CS 390 Unix Programming Environment

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

2. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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