ITEC 325
Lecture 28
Memory(5)
Review
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P2 coming on Friday
Exam 2 next Friday
Operating system / processes
Logical address / physical address
Pages / frames
Main memory  HD
Memory (5)
Demand
Paging
• Some programs hog memory resources: they
are huge and eat up too much physical
memory.
• Most systems attack this problem by only bringing into the
physical memory portions of the process that are actually
needed.
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In other words, some pages in the process are not loaded
into the physical memory – as long as they are not needed.
This is called demand paging.
Memory (5)
Page Table
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The page table maps between virtual memory and physical
memory.
Memory (5)
Address
translation
– Closer look at translating a virtual address into a physical
address:
Contains a
present
bit
Memory (5)
Demand Paging: only bring in the pages that are needed (e.g., page 7 for program A is not needed).
© Silberschatz and Galvin
Page Table When Some Pages Are Not in Main Memory
© Silberschatz and Galvin
Accessing when
not in memory
• If there is a reference to a page, first reference to that
page will trap to operating system:
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page fault
 Operating system looks at another table to decide:
– Invalid reference  abort
– Just not in memory
 Get empty frame – what if no empty
frame found?
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Swap page into frame
Reset tables
Set present bit = 1
Restart the instruction that caused the page fault
Memory (5)
Steps in Handling a Page Fault
© Silberschatz and Galvin
Performanc
e
• Page Fault Rate 0  p  1.0
– if p = 0 no page faults
– if p = 1, every reference is a fault
Effective Access Time (EAT)
EAT = (1 – p) x memory access
+ p (page fault overhead
+ swap page out
+ swap page in
+ restart overhead
Memory (5)
Example
• Memory access time = 200 nanoseconds
• Average page-fault service time = 8 milliseconds
= 8000000 nano seconds
• EAT = (1 – p) x 200 + p (8 milliseconds)
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= (1 – p) x 200 + p x 8,000,000
• If no page fault (p = 0), then EAT = 200 nano seconds
• If one access out of 1,000 causes a page fault, then
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EAT = 8.2 microseconds. = 8200 nanoseconds
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This is a slowdown by a factor of 41!!
• Hence, we need to reduce the number of page
faults.
Memory (5)
Performanc
e of paging
• Two factors effect performance:
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(a) Number of page faults:
(b) Fast access to the page table.
Next: Decreasing the number of page faults.
Memory (5)
Decreasing
page faults.
• To reduce the number of page faults, a key
factors is what pages to replace when a
page fault occurs.
– Two common algorithms:
• Least recently used
• First in First out.
• Optimal page replacement – replace the page that
will not be used for the longest period of time.
Memory (5)
Example
• When running a process, its virtual memory was divided
into 9 pages. However, there is only free space for 4
pages in the physical memory.
• Here is the order in which the pages are being accessed
as the process executes:
– 7, 0, 1, 2, 0, 3, 0, 4, 2, 3, 0, 3, 2, 1, 2, 0, 1, 7, 0, 1
– How many page faults occur with the:
• Optimal page replacement algorithm?
• Least Recently Used?
– Exercise in class….
Memory (5)
© Silberschatz
and Galvin
Faster
implementation
• Use a separate cache (other than L1, L2) to store a part of the
“page table”.
– Called Translation
look aside buffer (TLB):
• TLB is
– Hardware implementing associative memory
• Associative memory: similar to a hash table.
• Working of a TLB:
– First use page number to index into TLB,
• If page number found (TLB hit), read associated frame number
– This is faster than memory lookup since TLB is hardware implemented.
• If page number not found (TLB miss), go to main memory
Memory (5)
Paging Hardware With TLB
© Silberschatz
and Galvin
Advantage
of TLB
• When does TLB offer an advantage
– When most of the page numbers the process refers to
is in the TLB.
– Determined by what is called as hit ratio
– 0 <= hit ratio <= 1
– If hit ratio higher  greater chance of finding
page in TLB
– Example: Let memory access time be 100ns. Let TLB
access time be 20ns. Hit ratio: 0.8.
– What is the overhead of a lookup ?
Memory (5)
Thrashing
• Consider this scenario:
– The CPU is idle, lots of processes are waiting to execute.
– The OS (to increase multiprocessing) loads the processes into the pages.
– Now let us say, a process starts using more memory.
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Then it demands free frames in main memory.
What if there are no free frames?
Solution: steal from other process.
What if the other process needs more frames?
– Processes start page faulting – this causes the OS to think
that the CPU is idle and it loads even more processes
causes more thrashing.
– Example: the .NET runtime optimization service.
© Silberschatz
and Galvin
Memory (5)
Putting it All
Together
• An example TLB holds 8 entries for a system with 32 virtual pages and 16 page frames.
Memory (5)
Class exercise.
• Consider a memory architecture that contains a TLB, an L1
cache, an L2 cache and main memory.
• 30%: hit rate of TLB.
• 90%: hit rate of L1 cache
• 95%: hit rate of L2 cache
• 10% page fault rate.
• Also accessing
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TLB: 10 ns
L1 cache: 15 ns
L2 cache: 100 ns
Memory: 100 milliseconds.
Time to process a pagefault = 200 milliseconds
• What is the effective memory access time? (i.e., for any
logical address that the CPU requests, how long does it
take on average to read the data at that address?
Memory (5)
Summary
• Paging
• Faults
• TLB
Memory (5)