1. CS703 – Advanced Operating Systems
By Mr. Farhan Zaidi

2. Lecture No. 17

3. Some problems with multilevel queue concept
Can’t low priority threads starve?
Ad hoc: when skipped over, increase priority
What about when past doesn’t predict future?
e.g., CPU bound switches to I/O bound
Want past predictions to “age” and count less towards current view of the world.

4. Summary + simple - short jobs can get stuck behind long ones; poor I/O
FIFO:
+ simple
- short jobs can get stuck behind long ones; poor I/O
RR:
+ better for short jobs
- poor when jobs are the same length
STCF:
+ optimal (ave. response time, ave. time-to-completion)
- hard to predict the future
- unfair
Multi-level feedback:
+ approximate STCF
- unfair to long running jobs

5. Some Unix scheduling problems
How does the priority scheme scale with number of processes?
How to give a process a given percentage of CPU?
OS implementation problem:
OS takes precedence over user process
user process can create lots of kernel work: e.g., many network packets come in, OS has to process. When doing a read or write system call, ….

6. Linux Scheduling
Builds on traditional UNIX multi-level feedback queue scheduler by adding two new scheduling classes.
Linux scheduling classes:
- SCHED_FIFO: FCFS real-time threads
- SCHED_RR: round-robin real-time threads
- SCHED_OTHER: Other non-real-time threads
Multiple priorities may be used within a class
Priorities in real-time classes are higher than non-real-
time classes.

7. Linux Scheduling (2) Rules for SCHED_FIFO
1. The system will not interrupt a SCHED_FIFO thread
except in the following cases:
Another FIFO thread of higher priority becomes ready
The executing FIFO thread blocks on I/O etc.
The executing FIFO threads voluntarily gives up the CPU e.g. terminates or yields
2. When an executing FIFO thread is interrupted, it is placed in the queue associated with its priority

8. Linux Scheduling (3)
SCHED_RR class is similar to SCHED_FIFO except that there is a time-slice associated with an RR thread. On expiry of the time slice, if the thread is still executing, it is pre-empted and another thread from either SCHED_FIFO or SCHED_RR is selected for execution.
SCHED_OTHER class is managed by the traditional UNIX scheduling algorithms i.e. multi-level feedback queue.

9. Lottery scheduling: random simplicity
Problem: this whole priority thing is really ad hoc.
How to ensure that processes will be equally penalized under load?
Lottery scheduling! Very simple idea:
give each process some number of lottery tickets
On each scheduling event, randomly pick ticket
run winning process
to give process P n% of CPU, give it (total tickets)* n%
How to use?
Approximate priority: low-priority, give few tickets, high-priority give many
Approximate STCF: give short jobs more tickets, long jobs fewer. Key: If job has at least 1, will not starve

10. Grace under load change
Add or delete jobs (and their tickets):
affect all proportionally
Example: give all jobs 1/n of cpu?
4 jobs, 1 ticket each
each gets (on average) 25% of CPU.
Delete one job:
automatically adjusts to 33% of CPU!
Easy priority donation:
Donate tickets to process you’re waiting on.
Its CPU% scales with tickets of all waiters. 1 1

11. Classifications of Multiprocessor Systems
Loosely coupled or distributed multiprocessor, or cluster
Each processor has its own memory and I/O channels
Functionally specialized processors
Such as I/O processor
Controlled by a master processor
Tightly coupled multiprocessing
Processors share main memory
Controlled by operating system

12. Granularity of parallelism
Coarse and Very Coarse-Grained Parallelism
Synchronization among processes at a very gross level (after > 2000 instructions on the average)
Good for concurrent processes running on a multiprogrammed uniprocessor
Can by supported on a multiprocessor with little change

13. Granularity of parallelism
Medium grained parallelism
Single application is a collection of threads
Threads usually interact frequently
Fine-Grained Parallelism
Highly parallel applications
Specialized and fragmented area