1. Lecture 35: Review for Midterm 2

2. About Midterm 2
Cover scheduling, memory management, and file systems. We do have a couple of review questions for the materials covered in the 1st midterm exam.
A single-side A4 or US letter sheet of note is allowed
50 minutes

3. Overview
Process Scheduling
Memory Management
File System

4. Process scheduling What are the goals of process scheduling?
Scheduling Algorithms for Batch Systems
First come first serve
Shortest job first
Shortest remaining time next
Scheduling Algorithms for Interactive Systems
Round-Robin scheduling
Priority scheduling
Multiple queues
Lottery scheduling

5. Memory management Memory allocation Memory de-allocation
A new process comes into memory
Memory de-allocation
A process leaves memory
Address translation
Virtual address to physical address
Process isolation
One process cannot access the memory of other processes

6. Virtual memory Paging Address translation via page table
MMU
Page fault
Speed up address translation
TLB
Large page tables
Multi-level page table
Inverted page table

7. Page Replacement Algorithms
FIFO (First-In-First-Out)
NRU (Not-Recently-Used)
Second Chance
LRU (Least-Recently-Used)
Clock Algorithm(s)
Determining which pages to replace is a decision that could have significant ramifications to system performance. If the wrong pages are replaced, i.e. pages which are referenced again fairly soon, then not only must these pages be fetched again, but a new set of pages must be tagged for removal.
The FIFO scheme has the benefit of being easy to implement, but often removes the wrong pages. Now, we can’t assume that we have perfect knowledge of a process’s reference pattern to memory, but we can assume that most processes are reasonably “well behaved.” In particular, it is usually the case that pages referenced recently will be referenced again soon, and that pages that haven’t been referenced for a while won’t be referenced for a while. This is the basis for a scheme known as LRU—least recently used. We order pages in memory by the time of their last access; the next page to be removed is the page whose last access was the farthest in the past.
A variant of this approach is LFU—least frequently used. Pages are ordered by their frequency of use, and we replace the page which has been used the least frequently of those still in primary storage.
Both these latter two policies have the advantage that they behave better for average programs than FIFO, but the disadvantage that they are costly to implement. It is straightforward to implement LRU for the Unix buffer cache, since the time to order the buffers is insignificant compared to the time required to access them. But we cannot afford to maintain pages in order of last reference.

8. Belady's anomaly
Intuitively, more page frames the memory has, the fewer page faults a program will get.
Belady 1969

9. Files
Naming
Structure
Types
Access
Attributes
Operations

10. Directories
Directory systems
Path names
Operations

11. File system implmentation
Contiguous allocation
Linked list allocation
Linked list allocation with a table in memory
I-nodes

12. UNIX V7 Directory Implementation
The steps in looking up /usr/ast/mbox

13. Hard links vs. Symbolic links
Inode 134
links=2 . 12 .. 14 f1
134 f2
Hard link
inode 134 . 12 .. 14 f1
134 f2
208
special file
data
Symbolic link
/home/gong/f1

14. Sample Questions

15. Sample Questions RR, Q = 10 P1: 4 36 8 112 1 13 16 P2: 23 40 2 2013 16 20
P1:
P2:
P3:
P4:
Which process goes next, and when?
Gantt chart
P1 idle P2 P3 P4

16. Sample Questions MLFQ: Q0 = 8, Q1 = 16, Q2 = 40 q1 45
177 13 16 20
P1:
P2: 15
P3:
P4:
Gantt chart
1 idle

17. Sample Questions Which page frame will be replaced?
Time loaded
Time referenced
R bit
M bit 60
161 1
130
160 2 26
162 3 20
163
Which page frame will be replaced?
First-In-First-Out (FIFO)
Not-Recently-Used (NRU)
Second Chance
Least-Recently-Used (LRU)

18. Sample Questions Assume file “x” content is: 0123456789876543210
fdrw = open("x", O_RDWR);
fdr = open("x", O_RDONLY);
read(fdrw, buf, 3);
read(fdr, buf2, 4);

19. Sample Questions Assume file “x” content is: 0123456789876543210
fdrw = open("x", O_RDWR);
fdrw2 = dup(fdrw);
read(fdrw, buf, 3);
read(fdrw2, buf2, 4);

20. Sample Questions Assume file “x” content is: 0123456789876543210
fdrw = open("x", O_RDWR);
Fork();
Parent: read(fdrw, buf, 3);
Child: read(fdrw, buf2, 4);