Forking

1
Forking Process Scheduling

• Vivek Pai / Kai Li
• Princeton University

2
Mechanics

• Exams not graded
• Hopefully, this week
• Quick scan looked fine
• Printed slides dont exactly match
• Notebook problems, time limited
• Finish forking, start scheduling

3
A Quick Review

• What have we covered
• How to store data via files
• How to virtualize memory
• Every process gets uniform address space
• Lots of tricks can be played to share memory
• Whats left
• How to share/virtualize the processor
• Having processes communicate/cooperate

4
Whos Happy Right Now
5
How To Launch a New Process?

• Obvious choice start process system call
• But not all processes start the same
• testprogram versus testprogram gt outfile
  versus testprogram arg1 arg2 gt outfile
• The parent process wants to specify various
  aspects of the childs environment
• Next step add more parameters to specify
  environment

6
Can We Generalize?

• What happens as more information gets added to
  the processs environment more parameters?
  New system calls? This gets ugly
• Whats the most general way of setting up all of
  the environment?
• So, why not allow process setup at any point?
• This is the exec( ) system call (and its variants)

7
But We Want a Parent and a Child

• The exec call destroys the current process
• So, instead, destroy a copy of the process
• The fork( ) call duplicates the current process
• Better yet, dont tightly couple fork and exec
• This way, you can customize the childs
  environment
• So what does fork( ) entail?
• Making a copy of everything about the process
• Ouch!

8
What Gets Copied

• So far, weve covered the following
• VM system
• File system
• Signals
• How do we go about copying this information?
• What parts are easy to copy, and whats hard?
• Whats the common case with fork/exec?
• What needs to get preserved in this scenario?

9
Shared Memory

• How to destroy a virtual address space?
• Link all PTEs
• Reference count
• How to swap out/in?
• Link all PTEs
• Operation on all entries
• How to pin/unpin?
• Link all PTEs
• Reference count

w
. . .
. . .
Page table
. . .
Process 1
w
Physical pages
. . .
. . .
Page table
Process 2
10
Copy-On-Write

• Childs virtual address space uses the same page
  mapping as parents
• Make all pages read-only
• Make child process ready
• On a read, nothing happens
• On a write, generates an access fault
• map to a new page frame
• copy the page over
• restart the instruction

r
r
. . .
. . .
Page table
. . .
Parent process
r
r
Physical pages
. . .
. . .
Page table
Child process
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Issues of Copy-On-Write

• How to destroy an address space
• Same as shared memory case?
• How to swap in/out?
• Same as shared memory
• How to pin/unpin
• Same as shared memory

12
Process Creation Termination

• Four primitives
• Fork create a copy of this process
• Exec replace this process with this program
• Wait wait for child process to finish
• Kill (potentially) end a running process
• Processes form a tree what happens when parent
  disappears?

13
Signals

• Asynchronous event delivery mechanism
• Examples FPE, segv, ctrl-c, hang up, resume
• Default actions ignore, abort, core dump
• Handler program-specified routine for signal

14
A Signaling Sidebar

• Whats wrong with this program
• int randVal
• void SigHand(void)
• printf(your rand val is d\n, randVal)
• int main(int argc, char argv)
• set up ctrl-c handler
• while (1)
• randVal 0
• randVal 1
• randVal 9

15
Scheduling Primitives

• Block wait on some event/resource
• Network packet arrival
• Keyboard, mouse input
• Disk activity completion
• Yield give up running for now
• Directed (let my friend run)
• Undirected (let any process run)
• Synchronization
• We will talk about this later

16
Our Friend, The Transition Diagram
terminate
Scheduler dispatch
Running
Wait for resource
Create a process
Ready
Blocked
Resource becomes available
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Process State Transition ofNon-Preemptive
Scheduling
Terminate (call scheduler)
Running
Scheduler dispatch
Block for resource (call scheduler)
Yield (call scheduler)
Create a process
Ready
Blocked
Resource becomes available (move to ready queue)
18
Scheduler

• A non-preemptive scheduler invoked by explicit
  block or yield calls
• The simplest form
• Scheduler
• save current process state (into PCB)
• choose next process to run
• dispatch (load PCB and run)
• Does this work?

19
More on Scheduler

• Should the scheduler use a special stack?
• Yes, because a user process can overflow and it
  would require another stack to deal with stack
  overflow
• Should the scheduler simply be a kernel process?
• You can view it that way because it has a stack,
  code and its data structure
• This process always runs when there is no user
  process

20
Where Should PCB Be Saved?

• Save the PCB on its user stack
• Many processors have a special instruction to do
  it efficiently
• But, need to deal with the overflow problem
• When the process terminates, the PCB vanishes
• Save the PCB on the kernel heap data structure
• May not be as efficient as saving it on stack
• But, it is very flexible and no other problems

21
Physical Memory Multiprogramming

• Memory is a scarce resource
• Want to run many programs
• Programs need memory to run
• What happens whenM(a) M(b) M(c) gt physical
  mem?

22
Job Swapping
Swap in
Swap out
Partially executed swapped-out processes
Ready Queue
CPU
Terminate
I/O Waiting queues
I/O
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Add Job Swapping toState Transition Diagram
Swap out
Swap
Terminate (call scheduler)
Running
Scheduler dispatch
Block for resource (call scheduler)
Swap in
Yield (call scheduler)
Create a process
Ready
Blocked
Resource becomes available (move to ready queue)
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Think About Swapping

• Is swapping
• Necessary
• Desirable
• Good
• Ideal

• Things to consider
• Performance
• Complexity
• Efficiency