administrivia talking about caches today including another in class worksheet

Transcription

1 administrivia talking about caches today including another in class worksheet 1

2 memory hierarchy working on memory hierarchy now ranges from small and fast (registers) through big and almost fast (RAM) down to enormous and slow (disks) 2

3 Caches Cache goes between CPU and main memory 0.33ns CPU 70 ns RAM 3

4 Caches Cache goes between CPU and main memory 0.33ns CPU 70 ns RAM 3

5 Caches Cache goes between CPU and main memory 0.33ns CPU 5 ns cache 70 ns RAM 3

6 Caches data is held in main memory some data is more frequently used RAM A B C D E F G H I J K 4

7 Caches data is held in main memory some data is more frequently used Cache holds copy of some data from DRAM Cache A C D H J RAM A B C D E F G H I J K 4

8 Cache Hit If data looked for is in cache, called cache hit Cache A C D H J RAM A B C D E F G H I J K 5

9 Cache Miss If data looked for is not in cache, called cache miss Cache A C D H J RAM A B C D E F G H I J K 6

10 Cache line Cache is arranged as rows, each with address of first byte data starting with first byte data is called cache line ranges in size from 16 bytes to 256 bytes address always multiple of cache line size for 16 byte size, see 0, 16, 32, 48, 64,... address usually given in hex (0,10,20,30,40,...) 7

11 Cache example ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 8

12 finding data to read data from cache: 1) find cache line in cache (if its there) 2) find byte(s) in cache line 3) return bytes 9

13 finding ) find cache line in cache ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 10

14 finding ) find cache line in cache ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 10

15 finding data ) find byte(s) in cache line ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 11

16 finding data ) find byte(s) in cache line ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 11

17 Cache Miss If data is not in cache (cache miss) 1) choose line to replace 2) read in line from RAM (or lower cache) replacing chosen line 3) read data as before 12

18 Cache Miss 2000 line is not in cache ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 13

19 Cache Miss ) choose line to replace ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 14

20 Cache Miss ) read in line from RAM (or lower cache) ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 15

21 Cache Miss ) read in line from RAM (or lower cache) ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 15

22 Cache Miss ) read bytes from line as before A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 16

23 Fully Associative some caches allow any line to be anywhere must check all to find it can choose any line to replace called fully associative cache or set associative cache usually choose oldest line to replace longest since accessed least recently accessed 17

24 Direct Cache Direct cache only allows one place for any line usually driven off of address For address 21B4 assuming 16 byte cache line and 16 total lines in the cache 18

25 Direct Cache Direct cache only allows one place for any line usually driven off of address For address 21B4 4 is the byte within the cache line last digit gives 16 choices (0-F) for 16 bytes 19

26 Direct Cache Direct cache only allows one place for any line usually driven off of address For address 21B4 B (11) is which cache line gives 16 possible lines 2 digits would support 256 lines (1B in this case) 20

27 Direct Cache Direct cache only allows one place for any line usually driven off of address For address 21B4 21 become the tag (all the remaining parts) only data actually needed in address column my examples will use full address not tag 21

28 Direct Cache Example 3544 can only be one place ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 22

29 Direct Cache Example 3544 can only be one place ABCDEF FEDCBA A AAAAAAAA BBBBBBBB ABABABAB FFFFFFFF AABB CCDDEEFF 22

30 An example Lets try to work an example in groups 23

31 Cache hit rate Hit rate is % of mem accesses found in cache need good hit rate for computer to run well Improved by bigger cache smaller cache line fully associative 24

32 cache line size big cache line easier to find (fewer to look through) slower to read in from memory often brings in memory unwanted hurts cache hit rate small cache line harder to find (more to look through) faster to read in from memory bring in just what s needed improves cache hit rate 25