Lecture 24
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Reminders: Homework 5 due Wednesday.
Case study outline with references due
following Wednesday.
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Memory management project posted.
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Questions?
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CS 470 Operating Systems - Lecture 24
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Outline
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Review: Demand paging
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Storage management
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Page fault processing
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Page replacement
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CS 470 Operating Systems - Lecture 24
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Dynamic, Partial, Non-Contiguous Organization
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Recall: Virtual memory storage organization
provide support for the bolded design choices:
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single vs. multiple processes
complete vs. partial allocation
fixed-size vs. variable-size allocation
contiguous vs. fragmented allocation
static vs. dynamic allocation of partitions
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Review: Demand Paging
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Virtual memory (VM) commonly is implemented
using a demand paging technique. The idea
is fairly simple, only bring a logical page of a
process into memory when it is used.
As with complete allocation organizations, a
process's logical address space is loaded onto
a backing store (also called swap space) in a
contiguous manner to make loading into
memory easier. Backing store usually is a disk.
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Review: Demand Paging
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The PT is modified to have a valid/invalid bit
in each entry to indicate whether the page is in
memory.
If the entry is valid, it contains the physical
frame number as usual.
If the entry is invalid, it contains the disk
address on the backing store that holds the
page.
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Review: Address Translation
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Address translation must now handle the case
where the logical page is not in memory:
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Page number p is obtained from logical address
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If TLB hit, access memory
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If TLB miss, access PT
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If PT entry is valid, access memory
If PT entry is invalid, trap to OS and process goes
to the Wait Queue. This is called a page fault.
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Review: Address Translation
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Page fault processing consists of
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Issuing a disk transfer request to the backing store
Loading the requested page into a free frame in
physical memory
Updating the page table entry with valid bit and
frame number
OS issues an Event completion interrupt and
process goes to the Ready Queue to wait for
CPU. Then it attempts the same access that
caused the page fault.
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Review: Address Translation
log. addr.
phys. addr.
f
d
CPU
p
d
f
d
TLB
p#
f#
p
f
TLB hit
i/v
valid PT
entry
f#
main memory
p
TLB miss
update
page table
I/O completion interrupt
Monday, March 14
i/v m/f
invalid
PT entry
OS trap
page fault
page table
backing
store
CS 470 Operating Systems - Lecture 24
transfer page
from disk
to memory
8
Storage Management
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One advantage of VM is that more processes
can be run at any given time. (I.e., it increases
multi-programming.)
E.g., suppose processes each have 10 logical
pages and there are 40 physical frames. With
real (i.e., static and complete) memory
allocation, only 4 processes can be loaded. If
each process uses an average of 5 logical
pages, then can load 8 processes.
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Storage Management
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However, there is a chance that all 8 processes
will try to access all 10 of their pages at the
same time. Now need 80 physical frames, but
only have 40.
What should happen when there is a page fault
and no free frames?
Monday, March 14
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Page Replacement
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How is this done? Add to page fault handler
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Write page back to swap space
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Invalidate PT (and TLB, if any) entry
Note that now there are two disk transfers,
when there are no free frames.
Really only need to write back a page if it has
been modified. Add a dirty bit to each PT
entry to indicate if a page has been changed.
Only write back the modified frames.
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Page Replacement
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The page fault algorithm becomes
1. Find location of page p on swap disk
2. Find a free frame:
a. If there is a free frame, use it
b. If there is no free frame, use a page replacement
algorithm to select a victim frame
c. If the victim frame is dirty, write to disk
d. Change the PT of victim process accordingly
3. Read page p from swap disk into newly freed
frame; change PT of this process accordingly
4. Restart user process
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Replacement Algorithms
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There are many types of replacement
algorithms. Criteria for choosing includes:
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Low page fault rate
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Efficient in choosing victim frame
As with CPU scheduling, evaluate by simulating
on scenario data and comparing. For VM page
replacement, data is a string of logical page
references. Also need to know number of
physical frames available.
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Replacement Algorithms
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Generally, we expect that more physical frames
leads to fewer page faults. E.g., if have only
one physical frame, then nearly every memory
reference causes a fault.
Vice versa, if have as many physical frames as
logical pages, then only the first use of each
page causes a fault.
For most of the examples, will use 3 physical
frames.
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FIFO Replacement
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As usual, simplest algorithm is FIFO (first in,
first out). Associate a time with each frame.
Victim frame is the oldest one. Can be
implemented using a basic queue.
7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
7 7 7 2 2 2 2 4 4 4 0
0 0 0 0 3 3 3 2 2 2
1 1 1 1 0 0 0 3 3
*
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*
*
*
*
*
*
*
*
*
Easy to understand, but doesn't always give
th
good performance. E.g., 5 fault (page 3).
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Belady's Anomaly
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Consider following reference string with 3
physical frames that results in 9 faults:
1
2
3
4
1
2
5
1
1
1
4
4
4
2
2
2
1
3
3
3
1
2
3
4
5
5
5
1
1
3
3
2
2
2
4
5
Do it again with 4 frames; results in 10 faults!
1
2
3
4
1
1
1
1
2
2
2
3
3
1
2
5
1
2
3
4
5
4
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OPT Replacement
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OPT is the optimal algorithm - replace the page
that will be used farthest into the future.
7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
7 7 7 2
2
2
0 0 0
0
4
1 1
3
3
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OPT Replacement
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OPT is provably optimal for a finite reference
string.
Of course, generally do not know the entire
reference string, but it is good to know a
theoretical minimum so we can say things like
"at worst within 12% of optimal" or "4.7% of
optimal on average".
OPT is unimplementable in real systems, so
need to approximate. FIFO is not good.
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LRU Replacement
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LRU chooses the "least recently used" page as
the victim page. The theory is to use recent
past usage as a predictor of new future usage.
It replaces the page that has not been used
for the longest time.
7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1
7 7 7 2
2
4
0 0 0
0
0
1 1
3
3
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LRU Replacement
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The main problem for LRU is how to implement
it. There are a couple of feasible
implementations
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Counters: use CPU time counter and store it into
the PT on each reference. Replace the lowest
numbered entry. Need to search PT. Need to deal
with context switches and counter rollover
"Stack": keep a "stack" of page references. When a
page is referenced, move it to the top of stack.
Victim is bottom of stack. Since want to remove
from middle, usually use doubly-linked list.
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