Cache Miss Rate Computations - PowerPoint PPT Presentation

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Cache Miss Rate Computations

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Assumed Simple Cache 2 ints per block 2-way set associative 2 blocks total 1 set i.e., ... Do I re-use blocks over time? In what order am I accessing blocks? – PowerPoint PPT presentation

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Title: Cache Miss Rate Computations


1
Cache Miss Rate Computations
  • Topics
  • Simple examples

2
Assumed Simple Cache
Cache


Block 0
Block 1
  • 2 ints per block
  • 2-way set associative
  • 2 blocks total
  • 1 set
  • i.e., same thing as fully associative
  • Replacement policy Least Recently Used (LRU)

3
Array Access Key Questions
  • How many array elements are there per block?
  • Does the data structure fit in the cache?
  • Do I re-use blocks over time?
  • In what order am I accessing blocks?

4
Simple Array
Cache
for (i0iltNi) Ai


1 2 3 4
A
Miss rate misses / accesses
5
Simple Array
Cache
for (i0iltNi) Ai


1 2 3 4
A
Miss rate misses / accesses
(N//2) / N ½ 50
6
Simple Array w outer loop
Cache
for (k0kltPk) for (i0iltNi)
Ai


1 2 3 4
A
Assume A fits in the cache Miss rate
misses / accesses
7
Simple Array w outer loop
Cache
for (k0kltPk) for (i0iltNi)
Ai


1 2 3 4
A
Assume A fits in the cache First Visit Miss
rate misses / accesses
(N//2) / N ½ 50
8
Simple Array w outer loop
Cache
for (k0kltPk) for (i0iltNi)
Ai


1 2 3 4
A
Assume A fits in the cache Miss rate
misses / accesses ? 0 if P ? infinity
Lesson for sequential accesses with re-use, If
fits in the cache, first visit suffers all the
misses
9
Simple Array
Cache
for (i0iltNi) Ai


1 2 3 4 5 6 7 8
A
Assume A does not fit in the cache Miss rate
misses / accesses
10
Simple Array
Cache
for (i0iltNi) Ai
5 6
7 8
1 2 3 4 5 6 7 8
A
Assume A does not fit in the cache Miss rate
misses / accesses (N/2) / N ½ 50
Lesson for sequential accesses, if no-reuse it
doesnt matter whether data structure fits
11
Simple Array with outer loop
Cache
for (k0kltPk) for (i0iltNi)
Ai


1 2 3 4 5 6 7 8
A
Assume A does not fit in the cache Miss rate
misses / accesses
(N/2) / N ½ 50
Lesson for sequential accesses with re-use, If
the data structure doesnt fit, same miss rate as
no-reuse
12
2D array
Cache
for (i0iltNi) for (j0jltNj)
Aij
A


1 2
3 4
Assume A fits in the cache Miss rate
misses / accesses
(NN/2) / (NN) ½ 50
13
2D array
Cache
A
for (i0iltNi) for (j0jltNj)
Aij
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16


Assume A does not fit in the cache Miss rate
misses / accesses
50
Lesson for 2D accesses, if row order and
no-reuse, same hit rate as sequential, whether
fits or not
14
2D array
Cache
for (j0jltNj) for (i0iltNi)
Aij
A


1 2
3 4
Assume A fits in the cache Miss rate
misses / accesses (NN/2) / NN ½ 50
Lesson for 2D accesses, if column order and
no-reuse, same hit rate as sequential if entire
column fits in the cache
15
2D array
Cache
A
for (j0jltNj) for (i0iltNi)
Aij
1 2 3 4
5 6 7 8
9 10 11 12
13 14 15 16


Assume A does not fit in the cache Miss rate
misses / accesses
100
Lesson for 2D accesses, if column order, if
entire column doesnt fit, then 100 miss rate
(block (1,2) is gone after access to element 9).
16
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