1. COSC 1306 COMPUTER SCIENCE AND PROGRAMMING THE FIFTH ASSIGNMENT
J.-F. Pâris

2. Our objective We want to compare the costs of
Letting the programmer decide when to bring data in main memory through reads
Having the file system manage all data transfers between the disk and the main memory

3. Why it matters
Data transfers between disk and main memory are very expensive
Each disk access takes around 7ms
Corresponds to 10.5 million CPU cycles for a 1.5 GHz processor

4. Program-directed I/O
When a program requires to access data it issues a read
Most basic form is "read so many bytes from current location in this file"
We call these reads sequential reads because we access the contents of the file in sequence
Like reading a novel—or a manga
Most programs read data a few bytes at a time
Rarely more than 40 to 50 bytes, often less

5. Example
Consider a program that reads 15 bytes then 20 bytes then 20 bytes again.
If each read requires a disk access, we will have three disk accesses.

6. How the file system manages disk I/O
File contents are stored in fixed-size blocks
Most popular block size is 4 kilobytes
4,096 bytes
When a program issues a read, the file system always bring into memory at least one full block
Subsequent reads can get their data from that block until the program has read one full block

7. Example Read sizes (bytes) Total Bytes brought in Disk Access 2000
4096
Yes
1000
3000 -- No
4000
500
4500
8192
6500
7500
8500
12288
9000

8. Your output should be
Number of read requests: 8 Number of disk accesses: 3 Number of bytes being read: 9000 Number of bytes being brought into main memory: 12288

9. Another way to look at it
Program read requests
Data actually brought in
2000 bytes
4096 bytes

10. Another way to look at it
Program read requests
Data actually brought in
2000 bytes
1000
4096 bytes

11. Another way to look at it
Program read requests
Data actually brought in
2000 bytes
1000
1000
4096 bytes

12. Another way to look at it
Program read requests
Data actually brought in
2000 bytes
1000
1000
500
4096 bytes
4096 bytes

13. Another way to look at it
Program read requests
Data actually brought in
2000 bytes
1000
1000
500
2000 bytes
4096 bytes
4096 bytes

14. Another way to look at it
Program read requests
Data actually brought in
2000 bytes
1000
1000
500
2000 bytes
1000
4096 bytes
4096 bytes

15. Another way to look at it
Program read requests
Data actually brought in
1000
2000 bytes
500
4096 bytes
4096 bytes
4096 bytes