Memory caching

2
Outline
• General concepts
• 3 ways to organize cache memory
• Issues with writes
• Write cache friendly codes
• Cache mountain
• Suggested Reading: 6.4, 6.5, 6.6

4
Cache Memory
• History
– At very beginning, 3 levels
• Registers, main memory, disk storage
– 10 years later, 4 levels
• Register, SRAM cache, main DRAM memory, disk storage
– Modern processor, 4~5 levels
• Registers, SRAM L1, L2(,L3) cache, main DRAM memory,
disk storage
– Cache memories
• are small, fast SRAM-based memories
• are managed by hardware automatically
• can be on-chip, on-die, off-chip

5
Cache Memory
Figure 6.24 P488
main
memory
I/O
bridge
bus interfaceL2 cache
ALU
register file
CPU chip
cache bus system bus memory bus
L1
cache

6
Cache Memory
• L1 cache is on-chip
• L2 cache is off-chip several years ago
• L3 cache can be off-chip or on-chip
• CPU looks first for data in L1, then in L2, then
in main memory
– Hold frequently accessed blocks of main memory
are in caches

7
Inserting an L1 cache between the CPU and
main memory
a b c dblock 10
p q r sblock 21
...
...
w x y zblock 30
...
The big slow main memory
has room for many 4-word
blocks.
The small fast L1 cache has room
for two 4-word blocks.
The tiny, very fast CPU register file
has room for four 4-byte words.
The transfer unit between
the cache and main
memory is a 4-word block
(16 bytes).
The transfer unit between
the CPU register file and
the cache is a 4-byte block.
line 0
line 1

8
6.4.1 Generic Cache Memory Organization
Figure 6.25 P488
• • • B–110
• • • B–110
valid
valid
tag
tag
set 0:
B = 2b
bytes
per cache block
E lines
per set
S = 2s
sets
t tag bits
per line
1 valid bit
per line
• • •
• • • B–110
• • • B–110
valid
valid
tag
tag
set 1: • • •
• • • B–110
• • • B–110
valid
valid
tag
tag
set S-1: • • •
• • •
Cache is an array
of sets.
Each set contains
one or more lines.
Each line holds a
block of data.
pp.488

9
Addressing caches
Figure 6.25 P488
t bits s bits b bits
0m-1
<tag> <set index><block offset>
Address A:
• • •B–110
• • •B–110
v
v
tag
tag
set 0: • • •
• • •B–110
• • •B–110
v
v
tag
tag
set 1: • • •
• • •B–110
• • •B–110
v
v
tag
tag
set S-1: • • •
• • •
The word at address A is in the cache if
the tag bits in one of the <valid> lines in
set <set index> match <tag>.
The word contents begin at offset
<block offset> bytes from the beginning
of the block.

10
Cache Memory
Fundamental parameters
Parameters Descriptions
S = 2s
E
B=2b
m=log2(M)
Number of sets
Number of lines per set
Block size(bytes)
Number of physical(main memory)
address bits

11
Cache Memory
Derived quantities
Parameters Descriptions
M=2m
s=log2(S)
b=log2(B)
t=m-(s+b)
C=B×E ×S
Maximum number of unique memory address
Number of set index bits
Number of block offset bits
Number of tag bits
Cache size (bytes) not including overhead
such as the valid and tag bits

12
6.4.2 Direct-mapped cache
Figure 6.27 P490
• Simplest kind of cache
• Characterized by exactly one line per set.
valid
valid
valid
tag
tag
tag
• • •
set 0:
set 1:
set S-1:
E=1 lines per setcache block
cache block
cache block

13
Accessing direct-mapped caches
Figure 6.28 P491
• Set selection
– Use the set index bits to determine the set of
interest
valid
valid
valid
tag
tag
tag
• • •
set 0:
set 1:
set S-1:
t bits s bits
0 0 0 0 1
0m-1
b bits
tag set indexblock offset
selected set
cache block
cache block
cache block

14
• Line matching and word extraction
– find a valid line in the selected set with a matching
tag (line matching)
– then extract the word (word selection)

15
Figure 6.29 P491
1
t bits s bits
100i0110
0m-1
b bits
tag set index block offset
selected set (i):
=1?
= ?
(3) If (1) and (2), then
cache hit,
and block offset
selects
starting byte.
(1) The valid bit must be set
(2) The tag bits in the cache
line must match the
tag bits in the address
0110 w3w0 w1 w2
30 1 2 74 5 6

16
Line Replacement on Misses in Directed Caches
• If cache misses
– Retrieve the requested block from the next level in
the memory hierarchy
– Store the new block in one of the cache lines of
the set indicated by the set index bits

17
Line Replacement on Misses in Directed Caches
• If the set is full of valid cache lines
– One of the existing lines must be evicted
• For a direct-mapped caches
– Each set contains only one line
– Current line is replaced by the newly fetched line

18
Direct-mapped cache simulation P492
• M=16 byte addresses
• B=2 bytes/block, S=4 sets, E=1 entry/set

19
Direct-mapped cache simulation P493
1 0 m[1] m[0]
v tag data
1 1 m[13] m[12]
0 [0000] (miss)
(4)
1 1 m[9] m[8]
v tag data
1 1 m[13] m[12]
8 [1000] (miss)
(3)
1 0 m[1] m[0]
v tag data
1 1 m[13] m[12]
13 [1101] (miss)
(2)
1 0 m[1] m[0]
v tag data
0 [0000] (miss)
(1)
M=16 byte addresses, B=2 bytes/block, S=4 sets, E=1
entry/set
Address trace (reads):
0 [0000] 1 [0001] 13 [1101] 8 [1000] 0 [0000]
x
t=1 s=2 b=1
xx x

20
Direct-mapped cache simulation
Figure 6.30 P493
Address bits
Address
(decimal)
Tag bits
(t=1)
Index bits
(s=2)
Offset bits
(b=1)
Block number
(decimal)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
00
00
01
01
10
10
11
11
00
00
01
01
10
10
11
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
0
0
1
1
2
2
3
3
4
4
5
5
6
6
7

21
Why use middle bits as index?
• High-Order Bit Indexing
– Adjacent memory lines would
map to same cache entry
– Poor use of spatial locality
• Middle-Order Bit Indexing
– Consecutive memory lines
map to different cache lines
– Can hold C-byte region of
address space in cache at one
time
4-line Cache High-Order
Bit Indexing
Middle-Order
Bit Indexing
00
01
10
11
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
Figure 6.31 P497

22
6.4.3 Set associative caches
• Characterized by more than one line per set
valid tag
set 0: E=2 lines per set
set 1:
set S-1:
• • •
cache block
valid tag cache block
Figure 6.32 P498

23
Accessing set associative caches
• Set selection
– identical to direct-mapped cache
valid
valid
tag
tag
set 0:
valid
valid
tag
tag
set 1:
valid
valid
tag
tag
set S-1:
• • •
t bits s bits
0 0 0 0 1
0m-1
b bits
Selected set
cache block
cache block
cache block
cache block
cache block
cache block
Figure 6.33 P498

24
• Line matching and word selection
– must compare the tag in each valid line in the
selected set.
(3) If (1) and (2), then
cache hit, and
block offset selects
starting byte.
1 0110 w3w0 w1 w2
1 1001
t bits s bits
100i0110
0m-1
b bits
selected set (i):
=1?
= ?
(2) The tag bits in one
of the cache lines must
match the tag bits in
the address
(1) The valid bit must be set.
30 1 2 74 5 6
Figure 6.34 P499

25
6.4.4 Fully associative caches
• Characterized by all of the lines in the only
one set
• No set index bits in the address
set 0:
valid
valid
tag
tag
cache block
cache block
…
E=C/B lines in
the one and only set
t bits b bits
tag block offset
Figure 6.36 P500
Figure 6.35 P500

26
• Word selection
– must compare the tag in each valid line
0 0110
w3w0 w1 w2
1 1001
t bits
1000110
0m-1
b bits
tag block offset
=1?
= ?
(3) If (1) and (2), then
cache hit, and
block offset selects
starting byte.
(2) The tag bits in one
of the cache lines must
match the tag bits in
the address
(1) The valid bit must be set.
30 1 2 74 5 6
1
0
0110
1110
Figure 6.37 P500

27
6.4.5 Issues with Writes
• Write hits
– 1) Write through
• Cache updates its copy
• Immediately writes the corresponding cache block to
memory
– 2) Write back
• Defers the memory update as long as possible
• Writing the updated block to memory only when it is
evicted from the cache
• Maintains a dirty bit for each cache line

28
Issues with Writes
• Write misses
– 1) Write-allocate
• Loads the corresponding memory block into the cache
• Then updates the cache block
– 2) No-write-allocate
• Bypasses the cache
• Writes the word directly to memory
• Combination
– Write through, no-write-allocate
– Write back, write-allocate

29
6.4.6 Multi-level caches
size:
speed:
$/Mbyte:
line size:
8-64 KB
3 ns
32 B
128 MB DRAM
60 ns
$1.50/MB
8 KB
30 GB
8 ms
$0.05/MB
larger, slower, cheaper
MemoryMemory diskdisk
TLB
L1 I-cache
L1 D-cacheregs
L2
Cache
Processor
1-4MB SRAM
6 ns
$100/MB
32 B
larger line size, higher associativity, more likely to write back
Options: separate data and instruction caches, or a unified cache
Figure 6.38 P504

30
6.4.7 Cache performance metrics P505
• Miss Rate
– fraction of memory references not found in cache
(misses/references)
– Typical numbers:
3-10% for L1
• Hit Rate
– fraction of memory references found in cache (1 -
miss rate)

31
Cache performance metrics
• Hit Time
– time to deliver a line in the cache to the processor
(includes time to determine whether the line is in
the cache)
– Typical numbers:
1-2 clock cycle for L1
5-10 clock cycles for L2
• Miss Penalty
– additional time required because of a miss
• Typically 25-100 cycles for main memory

32
Cache performance metrics P505
• 1> Cache size
– Hit rate vs. hit time
• 2> Block size
– Spatial locality vs. temporal locality
• 3> Associativity
– Thrashing
– Cost
– Speed
– Miss penalty
• 4> Write strategy
– Simple, read misses, fewer transfer

33
6.5 Writing Cache-Friendly Code

34
Writing Cache-Friendly Code
• Principles
– Programs with better locality will tend to have
lower miss rates
– Programs with lower miss rates will tend to run
faster than programs with higher miss rates

35
Writing Cache-Friendly Code
• Basic approach
– Make the common case go fast
• Programs often spend most of their time in a few core
functions.
• These functions often spend most of their time in a few
loops
– Minimize the number of cache misses in each inner
loop
• All things being equal

36
Writing Cache-Friendly Code P507
8[h]7[h]6[h]5[m]4[h]3[h]2[h]1[m]Access order,
[h]it or [m]iss
i= 7i= 6i= 5i= 4i= 3i= 2i= 1i=0v[i]
Temporal locality,
These variables are usually put in registersint sumvec(int v[N])
{
int i, sum = 0 ;
for (i = 0 ; i < N ; i++)
sum += v[i] ;
return sum ;
}

37
Writing cache-friendly code
• Temporal locality
– Repeated references to local variables are good
because the compiler can cache them in the
register file

38
• Spatial locality
– Stride-1 references patterns are good because
caches at all levels of the memory hierarchy store
data as contiguous blocks
• Spatial locality is especially important in
programs that operate on multidimensional
arrays

39
Writing cache-friendly code P508
• Example (M=4, N=8, 10cycles/iter)
int sumvec(int a[M][N])
{
int i, j, sum = 0 ;
for (i = 0 ; i < M ; i++)
for ( j = 0 ; j < N ; j++ )
sum += a[i][j] ;
return sum ;
}

40
a[i][j] j=0 j= 1 j= 2 j= 3 j= 4 j= 5 j= 6 j= 7
i=0
i=1
i=2
i=3
1[m]
9[m]
17[m]
25[m]
2[h]
10[h]
18[h]
26[h]
3[h]
11[h]
19[h]
27[h]
4[h]
12[h]
20[h]
28[h]
5[m]
13[m]
21[m]
29[m]
6[h]
14[h]
22[h]
30[h]
7[h]
15[h]
23[h]
31[h]
8[h]
16[h]
24[h]
32[h]

41
Writing cache-friendly code P508
• Example (M=4, N=8, 20cycles/iter)
int sumvec(int v[M][N])
{
int i, j, sum = 0 ;
for ( j = 0 ; j < N ; j++ )
for ( i = 0 ; i < M ; i++ )
sum += v[i][j] ;
return sum ;
}

42
a[i][j] j=0 j= 1 j= 2 j= 3 j= 4 j= 5 j= 6 j= 7
i=0
i=1
i=2
i=3
1[m]
2[m]
3[m]
4[m]
5[m]
6[m]
7[m]
8[m]
9[m]
10[m]
11[m]
12[m]
13[m]
14[m]
15[m]
16[m]
17[m]
18[m]
19[m]
20[m]
21[m]
22[m]
23[m]
24[m]
25[m]
26[m]
27[m]
28[m]
29[m]
30[m]
31[m]
32[m]

43
6.6 Putting it Together: The Impact of
Caches on Program Performance
6.6.1 The Memory Mountain

44
The Memory Mountain P512
• Read throughput (read bandwidth)
– The rate that a program reads data from the
memory system
• Memory mountain
– A two-dimensional function of read bandwidth
versus temporal and spatial locality
– Characterizes the capabilities of the memory
system for each computer

45
Memory mountain main routine
Figure 6.41 P513
/* mountain.c - Generate the memory mountain. */
#define MINBYTES (1 << 10) /* Working set size ranges from 1 KB */
#define MAXBYTES (1 << 23) /* ... up to 8 MB */
#define MAXSTRIDE 16 /* Strides range from 1 to 16 */
#define MAXELEMS MAXBYTES/sizeof(int)
int data[MAXELEMS]; /* The array we'll be traversing */

46
int main()
{
int size; /* Working set size (in bytes) */
int stride; /* Stride (in array elements) */
double Mhz; /* Clock frequency */
init_data(data, MAXELEMS); /* Initialize each element in data to 1 */
Mhz = mhz(0); /* Estimate the clock frequency */

47
for (size = MAXBYTES; size >= MINBYTES; size >>= 1) {
for (stride = 1; stride <= MAXSTRIDE; stride++)
printf("%.1ft", run(size, stride, Mhz));
printf("n");
}
exit(0);
}

48
Memory mountain test function
Figure 6.40 P512
/* The test function */
void test (int elems, int stride) {
int i, result = 0;
volatile int sink;
for (i = 0; i < elems; i += stride)
result += data[i];
sink = result; /* So compiler doesn't optimize away the loop */
}

49
Memory mountain test function
/* Run test (elems, stride) and return read throughput (MB/s) */
double run (int size, int stride, double Mhz)
{
double cycles;
int elems = size / sizeof(int);
test (elems, stride); /* warm up the cache */
cycles = fcyc2(test, elems, stride, 0); /* call test (elems,stride) */
return (size / stride) / (cycles / Mhz); /* convert cycles to MB/s */
}

50
The Memory Mountain
• Data
– Size
• MAXBYTES(8M) bytes or MAXELEMS(2M) words
– Partially accessed
• Working set: from 8MB to 1KB
• Stride: from 1 to 16

51
The Memory Mountain
Figure 6.42 P514
s1
s3
s5
s7
s9
s11
s13
s15
8m
2m
512k
128k
32k
8k
2k
0
200
400
600
800
1000
1200
readthroughput(MB/s)
stride (words) working set size (bytes)
Pentium III Xeon
550 MHz
16 KB on-chip L1 d-cache
16 KB on-chip L1 i-cache
512 KB off-chip unified
L2 cache
Ridges of
Temporal
Locality
L1
L2
mem
Slopes of
Spatial
Locality
xe

52
Ridges of temporal locality
• Slice through the memory mountain with
stride=1
– illuminates read throughputs of different caches
and memory
Ridges: 山脊

53
Ridges of temporal locality
Figure 6.43 P515
0
200
400
600
800
1000
1200 8m
4m
2m
1024k
512k
256k
128k
64k
32k
16k
8k
4k
2k
1k
working set size (bytes)
readthrougput(MB/s)
L1 cache
region
L2 cache
region
main memory
region

54
A slope of spatial locality
• Slice through memory mountain with
size=256KB
– shows cache block size.

55
A slope of spatial locality
Figure 6.44 P516
0
100
200
300
400
500
600
700
800
s1 s2 s3 s4 s5 s6 s7 s8 s9 s10 s11 s12 s13 s14 s15 s16
stride (words)
readthroughput(MB/s)
one access per cache line

Memory caching

Recommended

Recommended

More Related Content

What's hot

What's hot (6)

Viewers also liked

Viewers also liked (20)

Similar to Memory caching

Similar to Memory caching (20)

More from Young Alista

More from Young Alista (20)

Recently uploaded

Recently uploaded (20)

Memory caching