TDC 311
Memory Management
Why Do We Need Memory Management?
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Protection - to protect one process from another; the
OS from processes; resident libraries from processes.
Sharing code - what if two different processes want to
share a piece of code?
Logical organization - allows for multiple,
independent modules to reside in memory at the same
time.
Virtual memory - processes are bigger than main
memory. Memory management allows a large process to
partially occupy main memory.
Historical Perspective
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Bare Machine - no memory management. User has
entire machine.
Resident Monitor - memory divided into 2 sections monitor and user. Protection hardware consists of one
fence register.
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User memory does not start at location 0. OS starts at 0.
User starts at location n. Are programs compiled/assembled at
location n? Probably not, so need relocation.
Static relocation - IBM 370 style.
Dynamic relocation - CDC 6600 style with relocation register.
Logical address space : What the user sees : 0 to max.
Physical address space : What the OS uses: R+0 to R+max for
fence register value R.
Historical Perspective Continued
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Swapping - memory still divided into monitor space and user
space, but you may have more than one job/process, so swap
one in, execute it, swap it out, swap next one, etc.
Overlapped swapping - memory divided into monitor,
buffer1, buffer2, and user space. Keep two jobs in buffer
spaces and swap them in and out of user space as needed.
Multiple partitions - memory is divided into regions, or
partitions. For a job to execute, it is loaded into a free
partition. When done, the job is removed and the partition is
marked free.
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Can have fixed (static) sized partitions (MFT) and variable (dynamic)
sized partitions (MVT) (IBM OS/360).
MFT
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Multiprogramming with Fixed-Sized Tasks
Problems:
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Asking for more memory
Internal fragmentation
MVT
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Multiprogramming with Variable-Sized Tasks
No fixed sized partitions. Fit job wherever it will fit. Best
fit? First fit? Worst fit?
Problems:
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Fragmentation
Garbage collection (compaction).
Simple Paging
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Divide memory into fixed sized frames
Divide user process into fixed sized pages
Take a page of user program and plug into empty frame
Each job has its own page table
No external fragmentation, only internal fragmentation
On average, ½ of a frame will be wasted
Shared pages - possible to share pages (like editor,
compiler, database code) if code is reentrant (does not
modify itself).
Protection - can add read only / read write / execute bits
to each page in the page table.
Simple Segmentation
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Like paging, but user job not broken into fixed sized pages
(physical division) but left as procedures / functions / data
sets
Protection - can now place access bits on entire segment.
A code segment can be accessed by many users. A data
segment can be accessed by only one user. Put an array
in a segment by itself and the hardware will check the
array bounds automatically!
Sharing - same as with paging
Virtual Memory
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Why load the entire program into memory at the start?
Just load the necessary routines / modules. Why? Error
routines are seldom called. Don’t always need entire
array, list or table. Certain sections of code may be
rarely used.
Resident set - portion of user process that is currently
in main memory.
Advantages of virtual memory - may have more
processes in memory since only part of each process is
resident. It is possible for a process to be larger than
main memory.
Virtual Memory Continued
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Disadvantages - if part of a process is not in memory,
process is moved to blocked state, disk I/O reads in
necessary piece of code, process is moved to Ready state,
process resumes execution
Thrashing - removing a piece to satisfy some other
process, but then needing that piece next.
Demand Paging
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Page table for each process must also include 1 bit for
each page – this bit tells if page is in memory, or resident
Also, one Modify bit for each page - tells if the page has
been modified
What if page is not resident?
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Take page number, look in page table.
Is the page resident? If yes, get frame number. If no, page fault.
Find a free frame. No free frame? Call replacement algorithm.
Load desired page from disk into memory.
When disk read completed, update page table.
Restart instruction.
Paging Issues
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Page size - the smaller the page, the less internal
fragmentation, but the bigger the page table.
If a page is 512 bytes in size (29) and we are using a
memory space 231, we will need 222 page table entries per
process! How do you handle such a large page table?
Keep only part of the page table resident, or use an
inverted page table.
Typical page sizes: IBM 370: 2048 / 4096;VAX: 512; INTEL
486: 4096; Motorola 68040: 4096.
The paging process can be time intensive. Make sure the
paging process is fast and doesn’t occur constantly.
Pure demand paging vs. prepaging
Page Replacement Algorithms
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If you need to bring a page in, but there are no available
frames, you have to remove a page and put it back to disk.
Who do you remove?
Let’s say we are given the following page reference string:
70120304230321201701
and 3 frames of memory
FIFO (first in first out):
LRU (least recently used):
Optimal (theoretical):
Other Issues
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How many pages at one time do you give to each process
(resident set size)? Can the resident set size change as
the process’s demands grow and shrink?
When you take a page out to make room for another
page, check its Modify bit. If it has not been set, the page
has not been modified (is not dirty) and there is no need
to write the page back to disk.
There is also demand segmentation.
Example – Windows NT
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Page size = 4096 bytes. Segmentation and paging are
optional and in combinations.
With unsegmented memory, user’s virtual space is 232 = 4
Gb.
With segmentation, there is a 16-bit segment reference
(of which 2 bits are used for protection) and a 32 bit
offset, for 246 = 64Tb !
Paging is done via 2 levels - first level is a page directory
containing 1024 entries. Each entry is a page table with
1024 entries. Each entry is 4 Kb.
Example – Linux
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Linux uses either 3-level paging or the more recent 4level paging
From IBM’s Explore the Linux Memory Model (3-level):
Example – Linux
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In 64-bit processors:
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The remaining 30 bits are divided into
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21 MSBs are unused
13 LSBs are represented by page offset
10 bits for Page Table
10 bits for Page Global Directory
10 bits for Page Middle Directory
Thus, 43 bits are used for addressing. This yields 243
bytes of available virtual memory space