1. COT 4600 Operating Systems Spring 2011
Dan C. Marinescu
Office: HEC 304
Office hours: Tu-Th 5:00-6:00 PM

2. Lecture 21 - Thursday April 7, 2011
Last time:
Scheduling
Today:
The scheduler
Multi-level memories
Next Time:
Memory characterization
Multilevel memories management using virtual memory
Adding multi-level memory management to virtual memory
Page replacement algorithms
Lecture 21

3. The scheduler
The system component which manages the allocation of the processor/core.
It runs inside the processor thread and implements the scheduling policies.
Other functions
Determines the burst
Manages multiple queues of threads
Lecture 20

4. CPU burst
CPU burst  the time required by the thread/process to execute
Lecture 20

5. Estimating the length of next CPU burst
Done using the length of previous CPU bursts, using exponential averaging
Lecture 20 5

6. Exponential averaging
 =0
n+1 = n
Recent history does not count
 =1
n+1 =  tn
Only the actual last CPU burst counts
If we expand the formula, we get:
n+1 =  tn+(1 - ) tn -1 + …
+(1 -  )j  tn -j + …
+(1 -  )n +1 0
Since both  and (1 - ) are less than or equal to 1, each successive term has less weight than its predecessor
Lecture 20 6

7. Predicting the length of the next CPU burst
Lecture 20 7

8. Multilevel queue
Ready queue is partitioned into separate queues each with its own scheduling algorithm :
foreground (interactive)  RR
background (batch)  FCFS
Scheduling between the queues
Fixed priority scheduling - (i.e., serve all from foreground then from background). Possibility of starvation.
Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e.,
80% to foreground in RR
20% to background in FCFS
Lecture 20 8

9. Multilevel queue scheduling
Lecture 20 9

10. Multilevel feedback queue
A process can move between the various queues; aging can be implemented this way
Multilevel-feedback-queue scheduler characterized by:
number of queues
scheduling algorithms for each queue
strategy when to upgrade/demote a process
strategy to decide the queue a process will enter when it needs service
Lecture 20 10

11. Example of a multilevel feedback queue exam
Three queues:
Q0 – RR with time quantum 8 milliseconds
Q1 – RR time quantum 16 milliseconds
Q2 – FCFS
Scheduling
A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.
At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2.
Lecture 20 11

12. Multilevel feedback queues
Lecture 20 12

13. Unix scheduler
The higher the number quantifying the priority the lower the actual process priority.
Priority = (recent CPU usage)/2 + base
Recent CPU usage  how often the process has used the CPU since the last time priorities were calculated.
Does this strategy raises or lowers the priority of a CPU-bound processes?
Example:
base = 60
Recent CPU usage: P1 =40, P2 =18, P3 = 10
Lecture 20 13

14. Comparison of scheduling algorithms
Round Robin
FIFO
MFQ
Multi-Level
Feedback Queue
SFJ
Shortest Job First
SRJN
Shortest Remaining Job Next
Throughput
Response
time
May be low is quantum is too small
Shortest average response
time if quantum chosen correctly
Not emphasized
May be poor
Good for I/O bound but poor for CPU-bound
processes
High
Good for short processes
But maybe poor for longer processes
Lecture 20 14

15. Shortest Remaining Job Next
Round Robin
FIFO
MFQ
Multi-Level
Feedback Queue
SFJ
Shortest Job First
SRJN
Shortest Remaining Job Next
IO-bound
Infinite postponement
No distinction
between
CPU-bound and
Does not occur
Gets a high priority if CPU-bound processes are present
May occur for CPU bound processes
May occur for processes with long estimated running times
Lecture 20 15

16. Shortest Remaining Job Next
Round Robin
FIFO
MFQ
Multi-Level
Feedback Queue
SFJ
Shortest Job First
SRJN
Shortest Remaining Job Next
Overhead
CPU-bound
Low
No distinction
between
CPU-bound and
IO-bound
The lowest
Can be high Complex data structures and processing routines
Gets a low priority if IO-bound processes are present
Can be high Routine to find to find the shortest job for each reschedule
Can be high Routine to find to find the minimum remaining time for each reschedule
Lecture 20 16

17. Memory virtualization
A process runs in its own address space; multiple threads may share an address space.
Process/tread management and memory management are important functions of an operating system
Virtual memory
Allows programs to run on systems with different sizes of real memory
It may lead to performance penalty.
A virtual address space has a fixed size determined by the number of bits in an address., e.g., if n=32 then size = 232 ~ 4 GB.
Swap area: image on a disk of the virtual memory of a process.
Page: a group of consecutive virtual addresses e.g., of size 4 K is brought from the swap area to the main memory at once.
Not all pages of a process are in the real memory at the same time
Caching
Blocks of virtual memory addresses are brought into faster memory.
Leads to performance improvement.
Lecture 21

18. Locality of reference
Locality of reference: when a memory location is referenced then the next references are likely to be in close proximity.
The programs include sequential code
The data structures include often data that are processed together; e.g., an array.
Spatial and temporal locality of reference.
Thus it makes sense to group a set of consecutive addresses into units and transfer such units at once between a slow but larger storage space to a faster but smaller storage. The sixe of the units is different
Virtual memory  Page size: KB
Cache  16 – 256 words.
Lecture 21

19. Virtual memory Several strategies Paging Segmentation
At the time a process/thread is created the system creates a page table for it; an entry in the page table contains
The location in the swap area of the page
The address in main memory where the page resides if the page has been brought in from the disk
Other information e.g. dirty bit.
Page fault  a process/thread references an address in a page which is not in main memory
On demand paging  a page is brought in the main memory from the swap area on the disk when the process/thread references an address in that page.
Lecture 21

20. Lecture 21

21. Lecture 21

22. Dynamic address translation
Lecture 20