Operating Systems: (Tue)

Transcription

1 Operating Systems: (Tue) Definition: An operating system is the set of software that controls the overall operation of a computer system. provides an interface between the application software and the hardware so that users and applications do not have to have direct communication with the lowlevel hardware devices. Examples: o Unix operating system and its various flavors like Solaris and Linux o Microsoft operating systems Windows 7 (XP, Vista) o MacOS Operating systems have three major functions: 1. Providing user interface for using the computer system 2. Providing software support for hardware devices 3. Managing access to resources like CPU, memory and the file space 1. User Interface. - Graphical or textual interface for users to access applications and resources - Algorithms involve graphics (we will cover that later) 2. Support for hardware devices. - Every hardware device connected to a computer system is under the control of a software program called the device driver - sends control signals to the device for doing its functions - sends data to the device from the memory - receives data to be stored in memory - attends to events occurring in the device that need attention from the computer

2 - Example: device driver for a printer performs the following kinds of actions: a) Check to see if the printer is idle. If it is busy, wait until it becomes available. If it has a fault (for example, if it has a paper jam), inform the user via the user interface. b) Issue control signal for the printer to print. There may also be other protocol involved for specifying the type of paper to be used and other print options. c) Supply data to the printer at a rate that it can receive. d) Wait for the end of printing and inform the user. - Other devices such as disk drives and modems have more actions and protocols to deal with. - Algorithms for hardware support involved low-level actions (too complex for this week s discussion) 2. Resource Management - OS manages resources used by applications in the computer system - Very interesting algorithms for this class because they tend to reflect problems that we deal with in our every day lives. - Today (Tue) we will consider some of the problems that Computer Scientists deal with in resource management. - Next time (Thu) we will look at some algorithms that provide solutions to these problems. - What are some resources that the OS has to manage? CPU scheduling. o Have students list resources? o CPU, memory, disks, etc. - The CPU (central processing unit) is the processor that executes all program instructions. o Can only execute one instruction at a time o In multi-tasking system (most are) need a way to determine which instructions to execute when. - The part of the operating system that manages processes and schedules their time on the CPU is called the scheduler. - Every program that wants to run on the CPU is called a process. - At any given time, every process in the system is in one of three states: o running - being actively executed by the CPU. (P1 in the figure)

3 o ready - not idle (P2,,P8 in the figure) o waiting - waiting for some event such as keyboard input (P9 in the figure) - ready processes in the system are maintained in a Ready Queue by the operating system - Each of the processes is given a turn to use the CPU to execute its instructions turn for the CPU again. - The job of a CPU Scheduling Algorithm is to determine which processes should access the CPU when and for how long. o We will discuss these algorithms on Thu Memory management. - Program is requested to be run, o operating system determines how much memory is required for its program code o estimates how much memory may be needed for its data area o allocates the required amount of memory for the program from the available pool of memory o If the required amount of memory is not available, then the program cannot run. o It must wait for other programs to complete and release memory under their control.

4 - memory is allocated to programs in small units called pages - If a program wants to use a page that is not currently present in the main memory o brought from the disk and loaded into the memory. - What if the main memory is already full with other pages? o One of the other pages resident in the main memory must be unloaded (sent back to the disk). o The decision of which page to be unloaded is determined by a page replacement policy. - On Thur we will discuss several algorithm solutions to the page replacement problem Concurrency and mutual exclusion. - When several processes run concurrently, they often need to access shared resources o e.g., the disk memory, the printer, or the network interface - The operating system must ensure that the resources are shared in a noninterfering fashion. o Example: A bank with ATMs at various locations. The Smith family has an account with $1000 in it

5 Mr. Smith goes to the ATM on Main St. Mrs. Smith goes to the ATM on Brown St. They both try to withdraw $100 (at the same time) Process: When the withdrawal occurs, subtract 100 from the account balance If Mr. Smith withdrawal process (P1) and Mrs. Smith withdrawal process (P2) both try to subtract at the same time we have the following: P1: Get balance balance = balance Write balance to account P2: Get balance balance = balance Write balance to account If they both get the balance=1000, then the final balance after BOTH withdrawals will be $900 when it should be $800 - Need a way to ensure that the resource (account balance here) is only accessed by one process at a time - Solution: Locking o When a process needs the resource, it requests a lock on it o If the resource is not already locked, the process gets the lock and then does what it needs to do, then releases the lock o ATM example: either P1 (Mr. Smith) or P2 (Mrs. Smith) will request the lock first, the other process will have to wait until the lock is released Deadlocks. - Locking as a solution to concurrency control can cause another problem, called Deadlock - Example: Mr. and Mrs. Smith both want to transfer $100 from checking to savings account. Need to lock both checking and savings balances to make the transfer. P1: Lock check_bal P2: Lock sav_bal Lock sav_balance Lock check_balance Subtract 100 from check_bal Subtract 100 from check_bal Add 100 to sav_bal Add 100 to sav_bal Unlock check_bal Unlock sav_bal Unlock sav_bal Unlock check_bal

6 - What if we have the following: P1: Lock check_bal then P2: Lock sav_bal o P1 will be waiting for P2 to unlock sav_bal and P2 will be waiting for P1 to unlock check_bal DEADLOCK - Deadlock: Occurs when two or more processes is each waiting for a resource that is locked by another process - Operating systems must detect and break deadlocks.

7 Operating System Algorithms: (Thu) CPU Scheduling - Recall the problem: How to determine which process to execute on the CPU when? - (Have the students think of solutions Discuss how it is like scheduling tasks in their own lives ) - FIFO First-in first-out o First process in the queue gets executed first until it is finished o Discuss advantages and disadvantages - Round Robin with time slice (see attached algorithm pseudocode) o Each process gets an equal amount of time to execute (time slice) o When time slice is done, switch to next process in queue o If process not finished, move to end of queue o Context switch switching from one process to the next Need to store the state of the current process so it can be resumed later Takes time to make the switch o Trade-off Longer time slice makes other processes wait Shorter time slice makes for too many context switches Time Slice CPU Scheduling Algorithm Given: A set of ready processes wanting to use the CPU. Find: A schedule of processes. ======================================================= get(time_slice) repeat get next process P from queue timer = 0 while (timer < time_slice) and (P not complete) execute instruction from P increment timer endwhile if (P not complete) put P in back of queue endif until (queue is empty) - Priority-based highest priority process executes first until finished o Used in mission-critical systems o Can have a hybrid algorithm

8 Several queues each queue FIFO Queues are numbered by prio Processes placed in queue based on its prio Memory Management - Recall the problem: How to decide what pages of memory to discard when bringing in pages from disk (page replacement algorithms) - (Ask students for ideas) - LRU - Least recently used - NRU not recently used - FIFO - Random - Not Frequently Used - (Discuss advantages and disadvantages of each) Deadlock Avoidance / Detection - Recall the problem: When two or more processes is each waiting for a resource held by another process. - To consider solutions, lets look at an analogy - Dining Philosophers Problem (draw the picture) o 7 (or n) philosophers are sitting around a table o Each philosopher has a bowl of rice in front of her o Each philosopher has one chopstick on her right and one on her left

9 o Each philosopher will spend some time thinking and some time eating o To each rice, a philosopher needs 2 chopsticks o What if each philosopher decides to pick up her right chopstick at the same time? o This is DEADLOCK o How can we avoid it? Or break it? o (Have the students think about solutions to the DP problem and discuss how it relates to the OS problem) o Be sure to at least mention the ordering resources solution Each chopstick (resource) is given a number Philosopher (process) must request chopsticks (resources) in order lowest to highest If lower numbered chopstick is not available, wait for it (Show how this will work see picture) Number the resources (chopsticks)

10 - If each philosopher is ready to eat at the same time, they each try to grab their first chopstick at the same time - Since philosopher A got chopstick 1 first, philosopher G has to wait until A is finished so deadlock will not occur because philosopher F will be able to get chopstick 7 and eat and finish. - Pseudocode for this solution is available on the web site on the Handouts link