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Operating Systems
CSE 411
CPU Management
Sept. 20 2006 - Lecture 7
Instructor: Bhuvan Urgaonkar
• Last class
– Dispatcher
– Context switch, FCFS, SJF, SRPT, RR, Priority, Quantum size
– Read on your own: Multi-level (feedback) queue based scheduling
• Today
– More on CPU scheduling
Proportional-Share Schedulers
• A general class of scheduling algorithms
• Process Pi given a CPU weight wi > 0
• The scheduler needs to ensure the following
– forall i, j, |Ti(t1, t2)/Tj(t1,t2) - wi/wj| ≤ e
– Given Pi and Pj were backlogged during [t1,t2]
• Who chooses the weights and how?
• Application modeling problem: non-trivial
– Approaches: analytical, empirical
• A part of my Ph.D. thesis
• Many PS schedulers developed in the 90s
– E.g., Start-time Fair Queueing (Qlinux UT-Austin/Umass-Amherst)
Lottery Scheduling
[Carl Waldspurger, MIT, ~1995]
• Perhaps the simplest proportional-share scheduler
• Create lottery tickets equal to the sum of the weights of all processes
– What if the weights are non-integral?
• Draw a lottery ticket and schedule the process that owns that ticket
– What if the process is not ready?
• Draw tickets only for ready processes
– Homework 1: Calculate the time/space complexity of the operations
Lottery scheduling will involve
– Likely question on Exam 1 !!!
Lottery Scheduling Example
P1=6
P2=9
1
4
7
10
13
2
5
8
11
14
3
6
9
12
15
9
Schedule P2
Lottery Scheduling Example
P1=6
P2=9
1
4
7
10
13
2
5
8
11
14
3
6
9
12
15
3
Schedule P1
Lottery Scheduling Example
P1=6
P2=9
1
4
7
10
13
2
5
8
11
14
3
6
9
12
15
11
• As t
∞, processes will get their share (unless they were blocked a lot)
• Problem with Lottery scheduling: Only probabilistic guarantee
• What does the scheduler have to do
– When a new process arrives?
– When a process terminates?
Schedule P2
Work Conservation
• Examples of work-conserving schedulers: All
schedulers we have studied so far
• Examples of non-work-conserving schedulers:
– DFS, a PS scheduler for multi-processors (Abhishek Chandra,
Umass, 2000, now at Univ. of Minnesota)
• Experiments showed the scheduler had become NWC!
• Fair Airport to convert into a WC scheme
NWC
Scheduler
WC
Scheduler
Reservation-based Schedulers
• Each process has a pair (x, y)
– Divide time into periods of length y each
– Guaranteed to get x time units every period
• Can be Non-work-conserving
Rate Regulation: Leaky Bucket
• A type of reservation-based scheduling
• Leaky bucket policing
– Rate ri for process Pi
– CPU cycles over period t  ri * t
CPU requirement
bi
ri
CPU the process gets
burst
Max. rate at which the
process may progress
Rate Regulation: Token Bucket
• A type of reservation-based scheduling
• Token bucket policing
– Rate ri and burst bi for process Pi
– CPU cycles over period t  ri * t + bi
bi tokens
CPU requirement
ri
burst
Deadline-based Scheduling
•
•
•
•
Can be NWC
Several variants NP-hard
Real-time systems
“Soft” real-time systems
– E.g., media servers: 30 MPEG-1 frames/sec
– A few violations may be tolerable
An Interesting Problem: An Invitation to
do Research on CPU Scheduling
• Can we achieve the effect of a scheduler that can provide
resource guarantees using a priority-based scheduler?
• You are welcome to talk to me during office hours if you
find this interesting or if you have any thoughts on this
Hierarchical Schedulers
Reservation-based
(4, 10)
(6, 10)
Round-robin
w=1
UNIX
Lottery
w=2
UNIX
Processes
• Variety of schedulers
• Subsets of processes with different
scheduling needs
Scheduler Considerations:
Context-Switch Overhead
•
Switching context
–
–
–
–
–
–
•
•
•
•
User mode to Kernel mode
Save PCB
Process interrupt (e.g, TCP/IP processing) if needed
Run scheduling algorithm
Load registers (e.g., PC) from PCB of chosen process
Flush TLB (will discuss this under memory management)
Switching to user mode
Jumping to the proper location in the user program to restart that program
Context switch time for Pentium/Linux: tens of microsec
So how often should the scheduler be invoked?
Scheduler Considerations:
Quantum Length
• The duration for which a CPU-intensive process will run before
being scheduled out in favor of another process
• Different processes may have different quantum lengths
– E.g., UNIX: Higher priority => Larger quantum
• Typically tens of msec in modern systems
• Small => Context switch overhead high
• Large => Bad responsiveness => Interactive processes suffer
Scheduler Considerations:
CPU Accounting
• Scheduler maintains CPU usage, last
scheduling instant etc. in each PCB
• Who should be charged for the CPU usage
during interrupt processing?
– Optional reading: Resource containers (Rice University, OSDI 1999,
Gaurav Banga et al.)
Scheduler Considerations:
Time and Space Requirements
• Run time (n processes)
– FCFS: O(1)
– RR: O(1)
– Deadline-based algos: NP-hard variants, poly-time heuristics
• Update time: Operations done when set of processes
changes (new, terminate, block, become ready)
• Space requirements
– Space to store various data structs
Scheduler Evaluation
• Optimize one or more of response time, waiting time,
throughput, fairness, utilization, ..
• Overheads
– Run time (time to pick the next process to schedule)
– Update time
– Space requirements
Analytical Approaches to
Scheduler Evaluation
• Example 1: Our evaluation of FCFS and SJF
• Example 2: Queueing Theory
– Little’s Law: applies to any scheduling discipline
– avg_num_processes_in_system = tput * avg_time_spent_in_system
yet to arrive
serviced
waiting
running
– Queuing theory provides expressions for mean response time for certain
scheduling policies (FCFS, PS, LCFS)
– Other statistical properties of response time only under restrictive
assumptions on arrival process and service time
Discussion: Analytical Approaches
• Pros:
– Intellectually satisfying!
– Quick, no code to be written
– May provide generally applicable results
• Cons:
– Often over-simplified
• Hard to model complex scheduling algorithms
• Lack of info about workloads
Simulation-based Evaluation
• Write software that mimics the behavior of the scheduler
• Subject it to realistic workload
• Observe output and make inferences about performance of
scheduler
• Only need to know the algorithms, no need to do complex
math
• Easier to implement than the actual system
Prototyping
• Build it!
Analysis, Simulation, and
Prototyping Compared
accuracy,
effort, time investment,
programming skills mathematical skills, generality
Analysis
Simulation
Prototyping
Hybrid Approaches
Analysis
Simulation
Prototyping
Threads
What is a Thread?
• A basic unit of CPU utilization like a process (not necessarily
known to the OS though)
• “Smaller” than a process
– Part of a process
– Shares code + data + some other OS resources with other threads that
belong to the same process
User Threads
• Thread management done by user-level threads library
• OS doesn’t know about the existence of these threads
• Three primary thread libraries:
– POSIX Pthreads
– Win32 threads
– Java threads
Kernel Threads
• OS sees and manages these threads
• OS provides system calls to create, terminate, etc. (just like
the system calls it provides for processes)
• Examples
– Windows XP/2000
– Solaris
– Linux
– Tru64 UNIX
– Mac OS X
Benefits
•
•
•
•
Responsiveness
Resource Sharing
Economy
Utilization of MP Architectures