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Memory Management
Operating Systems
Lecture 4, April 3, 2003
Mr. Greg Vogl
Uganda Martyrs University
April 3, 2003 Operating Systems: Memory Management
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Overview
Fixed and Variable Partition Paging Segmentation Virtual Memory and Page Replacement Segmented Paging Protection and Sharing DOS and UNIX Memory Management
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Sources
Ritchie Ch. 5-7 Burgess 5.1-2 Solomon Part 6
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What is stored in memory?
Operating system code and data User program code User program data
Process Control Blocks Stack for executing subroutines Memory mapped I/O: device drivers Screen/display memory (Video RAM)
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Memory management goals/tasks
Manage several processes at same time Load into memory, swap out to disk Run processes quickly, use available memory
Protect most processes from each other But allow some processes to share memory
Ease memory management for programmer Allocate memory in contiguous logical blocks Map logical addresses to physical addresses
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Fixed partition memory
Each process gets fixed partition of memory Usually the OS is put at the bottom (address 0)
Use different partition sizes Accommodates different possible process sizes
Don’t let a process harm another’s memory Check that the addresses are in its partition
Every partition has unused (wasted) space Not enough space for big new processes?
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Variable partition memory
Allocate the memory each process needs Free the space of a terminated process Put new processes in empty “holes”
Adjacent holes can be merged About 2x as many processes as holes Holes maybe not right size for new processes How to choose a hole to put in a new process?
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Storage placement policies
Best fit Put new process in smallest possible hole Remaining hole is as small as possible
Worst fit Put new process in largest possible hole Reduces number of big holes, creates few small ones
First fit (or next fit) Put new process in first (or next) hole big enough to fit No overhead of finding min/max hole sizes
Which is best? Tradeoff: speed vs. space
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Memory implementation
Functions to allocate, deallocate, reallocate UNIX uses malloc(), free(), realloc() C++ uses new and delete operators Lists keep track of allocated and free blocks
Keep lists of various-sized holes (powers of 2)
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Fragmentation
Gaps of unused storage space Occurs in all storage devices (memory, disk) Wastes space, may also reduce performance
Internal fragmentation Unused space within a process or block Can occur if word size > smallest data size
External fragmentation Unused space between processes or blocks
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Compaction (defragmentation)
Join together used memory; combine holes Move allocated blocks to an end of memory
Calculate distance the block will move Add to pointers, then move the data
Need to move a lot of things in memory Need to find and move all pointers Need to suspend processes until done Cannot use in time-critical systems
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When/how often to compact?
When any process terminates When there is no more free memory At fixed intervals At user(s) request
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Coalescing holes
Don’t coalesce Give entire hole (maybe used later by realloc)
Buddy system Combined buddies align in powers of 2 When (de)allocating do the buddy block too
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Garbage Collection
Find inaccessible blocks, add to free list Conservative
Treat pointer-like memory addresses as pointers Not all garbage found
Reference count Each block stores a count of pointers to itself When a block’s count is 0, free the block Does not detect circular lists of garbage
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Mark and sweep
Algorithm Mark used blocks using depth-first search Sweep (free) unused blocks and compact
Disadvantages Not helpful if memory is almost full Must load many swapped pages into memory
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Generational
Divide memory into spaces Objects are usually short- or long-lived Keep long-lived objects in their own spaces Clean out mostly empty spaces Copy objects to other spaces when accessed
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Paging
Each process composed of fixed-size pages Memory is also divided into pages (frames) Process pages can go anywhere in memory No external fragmentation Internal fragmentation ~ page size
(a process will not usually use all of each page) Need to keep track of page locations
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Implementing paging
Logical address = page number + displacement E.g. 32 bit addr. = 20 bit page no. + 12 bit disp. 4 GB of addresses: a million pages, 4 KB each
Page table translates page no. to frame no. Implemented as array of memory page numbers Page address + displacement physical address
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Segmentation
Each process has variable length pieces Segment size determined by programmer
Each subroutine or data takes one or more Reflects logical/modular process structure No internal fragmentation Improved performance (locality of reference) External fragmentation
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Implementing segmentation
Logical address = segment reference + displacement
Process segment table is like page table Each segment entry has base address and length Base address + displacement physical address If displacement > length, segmentation fault!
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Virtual memory
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Virtual memory
Process loaded in separate parts Dynamic address translation at run time Not need all of process in memory to run it Only currently accessed code & data pages Rest of process stays in secondary storage
Windows reserves a swap file (win386.swp) UNIX/Linux often uses a swap partition
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Virtual memory benefits
More processes keeping processor busy Memory and disk space more fully used Few pages of a process needed at one time
Modular programs have locality of reference Virtual memory > real memory A process memory can be > real memory Programmer not limited by real memory
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Virtual memory using paging
Resident set = a process’s pages in memory Demand paging: only load pages when needed When a required page is not in memory A page fault generates an interrupt to request it If no free page frames, replace an existing one
Separate page table for each process Maps page numbers to frame numbers Page table register points to process page table
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Virtual memory costs
Complexity, hardware and OS support Page table takes a lot of space
Must itself be stored in virtual memory Overhead of swapping is large
Too many page faults can cause thrashing Find optimum number of active processes Resident sets proportional to process sizes
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Page size
How large should pages be? Tradeoffs: If too small, page table is too big If too large, internal fragmentation is too big Must be a power of 2 for easy addressing
Pages in most systems are 2 or 4 KB
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Page replacement policies
A page is removed from memory, replaced Present bit: 1 if in real memory Modified bit: 1 if page is modified (“dirty”)
write to secondary storage before replacing Optimal policy can be known in retrospect If performance near optimal, good enough Policies: LRU, NRU, FIFO, Clock
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Least recently used (LRU)
Replace page not referenced the longest Frame is given time stamp when referenced Overhead for time stamp and finding oldest Linked list would also have big overhead
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Not recently used (NRU)
“page referenced” bit All bits are set to 0 periodically Bit is set to 1 when page is used Any page with 0 can be replaced
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First-in first-out (FIFO)
Remove page in memory longest Easy to implement using linked list queue Bad performance: evicts heavily used pages
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Clock or second chance
Circular linked list similar to queue Add used bit, set to 0 when loaded Set used to 1 when referenced Use pointer to head of list When replacement needed, look for a 0 Set any 1s to 0 Same as FIFO but leave recently used pages
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Translation lookaside buffer
A memory cache for page table entries Hardware buffer in fast storage If not in buffer, look in page table as usual Holds both virtual and real page numbers Associative lookaside buffer often also used
Maps virtual page no. to real page frame no.
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Virtual memory using segments
Facilitates use of dynamic memory Segments can grow or be relocated
Facilitates process sharing of code and data Logical structure ~ physical structure
Reinforces locality principle, good performance
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Implementing virtual segments
Virtual address = segment number + displacement Seg. table register points to current process Segment descriptor (table entries) include
Segment base address Segment size (limit) to check for address errors Bits: in memory, used, rwx access protection
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Paged segmented memory
Used in many modern operating systems Each segment has whole number of pages
Logical pages mapped to physical pages Programmer works with segments Operating system manages pages
Virtual address = segment no. + page no. + displacement
One segment table per process, s.t. register One page table per segment
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Paged segmented memory
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Sharing
Sharing Threads share process info. (PCB, code) Shared libraries e.g. dlls in Windows, stdio in C Segments shared by processes
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Memory Protection
Protection violations that produce errors: address < base address address > limit register + base address displacement > page/segment size page/segment no. > no. of pages/segments read/write/execute segment not permitted
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MS-DOS
Designed for Intel CPUs w/ 16-bit registers DOS limited at first to 64 KB, then 1 MB
16 bit addressing, left shifted 4 bits Processes have at least 4 64-KB segments
segment registers: code, data, stack, extra Process switching is possible
One awakens, the other goes to sleep First-fit to find free memory for each segment
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MS-DOS memory map
Early DOS memory had fixed uses 0-640 KB
DOS files, device drivers, user program(s) 640 KB-1 MB
Video RAM, ROM BIOS 1 MB-1MB + 64 KB
High Memory Area: parts of OS
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Overlays
Used to divide up a large program Process root module is always loaded Infrequently used routines put in overlays Separate modules use same memory area Only one can be loaded at a time
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Extended/expanded memory
How can DOS access > 1 MB memory? Extended memory: above 1 MB Expanded Memory System (EMS):
use memory board, expanded memory manager 1 MB has 64 16-KB page frames Up to 32 MB of additional 16-KB pages Address references redirected above 1MB
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Windows 3.1
Modes Real: Intel 8086, 640 KB Standard: 286, up to 16 MB, task switching Enhanced: 386 virtual memory, multitasking
16-bit segmented addressing (like DOS) Win16 API DLLs used by applications and Windows
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Windows 95, NT
32 bit memory, 4 GB total address space not segmented 2 GB process memory 2 GB system memory
paged, non-paged, physical addressing 64-bit processors and OS are now in use
Win32 API Win32s has same interface but uses 16 bit code
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UNIX
A process has three segments Text (executable code) Data (initialised, uninitialised) Stack (local procedure data and parameters)
Processes can share segments (text, data)
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UNIX virtual memory
pages (typically 4 KB) Page daemon counts number of free frames If too few, remove pages using clock If many page faults, remove LRU processes Reload processes swapped out a long time
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