Operating Systems
CSE 411
CPU Management
Sept. 18 2006 - Lecture 6
Instructor: Bhuvan Urgaonkar
Hmm .. Who should
I pick to run?
Running
OS (scheduler)
Ready
Lock
Waiting
Disk
First-Come, First-Served Scheduling
(FCFS)
Process
Run Time
P1
24
P2
3
P3
3
• Suppose that the processes arrive in the order: P1 , P2 , P3
The Gantt Chart for the schedule is:
P1
0
P2
24
• Waiting time for P1 = 0; P2 = 24; P3 = 27
• Average waiting time: (0 + 24 + 27)/3 = 17
P3
27
30
FCFS Scheduling (Cont.)
Suppose that the processes arrive in the order
P2 , P3 , P1
• The Gantt chart for the schedule is:
P2
0
•
•
•
•
P3
3
P1
6
Waiting time for P1 = 6; P2 = 0; P3 = 3
Average waiting time: (6 + 0 + 3)/3 = 3
Much better than previous case
Convoy effect short process behind long process
30
Shortest-Job-First (SJF) Scheduling
• Associate with each process the length of its next CPU
burst. Use these lengths to schedule the process with the
shortest time
• SJF is optimal for avg. waiting time – gives minimum
average waiting time for a given set of processes
– In class: Compute average waiting time for the previous example
with SJF
– Prove it (Homework 1, Will be handed out next Friday)
Architecture-dependent part
of the Scheduler: Dispatcher
• Dispatcher module gives control of the CPU to the
process selected by the scheduler; this involves:
– switching context
– switching to user mode
– jumping to the proper location in the user program to
restart that program
• Dispatch latency – time it takes for the dispatcher to stop
one process and start another running
– Also called the Context Switch time.
Costs/Overheads of
a Context Switch
• Direct/apparent
– Time spent doing the switch described in the last slide
– Fixed (more or less)
• Indirect/hidden costs
– Cache pollution
– Effect of TLB pollution (will study this when we get to
Virtual Memory Management)
– Workload dependent
Example from Linux 2.6.x
asmlinkage void __sched schedule(void)
{
[...]
prepare_arch_switch(rq, next);
prev = context_switch(rq, prev, next);
barrier();
finish_task_switch(prev);
[...]
}
task_t * context_switch(runqueue_t *rq, task_t *prev, task_t *next)
{
struct mm_struct *mm = next->mm;
struct mm_struct *oldmm = prev->active_mm;
/* Here we just switch the register state and the stack. */
switch_to(prev, next, prev);
return prev;
}
#define switch_to(prev,next,last) \
asm volatile(SAVE_CONTEXT
\
"movq %%rsp,%P[threadrsp](%[prev])\n\t" /* saveRSP */
\
"movq %P[threadrsp](%[next]),%%rsp\n\t" /* restore RSP */ \
"call __switch_to\n\t"
\
".globl thread_return\n"
\
"thread_return:\n\t"
\
"movq %%gs:%P[pda_pcurrent],%%rsi\n\t"
\
"movq %P[thread_info](%%rsi),%%r8\n\t"
\
LOCK "btr %[tif_fork],%P[ti_flags](%%r8)\n\t"
\
"movq %%rax,%%rdi\n\t"
\
"jc ret_from_fork\n\t"
\
RESTORE_CONTEXT
\
: "=a" (last)
: [next] "S" (next), [prev] "D" (prev),
\
\
[threadrsp] "i" (offsetof(struct task_struct, thread.rsp)), \
[ti_flags] "i" (offsetof(struct thread_info, flags)),\
[tif_fork] "i" (TIF_FORK),
\
[thread_info] "i" (offsetof(struct task_struct, thread_info)), \
[pda_pcurrent] "i" (offsetof(struct x8664_pda, pcurrent)) \
: "memory", "cc" __EXTRA_CLOBBER)
When is the scheduler invoked?
• CPU scheduling decisions may take place when a process:
1. Switches from running to waiting state
2. Switches from running to ready state
3. Switches from waiting to ready
4. Terminates
• Scheduling only under 1 and 4: nonpreemptive scheduling
– E.g., FCFS and SJF
• All other scheduling is preemptive
Why Pre-emption is Necessary
• To improve CPU utilization
– Most processes are not ready at all times during their lifetimes
– E.g., think of a text editor waiting for input from the keyboard
– Also improves I/O utilization
• To improve responsiveness
– Many processes would prefer “slow but steady progress” over “long
wait followed by fast process”
• Most modern CPU schedulers are pre-emptive
SJF: Variations on the theme
• Non-preemptive: once CPU given to the process it cannot be
preempted until completes its CPU burst - the SJF we already saw
• Preemptive: if a new process arrives with CPU length less
than remaining time of current executing process, preempt.
This scheme is know as Shortest-Remaining-Time-First (SRTF)
 Also called Shortest Remaining Processing Time (SRPT)
• Why SJF/SRTF may not be practical
 CPU requirement of a process rarely known in advance
Choosing the Right Scheduling
Algorithm/Scheduling Criteria
• CPU utilization – keep the CPU as busy as possible
• Throughput – # of processes that complete their execution
per time unit
• Turnaround time – amount of time to execute a particular
process
• Waiting time – amount of time a process has been waiting in
the ready queue
• Response time – amount of time it takes from when a
request was submitted until the first response is produced,
not output (for time-sharing environment)
• Fairness
Round Robin (RR)
• Each process gets a small unit of CPU time (time
quantum), usually 10-100 milliseconds. After this time
has elapsed, the process is preempted and added to the
end of the ready queue.
• If there are n processes in the ready queue and the
time quantum is q, then each process gets 1/n of the
CPU time in chunks of at most q time units at once.
No process waits more than (n-1)q time units.
• Performance
– q large => FCFS
– q small => q must be large with respect to context
switch, otherwise overhead is too high
Example of RR with Time
Quantum = 20
Process
P1
P2
P3
P4
CPU Time
53
17
68
24
• The Gantt chart is:
P1
0
P2
20
37
P3
P4
57
P1
77
P3
97 117
P4
P1
P3
P3
121 134 154 162
• Typically, higher average turnaround than SJF, but better response
Time Quantum and Context Switch Time
Turnaround Time Varies With Time Quantum
Priority-based Scheduling
• Associate with each process a quantity called its CPU priority
• At each scheduling instant
– Pick the ready process with the highest CPU priority
– Update (usually decrement) the priority of the process last running
•
•
•
•
Priority = Time since arrival => FCFS
Priority = 1/Size => SJF
Priority = 1/Remaining Time => SRPT
Priority = Time since last run => Round-robin (RR)
• UNIX variants
– Priority values are positive integers with upper bounds
– Decreased every quantum
• Fairness, avoid starvation
– Increased if the process was waiting, more wait => larger increase
• To make interactive processes more responsive
– Problems
• Hard to analyze theoretically, so hard to give any guarantees
• May unfairly reward blocking processes
Multilevel Queue
• Ready queue is partitioned into separate queues:
foreground (interactive)
background (batch)
• Each queue has its own scheduling algorithm
– foreground – RR
– background – FCFS
• Scheduling must be done 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
Multilevel Queue Scheduling
Multilevel Feedback Queue
• A process can move between the various queues;
aging can be implemented this way
• Multilevel-feedback-queue scheduler defined by
the following parameters:
–
–
–
–
–
number of queues
scheduling algorithms for each queue
method used to determine when to upgrade a process
method used to determine when to demote a process
method used to determine which queue a process will
enter when that process needs service
Example of Multilevel Feedback Queue
• 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.
Multilevel Feedback Queues