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Memory Management

Managing memory hierarchies

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Memory Management

• Ideally programmers want memory that is– large

– fast

– non volatile

– transparent

• Memory hierarchy – small amount of fast, expensive memory – cache

– some medium-speed, medium price main memory

– gigabytes of slow, cheap disk storage

• Memory manager handles the memory hierarchy

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Basic Memory ManagementMonoprogramming without Swapping or Paging

Three simple ways of organizing memory- an operating system with one user process

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Multiprogramming

• We usually want more than one program…– Multiprogramming!

• Flexibility

• Utilization (CPU vs. I/O)

• Multi-user environments

• First try: partition the memory!

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Multiprogramming with Fixed Partitions

• Fixed memory partitions– separate input queues for each partition– single input queue

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Multiprogramming

Sidestep: How much do we actually gain from multiprogramming?

5 tasks @ 80% idle = 100% CPU utilization?

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Modeling Multiprogramming

Degree of multiprogramming

np1on UtilizatiCPU

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Analysis of Multiprogramming System Performance

• Arrival and work requirements of 4 jobs• CPU utilization for 1 – 4 jobs with 80% I/O wait• Sequence of events as jobs arrive and finish

– note numbers show amount of CPU time jobs get in each interval

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Where to place a program?

• Where in the memory should we place our programs?– Memory starts at 0 and ends at 232!– We might want more than one program!– Programs might have different sizes!– Program size might grow (or shrink)!

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Swapping (1)

Memory allocation changes as – processes come into memory– leave memory

Solution for relocation needed

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Swapping (2)

• Allocating space for growing data segment• Allocating space for growing stack & data segment

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Memory Management with Bit Maps

• Part of memory with 5 processes, 3 holes– tick marks show allocation units– shaded regions are free

• Corresponding bit map• Same information as a list

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Memory Management with Linked Lists

Four neighbor combinations for the terminating process X

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Issues with memory

• We want:– A lot of memory (more than RAM)– Transparency (Relocation)– Protection (rouge processes)– Speed (Cache is fast, RAM is OK, disk is slow)

• (Partial) Solution: Virtual Memory

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Virtual Memory

• Separates:– Virtual (logical) addresses– Physical addresses

• Requires a translation at run time– Virtual Physical– Handled in HW (MMU)

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Virtual MemoryPaging

The position and function of the MMU

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Paging

• The relation betweenvirtual addressesand physical memory addres-ses given bypage table

• One page table per process is needed (per thread?)

• Page table needs to be reloaded at context switch

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Address Translation

3 2 1 011 10 9 815 14 13 1231 30 29 28 27

Page offsetVirtual page number

Virtual address

3 2 1 011 10 9 815 14 13 1229 28 27

Page offsetPhysical page number

Physical address

Translation

© Patterson & Hennesy 1998

2048211

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Page Tables

Internal operation of MMU with 16 4 KB pages

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Hierarchical Page Tables

• 32 bit address with 2 page table fields

• Two-level page tables

Second-level page tables

Top-level page table

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Page Tables

Typical page table entry

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Paging

Every memory lookup:

1. Find the page in the page table

2. Find the (physical) memory location

Now we have two memory accesses (per reference)

Solution: Translation Lookaside Buffer (TLB)

(again a cache…)

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TLBs – Translation Lookaside Buffers

A TLB to speed up paging

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TLB Handling

Memory lookup:

• Look for page in TLB (fast)– If hit, fine go ahead!– If miss, find it and put it in the TLB:

• Find the page in the page table (hit)

• Reload the page from disk (miss)

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Inverted Page Tables

Comparison of a traditional page table with an inverted page table

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So where is the cache?

Valid Tag Data

Page offset

Page offset

Virtual page number

Virtual address

Physical page numberValid

1220

20

16 14

Cache index

32

Cache

DataCache hit

2

Byteoffset

Dirty Tag

TLB hit

Physical page number

Physical address tag

TLB

Physical address

31 30 29 15 14 13 12 11 10 9 8 3 2 1 0

© Patterson & Hennesy 1998

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Paging

• So we know how to find a page table entry

• If it is not in the page table– Get it from disk

• What if the physical memory is full?– Throw some page out!– Remember cache replacement policies?

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Page Replacement Algorithms

• Page fault forces choice – which page must be removed– make room for incoming page(s)

• Modified page must first be saved– unmodified just overwritten

• Better not to choose an often used page– will probably need to be brought back in soon

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Optimal Page Replacement Algorithm

• Replace page needed at the farthest point in future– Optimal but unrealizable

• Estimate by …– logging page use on previous runs of process– although this is impractical

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Not Recently Used Page Replacement Algorithm

• Each page has Reference bit, Modified bit– Bits are set by HW when page is referenced, modified– Reference bit is periodically unset (at clock ticks)

• Pages are classifiedClass 0: not referenced, not modifiedClass 1: not referenced, modifiedClass 2: referenced, not modifiedClass 3: referenced, modified

• NRU removes page at random– From lowest numbered non empty class

• NRU is simple and gives decent performance

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FIFO Page Replacement Algorithm

• Maintain a linked list of all pages – in the order they came into memory

• Page at beginning of list replaced

• Disadvantage– page in memory the longest may be used often

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Second Chance Page Replacement Algorithm

• Operation of a second chance– pages sorted in FIFO order– Inspect the R bit, give the page a second chance if R=1– Put the page at the end of the list (like a new page)– Eventually the page might be selected anyway (how?)Example: Page fault @ 20, A has R bit set

• 2nd Chance moves pages around in the list …

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The Clock Page Replacement Algorithm

How does this compare with second chance?

Hand points to oldest page

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Locality

• Locality in:– Time

• A page recently used, will be used soon again

– Space• Adjacent pages are likely to be used together

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Least Recently Used (LRU)

• Locality: pages used recently will be used soon– throw out the page that has been unused longest

• Must keep a linked list of pages– most recently used at front, least at rear– update this list every memory reference !!

• Alternative: counter (64 bit) in the page table entry– Update on memory reference– choose page with lowest value counter– periodically zero the counter

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Least Recently Used

LRU using a matrix – pages referenced in order 0,1,2,3,2,1,0,3,2,3

1. Set row k to 12. Set column k to 0

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Simulating LRU in Software

• Both solutions (counter, matrix) require extra hardware

• and they don’t scale very well with large page tables …

• Approximations of LRU can be done in SW

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Simulating LRU in Software

NFU (Not Frequently Used)

• A counter is associated with each page

• At each clock interrupt add R to the counter

• Problems?

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NFU

• NFU tends to have a long memory!

• Pages frequently used in pass 1 may still be hot in later passes, even though they are not used!

• Solution: instead of adding R, shift R from the left (R=1, c=010 c’=101)

• This algorithm is called aging

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Simulating LRU in Software

• The aging algorithm simulates LRU in software• Note 6 pages for 5 clock ticks, (a) – (e)

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Aging

• This algorithm works fine, but consider– Granularity

• Page accesses between ticks?

– Memory• How long is the memory?

• Tannenbaum: 8 bits @ 20ms ticks works ok

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Memory Management

• Managing hierarchies of memory– Caches– RAM– Disk

• Paging– Page tables– Page replacement algorithms

Virtual Memory