1. CS 390 Unix Programming Environment
Topics to be covered:
Memory Management
Process Scheduling
CS390 - Unix Programming Environment

2. CS390 - Unix Programming Environment
Memory Management
Memory is an important resource that has to be managed with the help of memory manager
Memory Manager is a component of the OS responsible for managing memory
Allocate and deallocate processes to and from the memory
Allocate memory to processes that are currently being used
Deallocate memory from processes that have ended
Unlike PC’s, UNIX OS use very sophisticated memory management algorithms to make efficient use of memory resources
CS390 - Unix Programming Environment

3. CS390 - Unix Programming Environment
Types of Memory
Main Memory
The physical RAM, also called Real memory
processes can only be run when both the program code, and program data resides in main memory
Cache Memory
The cache is a small amount of high-speed memory, usually with a memory cycle time comparable to the time required by the CPU to fetch one instruction.
The cache is usually filled from main memory when instructions or data are fetched into the CPU.
Swap space
Disk memory used to hold data that is not in Real or File System
CS390 - Unix Programming Environment

4. CS390 - Unix Programming Environment
Virtual Memory (VM)
Used to enlarge the available address space beyond the physical address space
Total Memory = Virtual Memory + Main Memory
It is the responsibility of the Memory manager to keep track of processes, whether they are in Main or Virtual Memory
All current running processes must be in the Main Memory
CS390 - Unix Programming Environment

5. CS390 - Unix Programming Environment
Virtual Memory Contd.
Most modern computers have special hardware called a memory management unit (MMU).
This unit sits between the CPU and the memory unit
If CPU wants to access memory (whether it is to load an instruction or load or store data), it sends the desired memory address to the MMU, which translates it to another address
The address generated by the CPU is called a virtual address, and the address it gets translated to by the MMU is called a physical address.
CS390 - Unix Programming Environment

6. CS390 - Unix Programming Environment
Memory Manager Issues
Often the number of processes that an OS needs to manage have memory requirements that exceed the amount of available memory
Move processes to and from disk to the main memory
Disk access is much slower than main memory
Needs efficient algorithms to effectively manage all system processes
Common techniques
Swapping
Demand Paging
CS390 - Unix Programming Environment

7. CS390 - Unix Programming Environment
Swapping
When the number of processes have memory requirements that exceed the amount of main memory, some processes need to be moved to the disk
Swapping systems require that the entire process has to be in main memory in order to run the process
Consists of three parts
Managing space in the swap device (secondary memory)
Swapping process out of main memory
Swapping process back into the main memory
CS390 - Unix Programming Environment

8. CS390 - Unix Programming Environment
Demand Paging
Virtual address space is divided into equally sized units called pages
Pages typically range from 1K to 8k
Now the entire process need not to be in the main memory to execute
The act of moving back and forth the pages from the Main memory to the virtual memory is called Paging
If a process accesses a page and that page is not in Main memory a page fault occurs
Move the page from the virtual memory to the main memory
CS390 - Unix Programming Environment

9. CS390 - Unix Programming Environment
Processes
A task in operation is called as a Process
A process is created by fork system call at the lowest level
One can check what processes are running using the ps command
We also know how to terminate a process using the kill command
Lets now see how processes can be scheduled
CS390 - Unix Programming Environment

10. CS390 - Unix Programming Environment
Process scheduling
As a time sharing system, the UNIX system kernel directly controls how processes are scheduled
The Kernel suspends a process when its quantum time expires
If a process wants to get the attention of the kernel, it uses Interrupts
CS390 - Unix Programming Environment

11. CS390 - Unix Programming Environment
Interrupts
When an Interrupt occurs, the current process is suspended and the kernel examines the cause and services the interrupt
After the interrupt has been handled, the suspended process either resumes execution or preempted
Pre-emption is a process of taking away the control
CS390 - Unix Programming Environment

12. CS390 - Unix Programming Environment
Process state Diagram
Interrupt,
Interrupt return
User Running
System call,
interrupt
Kernel Running
return
Return to user
Zombie
Preempt
exit
Preempted
sleep
Reschedule
process
Asleep
in memory
Wakeup
Ready to run
Enough memory
Swap out
Swap in,
Swap out
Created using fork
Not enough memory
Wakeup
Sleep Swapped
Ready to run, swapped
CS390 - Unix Programming Environment

13. CS390 - Unix Programming Environment
Interrupt contd…
Current Process
Suspend
resume
time
Terminal interrupt handler
Process interrupt
completed
Terminal interrupt
CS390 - Unix Programming Environment

14. Process Scheduling contd…
A user can specify when a process should run using user-level commands
at command
batch command
nice command
sleep command
wait command
nohup command
CS390 - Unix Programming Environment

15. CS390 - Unix Programming Environment
at command
One can specify when the commands should run using the at command
$ at 0500 Jan 18
echo “Don’t forget the meeting” | mail you
CNTL-D
The above command will mail you a reminder at 5:00 am on Jan 18 about the meeting
-f option: run a sequence of commands contained in a file
$ at –f scriptfile 6 am Monday
This will run scriptfile at 6 A.M. on Monday
CS390 - Unix Programming Environment

16. CS390 - Unix Programming Environment
at command contd…
You can see the listing of all at jobs you have scheduled, using the –l option, which returns ID number and scheduled time for each of your at jobs
$ at –l
To remove a job from the queue use –r option
CS390 - Unix Programming Environment

17. CS390 - Unix Programming Environment
batch command
The batch command lets the user defer the execution, but does not give control as to when it is supposed to run
$ batch <<!
cat mybook | lp
CS390 - Unix Programming Environment

18. CS390 - Unix Programming Environment
Process Priorities
The kernel assigns the CPU to a process for a time slice
When the time of a process is elapsed it is placed in one of the several priority queues.
All the processes have some priority associated with them
CS390 - Unix Programming Environment

19. CS390 - Unix Programming Environment
The nice command
The nice command allows a user to execute a command with a lower-than-normal priority
Priority = priority index + nice value
You can decrease the priority of a command by using nice. It will run slower and takes less CPU time.
Advantage: you are being nice to your neighbors
$ nice –19 profit
CS390 - Unix Programming Environment