and buffers (e.g, file descriptor blocks)
  which require dynamic memory allocation
• These tables and buffers are much smaller than
  typical machine page size paging mechanism would
  be inefficient
• Solution SVR4 uses the lazy buddy system
• Observation demand for blocks of particular size
  varies slowly in time
• Solution defer coalescing blocks until it seems
  likely that it is needed and then coalesce as
  many blocks as possible
• Strategy try to maintain a pool of locally free
  blocks and only invoke coalescing if the number
  of free blocks exceeds a threshold
• Criterion for coalescing the number of locally
  free blocks of a given size should not exceed the
  number of allocated blocks of that size

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Linux Memory Management

• Characteristics
• Shares many characteristics from the other UNIX
  implementations
• Has some unique features and is quite complex
• Linux virtual memory
• Virtual memory addressing
• Three-level page table structure
• Page directory one per active process, size of
  one page
• Page middle directory
• Page table
• Virtual address space has four fields
• Index into the page directory
• Index into the page middle directory
• Index into the page table
• Offset within the selected page of memory
• Page table structure is platform independent and
  was designed to accommodate the 64-bi Alpha
  processor

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Linux Memory Management (cont.)

• Linux virtual memory (cont.)
• Virtual memory addressing (cont.)
• Page allocation
• A buddy system is used to improve the efficiency
  of reading and writing contiguous blocks of pages
  mapped into contiguous blocks of page frames
• Kernel maintains a list of contiguous page frame
  groups of fixed size 1, 2, 4, 8, 16, 32 page
  frames
• For allocation and deallocation of pages in main
  memory, the available groups are split and merged
  using the buddy system
• Page replacement algorithm
• Variation of the clock algorithm least
  frequently used policy
• The use bit is replaced with an 8-bit age
  variable
• Age variable incremented each time page is
  accessed
• Periodically, Linux sweeps through the global
  page pool and decrements the age variable for
  each page
• A page with a low age variable is a candidate for
  replacement

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Linux Memory Management (cont.)

• Kernel memory allocation
• Based on the page allocation mechanism used for
  virtual memory management
• Buddy algorithm is used to allocate and
  de-allocate kernel memory in units of one or more
  pages
• Since the kernel needs small (smaller than a
  page) chunks of memory for short intervals,Linux
  uses slab allocation within an allocated page
• Example on a Pentium/x86 machine page size is 4
  Kbytes and chunks can be allocated in sizes 32,
  64, 128, 252, 508, 2040, and 4080 bytes
• Slab allocator implementation
• A set of linked lists are maintained, one for
  each size of chunk
• Chunks can be split, aggregated, and moved
  between lists like in the buddy algorithm

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Windows 2000 Memory Management

• Characteristics
• Designed to operate over a variety of platforms
  and use page sizes from 4 Kbytes to 64 Kbytes
• Current platforms
• Intel, PowerPC, and MIPS platforms have 4,096
  bytes per page
• DEC Alpha platforms have 8,192 bytes per page
• W2K virtual address map
• 2 Gbytes of available virtual address space for
  user (increased optionally to 3 Gbytes)
• 2 Gbytes for the operating system (W2K Executive,
  microkernel, and device drivers)

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Windows 2000 Memory Management (cont)
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Windows 2000 Memory Management (cont.)

• W2K paging
• When a process is created, it can use up to 2
  Gbytes (minus 128 Kbytes) of virtual space,
  divided in fixed-size pages
• Page states
• Available Pages not currently used by the
  process
• Reserved Set of contiguous pages set aside for
  the process, but not counted as part of process
  quota until used. Committed to to process when it
  needs to write into memory
• Committed Pages for which space on the paging
  file has been set aside (e.g., the disk file used
  when removing pages from memory)
• Resident set management scheme
• Variable allocation, local scope
• When process first activated, given a default
  working set if process references a page not in
  memory, one of the resident pages is swapped out
• Working set of active processes is adjusted as
  follows
• When memory plentiful, active processes are
  allowed to grow
• When memory becomes scarce, resident sets for
  each process are reduced