Operating System11

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Operating System(11)
Minyi Guo Dept. of Computer Science and
Engineering, Shanghai Jiaotong University
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Memory Management

• Basic memory management
• Swapping
• Virtual memory
• Page replacement algorithms
• Implementation issues
• Segmentation

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Swapping

• Memory allocation changes as
• Processes come into memory
• Processes leave memory
• Take processes out from memory and bring process
  into memory are called Swapping

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Swapping
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Swapping

• Problem with previous situation?
• Difficult to grow process space
• Solution
• Allocating space for growing data segment
• Allocating space for growing stack data segment

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Swapping
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Memory Mgmt Issues

• Need keep track of memory used and available
• Bit map approach
• Linked list approach

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Memory Mgmt

• Divide memory into allocation units
• Bit map approach Use one bit to mark if the unit
  is allocated or not
• Linked list approach Use a linked list to
  indicate allocated and available units in chunks

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Memory Mgmt
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Memory Mgmt with Linked Lists

• What happens when units are released?
• May need to collapse linked list appropriately

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Memory Mgmt with Linked Lists
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Strategies to find a memory region

• Best fit
• Allocate the smallest memory region that can
  satisfy the request (least amount of wasted
  space)
• First fit
• Allocate the memory region that you find first
  that can satisfy the request

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External Fragmentation

• Processes come and go
• Can leave a mishmash of available memory regions
• Some regions may be too small to be of any use
• Merging
• Dynamical address translation

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Virtual Memory

• An address space can be larger than the amount of
  physical memory on the machine
• Load part of the address space into the memory,
  and others stay in disk
• Amount of the virtual memory?

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Paging

• Most virtual memory use paging technology
• Allocate physical memory in terms of fixed-size
  chunks of memory, any free physical page frame
  can store any virtual page
• Virtual address
• virtual page number (high bits of address, e.g.
  bits 31-12 for 4 KB page)
• offset (low bits of address, e.g. bits 11-0 for 4
  KB page)

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Paging
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Paging

• Processes access memory by virtual addresses
• Each virtual memory reference is translated into
  physical memory reference by the MMU

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Paging
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Paging

• Page fault
• The page is unmapped
• Trap to kernel, then the OS allocate a free page
  frame and load the page to the memory. If there
  is no free page frame for this process, pick a
  little-used page frame and write its contents
  back to the disk

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Page Table

• Used to keep track of virtual-physical page map
• One entry for each virtual page
• Store the position of the page, and also keep
  information concerning other relevant information
  such read/write/execute, present, etc.
• MMU use it to perform addresses translation

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Paging

• What must be changed on a context switch?
• Page table, registers, cache image
• Possibly memory images
• Each virtual page can be in physical memory or
  disk

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Paging

• How does processor know that virtual page is not
  in physical memory?
• Through a present/absent bit in page table
• Pages can have different protections
• e.g. read, write, execute
• These information is also kept in page table

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Page Table
Typical page table entry
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The internal operation of the MMU with 16 4KB
pages
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Pros of paging

• simple memory allocation
• can share lots of small pieces of an address
  space
• easy to grow the address space
• Simply add a virtual page to the page table and
  find a free physical page to hold the virtual
  page before accessing it

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Cons of Paging

• The size of page table could be enormous
• Take 32 bits virtual address for example, assume
  the size of page is 4KB, then there are 65536
  virtual pages
• For a 64 bit virtual address?
• solution
• Use multi-level translation