1. Practical Session 2, Processes and Scheduling
Operating Systems
Practical Session 2,
Processes and Scheduling

2. Five-state Process Model
New
Ready
Running
Exit
Blocked
Admit
Event
Occurs
Dispatch
Release
Time-out
Wait

3. Two types of scheduling
Preemptive scheduling A task may be rescheduled to operate at a later time (for example, it may be rescheduled by the scheduler upon the arrival of a “more important” task).
Non Preemptive scheduling (cooperative) Task switching can only be performed with explicitly defined system services (for example: the termination task, explicit call to yield() , I/O operation which changes the process state to blocking, etc’…).

4. Scheduling algorithms
FCFS (First – Come, First – Served)
Non preemptive
Convoy effect
SJF (Shortest Job First)
Provably minimal with respect to the minimal average turn around time
No way of knowing the length of the next CPU burst
Can approximate according to: Tn+1=αtn+(1- α)Tn
Preemptive (Shortest Remaining Time - SRT) or non preemptive
Round Robin
When using large time slices it imitates FCFS
When using time slices which are closer to context switch time, more CPU time is wasted on switches
Guaranteed scheduling
Constantly calculates the ratio between how much time the process has had since its creation and how much CPU time it is entitled to
CPU time entitled to = Time since process creation Total number of processses
Guarantees 1/n of CPU time per process / user

5. Priority Scheduling algorithms
Priority scheduling (naïve)
A generalization of SJF
Multi Level Queue scheduling
Partition the ready queue
Each partition employs its own scheduling scheme
A process from a lower priority group may run only if there is no higher priority process
May cause starvation!
Regarding Guaranteed scheduling: Tanenbaum second edition p. 170

6. Priority Scheduling algorithms (cont.)
Dynamic Multi Level scheduling
Promote (starving) low priority processes
Can be implemented by counting the time from last execution of the process
Demote processes running longer
Priority may become balanced eventually

7. Two Level scheduling First level: Choose process to run
Second level: Choose the process to swap out
CPU
Memory
disk
Memory scheduler
CPU scheduler

8. The Completely Fair Scheduler
CFS models an “ideal, precise multitasking CPU”
Can be thought of as a combination of guaranteed scheduling and dynamic priority scheduling
It measure how much runtime each task has had and try and ensure that everyone gets their fair share of time
The process’s runtime held in a vruntime variable
A lower vruntime indicates that the process has had less time to compute, and therefore has more need of the processor

9. CFS Priorities
CFS doesn't use priorities directly but instead uses them as a decay factor for the time a process is permitted to execute
Lower-priority processes have higher factors of decay, while higher-priority processes have lower factors of decay
Decay factor is inverse to the priority
A higher priority process will accumulate vruntime more slowly
Likewise, a low-priority process will have its vruntime increase more quickly

10. Quality criteria measures
Throughput – The number of completed processes per time unit.
Turnaround time – The time interval between the process submission and its completion.
Waiting time – The sum of all time intervals in which the process was in the ready queue.
Response time – The time taken between submitting a command and the generation of first output.
CPU utilization – Percentage of time in which the CPU is not idle.

11. XV6

12. Five-state Process Model
New
Ready
Running
Exit
Blocked
Admit
Event
Occurs
Dispatch
Release
Time-out
Wait
proc.h
enum procstate { UNUSED, EMBRYO, SLEEPING, RUNNABLE, RUNNING, ZOMBIE };
// Per-process state
struct proc {
uint sz; // Size of process memory (bytes)
pde_t* pgdir; // Page table
char *kstack; // Bottom of kernel stack for this process
enum procstate state; // Process state
int pid; // Process ID
struct proc *parent; // Parent process
struct trapframe *tf; // Trap frame for current syscall
struct context *context; // swtch() here to run process
void *chan; // If non-zero, sleeping on chan
int killed; // If non-zero, have been killed
struct file *ofile[NOFILE]; // Open files
struct inode *cwd; // Current directory
char name[16]; // Process name (debugging) };

13. Creation of a Process (fork)
New
Ready
Running
Exit
Blocked
Admit
Event
Occurs
Dispatch
Release
Time-out
Wait
np->sz = curproc->sz;
np->parent = curproc;
*np->tf = *curproc->tf;
// Clear %eax so that fork returns 0 in the child.
np->tf->eax = 0;
for(i = 0; i < NOFILE; i++)
if(curproc->ofile[i])
np->ofile[i] = filedup(curproc->ofile[i]);
np->cwd = idup(curproc->cwd);
safestrcpy(np->name, curproc->name, sizeof(curproc->name));
pid = np->pid;
acquire(&ptable.lock);
np->state = RUNNABLE;
release(&ptable.lock);
return pid; }
proc.c
int
fork(void) {
int i, pid;
struct proc *np;
struct proc *curproc = myproc();
// Allocate process.
if((np = allocproc()) == 0){
return -1;
// Copy process state from p.
if((np->pgdir = copyuvm(curproc->pgdir, curproc->sz)) == 0){
kfree(np->kstack);
np->kstack = 0;
np->state = UNUSED;

14. Transition to a Running state
New
Ready
Running
Exit
Blocked
Admit
Event
Occurs
Dispatch
Release
Time-out
Wait
// Switch to chosen process. It is the process's job
// to release ptable.lock and then reacquire it
// before jumping back to us.
c->proc = p;
switchuvm(p);
p->state = RUNNING;
swtch(&(c->scheduler), p->context);
switchkvm();
// Process is done running for now.
// It should have changed its p->state before coming back.
c->proc = 0; }
release(&ptable.lock);
proc.c
void
scheduler(void) {
struct proc *p;
struct cpu *c = mycpu();
for(;;){
// Enable interrupts on this processor.
sti();
// Loop over process table looking for process to run.
acquire(&ptable.lock);
for(p = ptable.proc; p < &ptable.proc[NPROC]; p++){
if(p->state != RUNNABLE)
continue;

15. Transition to a Running state
New
Ready
Running
Exit
Blocked
Admit
Event
Occurs
Dispatch
Release
Time-out
Wait
# Switch stacks
movl %esp, (%eax) // esp points to a context like data
// store it into old
movl %edx, %esp // esp gets edx = switch kernel stack
# Load new callee-save registers
popl %edi
popl %esi
popl %ebx
popl %ebp
ret
swtch.S
# Context switch
# void swtch(struct context **old, struct context *new);
# Save current register context in old
# and then load register context from new.
.globl swtch
swtch:
movl 4(%esp), %eax // eax gets old
movl 8(%esp), %edx // edx gets new
# Save old callee-save registers
pushl %ebp
pushl %ebx
pushl %esi
pushl %edi
struct context {
uint edi; uint esi; uint ebx; uint ebp; uint eip; };

16. Transition to a Ready state
New
Ready
Running
Exit
Blocked
Admit
Event
Occurs
Dispatch
Release
Time-out
Wait
void
sched(void) {
int intena;
struct proc *p = myproc();
if(!holding(&ptable.lock))
panic("sched ptable.lock");
if(mycpu()->ncli != 1)
panic("sched locks");
if(p->state == RUNNING)
panic("sched running");
if(readeflags()&FL_IF)
panic("sched interruptible");
intena = mycpu()->intena;
swtch(&p->context, mycpu()->scheduler);
mycpu()->intena = intena; }
proc.c
yield(void)
acquire(&ptable.lock); //DOC: yieldlock
myproc()->state = RUNNABLE;
sched();
release(&ptable.lock);

17. Transition to a Blocked state
proc.c
void
sleep(void *chan, struct spinlock *lk) {
struct proc *p = myproc();
if(p == 0)
panic("sleep");
if(lk == 0)
panic("sleep without lk");
if(lk != &ptable.lock){ //DOC: sleeplock0
acquire(&ptable.lock); //DOC: sleeplock1
release(lk); }
// Go to sleep.
p->chan = chan;
p->state = SLEEPING;
sched();
// Tidy up.
p->chan = 0;
// Reacquire original lock.
if(lk != &ptable.lock){ //DOC: sleeplock2
release(&ptable.lock);
acquire(lk);
New
Ready
Running
Exit
Blocked
Admit
Event
Occurs
Dispatch
Release
Time-out
Wait

18. Transition to a Ready state
static void
wakeup1(void *chan) {
struct proc *p;
for(p = ptable.proc; p < &ptable.proc[NPROC]; p++)
if(p->state == SLEEPING && p->chan == chan)
p->state = RUNNABLE; }
proc.c
void
wakeup(void *chan)
acquire(&ptable.lock);
wakeup1(chan);
release(&ptable.lock);
New
Ready
Running
Exit
Blocked
Admit
Event
Occurs
Dispatch
Release
Time-out
Wait

19. Warm up Why bother with multiprogramming?
Assume processes in a given system wait for I/O 60% of the time.
What is the approximate CPU utilization with one process running?
What is the approximate CPU utilization with three processes running?

20. Warm up
If a process is blocking on I/O 60% of the time, than there is only 40% of CPU utilization.
At a given moment, the probability that all three processes are blocking on I/O is (0.6)3. That means that the CPU utilization is (1-(0.6)3)=0.784, or roughly 78%.

21. Preemptive dynamic priorities scheduling
(Taken from Silberschatz, 5-9)
Consider the following preemptive priority scheduling algorithm with dynamically changing priorities: When a process is waiting for the CPU (in the ready queue, but not running), its priority changes at rate α; when it is running, its priority changes at rate β. All processes are given a priority of 0 when they enter the ready queue. The parameters alpha and beta can be set. Higher priority processes take higher values.
What is the algorithm that results from β> α > 0?
What is the algorithm that results from α < β< 0?
Is there a starvation problem in 1? in 2? explain.

22. Preemptive dynamic priorities scheduling
β>α>0.
To get a better feeling of the problem, we will create an example: C, P1, P2, P3 arrive one after the other and last for 3 TU, α=1 and β=2(bold marks the running process):
The resulting schedule is FCFS.
Slightly more formal: If a process is running it must have the highest priority value. While it is running, it’s priority value increases at a rate greater than any other waiting process. As a result, it will continue it’s run until it completes (or waits on I/O, for example). All processes in the waiting queue, increase their priority at the same rate, hence the one which arrived earliest will have the highest priority once the CPU is available.
Time 1 2 3 4 5 6 7 8 9 P1 P2 P3

23. Preemptive dynamic priorities scheduling
α <β<0.
We will use (almost) the same example problem as before, but this time α=- 2, β=-1:
The resulting schedule is LIFO.
More formally: If a process is running it must have the highest priority value. While it is running, it’s priority value decreases at a much lower rate than any other waiting process. As a result, it will continue it’s run until it completes (or waits on I/O, for example), or a new process with priority 0 is introduced. As before, all processes in the waiting queue, decrease their priority at the same rate, hence the one which arrived later will have the highest priority once the CPU is available.
Time 1 2 3 4 5 6 7 8 9 P1 -1 -3 -5 -7 -9
-11
-13
-14 P2 -8 P3 -2

24. Preemptive dynamic priorities scheduling
In the first case it is easy to see that there is no starvation problem. When the kth process is introduced it will wait for at most (k-1)⋅max{timei} Time Units. This number might be large but it is still finite.
This is not true for the second case - consider the following scenario: P1 is introduced and receives CPU time. While still working a 2nd process, P2, is initiated. According to the scheduling algorithm in the second case, P2 will receive the CPU time and P1 will have to wait. As long as new processes will keep coming before P1 gets a chance to complete its run, P1 will never complete its task.