Sean Barker

Data Storage

•Disks• Hard disk (HDD)• Solid state drive (SSD)

•Random Access Memory• Dynamic RAM (DRAM)• Static RAM (SRAM)

•Registers• %rax, %rbx, ...

1

Sean Barker

The CPU-Memory Gap

2

0.0

0.1

1.0

10.0

100.0

1,000.0

10,000.0

100,000.0

1,000,000.0

10,000,000.0

100,000,000.0

1985 1990 1995 2000 2003 2005 2010 2015

Tim

e (n

s)

Year

Disk seek time

SSD access time

DRAM access time

SRAM access time

CPU cycle time

Effective CPU cycle time

DRAM

CPU

SSD

Disk

Sean Barker

Caching

3

0 1 2 3

4 5 6 7

8 9 10 11

12 13 14 15

8 9 14 3Cache

MemoryLarger,slower,cheapermemoryviewedaspar@@onedinto“blocks”

Dataiscopiedinblock-sizedtransferunits

Smaller,faster,moreexpensivememorycachesasubsetof

theblocks

4

4

4

10

10

10

Sean Barker

Cache Hit

4

0 1 2 3

4 5 6 7

8 9 10 11

12 13 14 15

8 9 14 3Cache

Memory

Request:14

14

Sean Barker

Cache Miss

5

0 1 2 3

4 5 6 7

8 9 10 11

12 13 14 15

8 9 14 3Cache

Memory

Request:12

Request:12

12

12

12

Sean Barker

Locality

6

¢  Temporallocality:

¢  Spa0allocality:

Sean Barker

Locality Example (1)

7

sum = 0; for (i = 0; i < n; i++)

sum += a[i]; return sum;

Sean Barker

Locality Example (2)

8

int sum_array_rows(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; }

Sean Barker

Locality Example (3)

9

int sum_array_cols(int a[M][N]) { int i, j, sum = 0; for (j = 0; j < N; j++) for (i = 0; i < M; i++) sum += a[i][j]; return sum; }

Sean Barker

The Memory Hierarchy

10

The Memory Hierarchy

Local secondary storage (disk)

Larger Slower Cheaper per byte

Remote secondary storage (tapes, Web servers / Internet)

~100 M cycles to access

On Chip

Storage

Smaller Faster Costlier per byte

Main memory (DRAM)

~100 cycles to access

CPU instrs

can directly access

slower than local disk to access

Registers 1 cycle to access

Cache(s) (SRAM)

~10’s of cycles to access

Flash SSD / Local network

L1, L2