Paging techniques Structure of the Page Tabletami/COP4610M12Slides.pdfPRACTICAL PROBLEMS OF PAGING...
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Operating systems Module 12 Main memory and Virtual memory Part 1 Paging techniques Structure of the Page Table 1 Tami Sorgente
Transcript of Paging techniques Structure of the Page Tabletami/COP4610M12Slides.pdfPRACTICAL PROBLEMS OF PAGING...
Operating systemsModule 12
Main memory and
Virtual memory
Part 1
Paging techniques
Structure of the Page Table
1Tami Sorgente
PAGING
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Logical/virtual memory
pages
Page table
Frame #
Physical memory
frames
X
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PRACTICAL PROBLEMS OF PAGING
Each process must have its own page table –
page tables must reside in Memory –o much slower than registers – every time need to
address, have to go to page table – each instruction must then read memory once or twice – use special hardware for solution (TLB)
Page size? – fixed, but how big?
Small pages – mean many entries in tables
Big pages – internal fragmentation – memory waste
When page tables are very large then we have search problems
3Tami Sorgente
TLB CACHING PAGE/ FRAME PAIRS
All gains require hardware support Solution: use cache with (page#, frame#) pairs-TLB (Translation look-aside buffer)
Find frame number in TLB – 20 ns
Find frame number in PT – 100 ns
Access memory– 100 ns
Effective Access Time (EAT) calculation:
80% hit ratio – 0.80x120 + 0.20x220 = 140 ns
40% overhead of paging
98% hit ratio –0.98x120 + 0.02x220 = 122 ns
22% overhead of paging
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ADDRESS TRANSLATION SCHEME Address generated by CPU is divided into:
o Page number (p) – used as an index into a page table which contains base address of each page in physical memory
o Page offset (d) – combined with base address to define the physical memory address that is sent to the memory unit
o For given logical address space 2m and page size 2n
o For given physical address space
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page number page offset
p d
m - n n
f d
frame number frame offset
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PAGING EXAMPLE
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4 byte pages
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Page table
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32 byte M
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001 10
Logical address
of letter k
Physical address
of letter k
Operating systemsModule 12
Virtual memory paging address translation
Part 2
7
CONTENTS OF PAGE TABLE
Contents of Page Table:
Valid bit –if page is loaded (is in frame)
Frame number
modified bit -indicates if some part of the page has been written
referenced bit - if it has been referenced (victims)
protection bits –
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frame #
valid bit – protection bits – modified (dirty) bit
1
0
write back to disk
no need to write
back (save time)
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Size of virtual address (32 bits)Size of physical address (27 bits)
000 0000 0000 0010 1101 0110 1010
PAGE FAULT
If there is a reference to a page, first reference to that page will trap to operating system:
page fault
1. Operating system looks at another table to decide:o Invalid reference aborto Just not in memory
2. Get empty frame3. Swap page into frame via scheduled disk
operation4. Reset tables to indicate page now in memory
Set validation bit = v5. Restart the instruction that caused the page fault
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STEPS IN HANDLING A PAGE FAULT
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Operating systemsModule 12
Virtual memory
Part 3
Page replacement
12Tami Sorgente
VIRTUAL MEMORY REVIEW
Code needs to be in memory to execute,
but entire program rarely used
o Error code, unusual routines, large data
structures
Entire program not needed at same time
Ability to execute partially-loaded program
o Program no longer constrained by limits of
physical memory
o Programs could be larger than physical memory
13Tami Sorgente
VIRTUAL MEMORY REVIEW
Virtual memory – separation of user logical memory from physical memoryo Only part of the program needs to be in memory for
execution
o Logical address space can therefore be much larger than physical address space
o Allows address spaces to be shared by several processes
o Allows for more efficient process creation
o More programs running concurrently
o Less I/O needed to load or swap processes
Virtual memory can be implemented via:o Demand paging
o Demand segmentation
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DEMAND PAGING
Bring a page into memory only when it is needed
o Less I/O needed, no unnecessary I/O
o Less memory needed
o Faster response
o More users
Page is needed reference to it
o invalid reference abort
o not-in-memory bring to memory
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ASPECTS OF DEMAND PAGING
Extreme case – start process with no pages in memoryo OS sets instruction pointer to first instruction of
process, non-memory-resident -> page fault
o And for every other process pages on first access
o Pure demand paging
Actually, a given instruction could access multiple pages -> multiple page faultso locality of reference
Hardware support needed for demand pagingo Page table with valid / invalid bit
o Secondary memory (swap device with swap space)
o Instruction restart
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BASIC PAGE REPLACEMENT
1. Find a free frame:- If there is a free frame, use it- If there is no free frame, use a page
replacement algorithm to select a victim page- Write victim page to disk if dirty
2. Bring the desired page into the (newly) free frame; update the page and frame tables
3. Continue the process by restarting the instruction that caused the trap
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PAGE REPLACEMENT
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GLOBAL VS. LOCAL ALLOCATION
Global replacement – process selects a replacement frame from the set of all frames; one process can take a frame from another
o But then process execution time can vary greatly
o But greater throughput so more common
Local replacement – each process selects from only its own set of allocated frames
o More consistent per-process performance
o But possibly underutilized memory
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