Virtual Memory
Page Replacement Concepts
Welcome!
Generic Cache Memory Access Time
Paging
In Parallel
How long does it take to access?
No
Line in
Cache?
(Miss)
Load Line
(Some Cycles)
Yes
(Hit)
Get Line
(few cycles)
!
There is something
strange with this
formula.
Memory Access Time
(1-P CM)*T C+P CM*(T MM)
T C:
T MM :
P CM :
1-P CM :
Time to access cache
Time to access main memory
Probably of cache miss
Probably of cache hit
TLB
How much time does it take?
EAT(effective access time) =
P x hit memory time + (1-P) x miss memory time
•
Consider that the page is in main memory, not on disk.
•
TLB hit rate: 80%
•
TLB read time: 20 nanoseconds
•
RAM read time: 100 nanoseconds
EAT(effective access time) =
0.80 x (20read TLB + 100read page from ram) +
0.20 x (100 bring TLB entry from ram + 20read TLB + 100read page from ram)
= 140 nanoseconds
(The Pennsylvania State University, 2014)
Page Replacement
But what if the main memory
is full?
Paging Replacement
Tracking Page Data
int main()
{
char chunkA[3072];
char chunkB[2048];
char* chunkC = new char[2048];
chunkB[42] = 9001;
chunkA[0] = 42;
chunkC[1] = chunkA[0];
}
Reference
String
0,7 ,6 ,2 ,6 ,7 ,7 ,2
We can remove
consecutive
references (as they
are redundant) to
get a reduced
reference string.
[7]
[6]
Stack
28-32KiB
24-28KiB
[5]
20-24KiB
[4]
16-20KiB
[3]
12-16KiB
[2]
Heap
8-12KiB
[1]
Data
4-8KiB
[0]
Text
0-4KiB
0,7 ,6 ,2 ,7 ,7 ,6 ,2
0,7 ,6 ,2 ,7 ,6 ,2
Paging Replacement Algorithms
Green: Page Fault
Optimal (Read: Magic)
Blue: Already in Memory
•
A hypothetical optimal solution shows us the best possible page fault rate (lowest page fault rate).
Page Frames
Reduced Reference String
0
1
2
0
2
3
1
0
4
1
2
0
3
0
3
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
-
1
1
1
1
1
1
1
1
1
1
1
1
1
1
-
-
2
2
2
3
3
3
4
4
2
2
3
3
3
Time
•
The Page Fault Rate is 7/15 (46.7%).
Green: Page Fault
Paging Replacement Algorithms
Blue: Already in Memory
First-In-First-Out (FIFO)
Red: Oldest Page
•
With First-In-First-Out (FIFO) page replacement, the oldest page is evicted first.
Page Frames
Reduced Reference String
0
1
2
0
2
3
1
0
4
1
2
0
3
0
3
0
0
0
0
0
3
3
3
3
1
1
1
3
3
3
-
1
1
1
1
1
1
0
0
0
2
2
2
2
2
-
-
2
2
2
2
2
2
4
4
4
0
0
0
0
Time
•
The Page Fault Rate is 10/15 (66.7%).
!
For FIFO, even if we increase the number of frames,
there are always situations where the fault rate
increases. This is called the Bélády’s anomaly.
Green: Page Fault
Paging Replacement Algorithms
Blue: Already in Memory
Least Recently Used (LRU)
Red: Oldest Page
•
The Least Recently Used (LRU) algorithm evicts the most “stale” page.
Page Frames
Reduced Reference String
0
1
2
0
2
3
1
0
4
1
2
0
3
0
3
0
0
0
0
0
1
1
1
1
1
1
1
3
3
3
-
1
1
1
1
3
3
3
4
4
4
0
0
0
0
-
-
2
2
2
2
2
0
0
0
2
2
2
2
2
Time
•
The Page Fault Rate is 10/15 (66.7%).
!
LRU is superior because it does not suffer weird edge cases
like FIFO (Bélády’s anomaly). However, it’s impractical /
inefficient (requires in-place list of modifications).
Thank You
References
• The Pennsylvania State University. (2014). Diagram of TLB [Online Image]. The Pennsylvania State
University. https://www.cse.psu.edu/~deh25/cmpsc473/notes/OSC/MemoryManagement.html