04. Memory Management

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SIT222Operating SystemsSession 04. Memory management


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Outline

Introduction to memory management

Virtual memory and paging Practical task

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The main memory (RAM) of a computer must becarefully managed Many processes compete for memory Demand for memory often exceeds physical

memory available Disk can be used to supplement memory,

but this introduces new problems Processes expect a consistent view of memory

How to achieve this without conflicts?

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Historically, computers would load and run a singleprogram at a time Processes would run until they terminate Processes access physical memory directly

Can corrupt the operating systems memory easilyas a result

Can this be extended to multiple programs to run? Yes, through swapping

Save out memory contents to disk Load next programs memory contents from disk

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Swapping is very inefficient Only one program is loaded at any one time

Obvious improvement is to load multiple programs intomemory at the same time!

The problem: programs are working directly with physicalmemory, i.e., they load and store data at physical locationsin memory

One solution is static relocation

Modifies all memory references used by the program Longer loading time as addresses must be updated Processes can still access/modify each others

memories

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To properly solve the problem of memorymanagement, we need to implement amemory abstraction Processes have an abstract view of memory

(logical address space) Every logical memory address is then mapped

to a physical address (physical address space) i.e., processes do not work directly with

physical memory Uses hardware support to improve efficiency

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The simplest memory abstraction uses two registers Base register stores the location in memory where

a program is loaded Base register is added to all memory references

(logical address + base = physical address) Known as dynamic relocation

Limit register stores the amount of memory used

by that process If a process references memory outside thelimit it is terminated(logical address + base > limit == error)

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Having several processes in memory increases thelikelihood that the demand for memory will exceedphysical memory We have already considered swapping as a

solution earlier Swapping moves whole processes in/out of

memory Idle processes can be swapped out to allow

room for ready processes

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Swapping processes leads to the need for analgorithm to manage the free regions of memory If there are several free regions, which one to

you allocate to a new/swapped in process?

To solve this problem, the location and size of free

memory regions are usually stored in a (linked) list

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Algorithms for allocating free memory regions: First fit

Traverse the list until you find the first freememory region whose size is big enough

Memory region is then broken into two units,the first for the new/swapped in process, the

second is returned to the free list

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Algorithms for allocating free memory regions: Next fit

Almost identical to first fit, but keep yourplace in the list rather than searching fromthe start of the list each time

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Algorithms for allocating free memory regions: Best fit

Search the entire list for the smallest freememory region that is big enough

Unfortunately, because best fit selects thesmallest regions, it leaves very tiny free

regions behind and tends to perform poorly

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Algorithms for allocating free memory regions: Quick fit

Keeps several lists of free regions, each ofdifferent sizes, e.g., 4K, 8K, etc.

Very fast to find a matching hole

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Algorithms for allocating free memory regions: All of these algorithms suffer from the need to

search the list after every deallocation Two adjacent free regions must be

recombined into a single free region Can be done periodically and/or when a large

allocation request cannot be serviced.

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Because processes are variable sized,fragmentation becomes a problem over time After swapping out a process, a smaller process

is loaded in its place, leaving a gap This is known as external fragmentation

The fragments of unused memory are located

outside of allocated memory regions

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Compaction is possible in some circumstances,e.g., move process and adjust base and limitregisters Compaction moves the processes in memory to

remove the gaps This is an intensive operation because

effectively you are copying most/all memory Unfortunately, processes also tend to grow over

time (dynamic memory allocation)

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Virtual memory and paging

A better solution to the problem is to usevirtual memory Processes access virtual address space Virtual address space is mapped to the physical

address space by: Hardware: The MMU translates virtual

memory addresses to physical addresses Software: The operating system configuresand manages the MMU

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The virtual address space is broken up into small,fixed size chunks known as pages Virtual address space is limited only by the size

of a memory reference e.g., 32-bit platform = 2 32 addresses (4GB)

Do not need to use all pages Do not need to have all pages used by a process

loaded in physical memory i.e., processes can be partially loaded in

memory whereas under swapping wholeprocesses are loaded/unloaded

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Virtual address space is mapped to the physicaladdress space by the MMU, which is similarlydivided into page frames Page frames are generally the same size as the

pages

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Page/page frame sizes are always 2 n , e.g., 2KB,4KB, 8KB, etc., as this simplifies mapping Consider a memory address in binary, for a 4KB

page size:

Mapping only requires substituting the page

number for the frame number:

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The record of whatpage is stored in whatpage frame is kept inthe page table for aprocess

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A typical page table entry

Referenced bit is set whenever the page has

been accessed (read or write) Can reset and record periodically to tell

whether the page has been recently used

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Modified bit (dirty bit) is set whenever the

page has been changed/written to

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Protection has read/write/execute rights, e.g.,

Code region read + execute Data region read + write Heap region read + write

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Present indicates whether that page is currently

loaded into a page frame or not Where a page is not loaded a page fault

exception will result

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Problem 1 of using page tables:The page table itself is usually stored in memory Every memory reference made by a program

takes two physical memory references: One reference to read the page table One reference to complete the request made

by the process

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Solution: Translation Lookaside Buffers (TLBs)

Also called associative memory Very fast hardware cache (expensive!)

Usually only a small number of entries Fortunately, most processes only access a

small number of pages at any one time Referred to as the working set of pages

due to locality of reference (more later)

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Problem 2 of using page tables:The page table itself can be very large

Assume a page table entry is ~32 bits/4 bytes 4KB pages on a 32-bit platform will require

2 20 page table entries (1,048,576 entries) Page table is approx. 4MB per process

4KB pages on a 64-bit platform will require

2 52 page table entries(4,503,599,627,370,496 entries) Page table is approx. 16 petabytes per

process!!! WOW!!!

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Solution 1: Multi-level page table

Still uses one page table entry per pageframe

Rather than one table containing all entries,break the table into parts and index thoseparts with another table

Does not require all parts to be present Can have more than two levels of page

tables

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Solution 1: Multi-level page table

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Solution 2: Inverted page table

One page table entry per physical frame (notper page!)

Result is a much smaller table However, it is much harder to find the correct

entries in the page table

Page number can no longer be used toindex the table Must perform a manual search instead

TLBs still help here

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Solution 2: Inverted page table

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Practical task

Package management Monitoring memory usage Monitoring processes Creating and managing swap space

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Summary

Introduction to memory management Virtual memory and paging Practical task

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Documents

04. Memory Management