Operating Systems

1. Operating Systems Dr. Jerry Shiao, Silicon Valley University SILICON VALLEY UNIVERSITY CONFIDENTIAL

2. Processes • Overview • Process Concepts • Process is Program in Execution • Process Control Blocks ( PCB ) • Process State • Operating System Responsibilities: • Process Scheduling • Short-Term, Long-Term, Medium-Term Schedulers • Operations on Process • Process Creation / Termination • InterProcess Communication • Shared-Memory Systems • Message-Passing Systems • Communication in Client-Server Systems • Sockets • Remote Procedure Calls • Pipes SILICON VALLEY UNIVERSITY CONFIDENTIAL

3. Processes • Process Concept • Process is a unit of work in a Time-Sharing System. • Process contains: • Text Section – Program code. • CPU Program Counter • CPU Registers • Stack – Temporary data ( function parameters, return addresses, local variables ). • Data Section – Global variables. • Heap – Dynamically allocated memory during execution. Max Stack Process in Memory Heap Data Text 0 SILICON VALLEY UNIVERSITY CONFIDENTIAL

4. Processes • Process Concept • Process State • New – Process is created. • Running – Instructions are being executed. • Waiting – Process is waiting for some event ( I/O or signal ). • Ready – Process is waiting to be assigned to CPU. • Terminated – Process finished execution. New Terminated Admitted Exit Interrupt Ready Running Scheduler Dispatch I/O or Event Completion I/O or Event Wait Waiting SILICON VALLEY UNIVERSITY CONFIDENTIAL

5. Processes • Process Concept • Process Control Block ( PCB ) • Process State: • New/Ready/Running/Waiting/Terminated • Program Counter: • Address of instruction to be executed. • CPU Registers: • Accumulators, index registers, stack pointers, general purpose. • CPU-Scheduling Information: • Process priority, pointer so scheduling queues. • Memory-Management Information: • Base and limit registers, page tables. • Accounting Information: • Amount of CPU time used, accounting numbers. • I/O Status Information: • List of I/O devices allocated to the process, list of open files. SILICON VALLEY UNIVERSITY CONFIDENTIAL

6. Processes • Process Concept • Threads • Process supporting multiple threads of execution. Process P0 Operating System Process P1 Executing Interrupt or System Call Save State into PCB0 . . . Reload State From PCB1 Executing Interrupt or System Call Save State into PCB1 . . . Executing Reload State From PCB0 SILICON VALLEY UNIVERSITY CONFIDENTIAL

7. Processes • Process Scheduling • Scheduling Queues • Objectives: • For Multi-Programming, maximize CPU utilization. • For Time Sharing, switch CPU amount processes • Ready Queue: • Processes waiting to execute. • Device Queues: • Processes waiting for I/O Request to complete. PCB 0 PCB 2 NULL Ready Queue Head Head Head Tail Registers Registers . . . . . . PCB 3 Disk Unit 0 Head Head NULL Tail Registers . . . SILICON VALLEY UNIVERSITY CONFIDENTIAL

8. Processes • Process Scheduling • Scheduling Queues • Queuing Diagram Process Scheduled CPU Ready Queue I / O I/O Request I/O Queue Time Slice Expired Child Executes Fork a Child Interrupt Occurs Wait for an Interrupt SILICON VALLEY UNIVERSITY CONFIDENTIAL

9. Processes • Process Scheduling • Schedulers • Short-Term Scheduler • Selects a new process for the CPU frequently ( every 100 milliseconds ). • I/O bound processes - Interactive processes are I/O-bound, spending a lot of time waiting for I/O operations. • Interactive processes waiting for human input. • Response time must be quick (latency 50-150ms). • Command shells, text editors, graphical applications, mouse and keyboard processes. • Time-Sharing systems, UNIX and Microsoft Windows. • Long-Term Scheduler • Selects a new process less frequently ( order of minutes ). • Controls the degree of multi-programming ( number of processes in memory ). Ideally, new process starts as a process exits. • CPU bound processe - Batch processes are CPU-bound, heavily ultilize CPU time for computations. • Batch processes typically executing in background, i.e. compilers, database search engines, and scientific computiations. • No user interactions. • Batch systems, IBM mainframe systems. SILICON VALLEY UNIVERSITY CONFIDENTIAL

10. Processes • Process Scheduling • Schedulers • Medium-Term Scheduler • Time-Sharing systems needs combination of short and long term schedulers. • Reduce the degree of mult-programming ( remove processes from memory ) and contention for CPU by swapping processes out to disk. • Overcommitted available memory. • Process swapped in later when process mix is more favorable. Swap In Partially Executed Swapped-Out Processes Swap Out Process Terminate Process To Be Scheduled Scheduled CPU Ready Queue I/O I/O Waiting Queues SILICON VALLEY UNIVERSITY CONFIDENTIAL

11. Processes • Process Scheduling • Context Switch • Interrupts switch CPU from executing process to Kernel process. • Need to save and restore interrupted process. • PCB contains context of the process • CPU Registers • Process State • Memory-Management Information • Switch the CPU to another process • All Overhead, speed depends on: • Memory Speed • Number of registers • Hardware assist • Single instruction to load or restore all registers . • CPU contains multiple sets of registers ( Sun UltraSPARC ). • Preserve and restore process Page Table. SILICON VALLEY UNIVERSITY CONFIDENTIAL

12. Processes • Operations on Processes • Process Creation • Process ( parent process ) creates new processes ( child process ). • Process Identifier ( PID ) uniquely identifies a process. • UNIX: fork ( ) System Call. • Created process used to execute another program in parallel ( parent process continue to execute ) or parent program will wait for child process to complete. • exec ( ) system call loads binary image into memory to execute, but loaded over the process that called exec ( ). fork( ) Parent wait( ) Parent Resumes Child exec(‘cmd’ ) exit( ) SILICON VALLEY UNIVERSITY CONFIDENTIAL

13. Processes • Operations on Processes • Process Creation • Windows: CreateProcess ( ) System Call. • Similar to UNIX fork( ), but CreateProcess( ) is passed, as a parameter, the program to load and execute. • Process Termination • Child exceeded resource usage ( shared with parent and other child processes ). • Child’s program execution not needed. • Parent termination. WaitForSingleObject( ) CreateProcess ( ) Parent Parent Resumes Child ‘cmd’ exit( ) SILICON VALLEY UNIVERSITY CONFIDENTIAL

14. Processes • Interprocess Communication • Process is independent if it cannot affect or be affected by other processes in the system. • Process is cooperating if it can affect or be affected by other processes in the system. • Concurrent access to shared data. • Computational speedup with subtasks. • Modularity in system design with separate processes or threads. • Convenience with subtasks. • Cooperating Process and InterProcess Communcation ( IPC ) • Shared-Memory Systems • Region of memory shared between processes to exchange messages and data. • Message Passing Systems • Messages exchanged through System Calls to the Kernel. SILICON VALLEY UNIVERSITY CONFIDENTIAL

15. Processes Message Passing Shared memory • Interprocess Communication Process A Process A M Process B Process A Shared M Kernel Kernel M SILICON VALLEY UNIVERSITY CONFIDENTIAL

16. Processes • Interprocess Communication • Shared-Memory Systems • Shared Region exists in the address space of the process creating the shared-memory segment. • Processes must handle synchronizing shared-memory segment read and write. • Cooperating Processes: Producer – Consumer • Producer ( Server ): Produces information • Web Server creates and sends HTML files and images. • Consumer ( Client ): Consumes information • Web Browser reads the HTML files and images. • Producer – Consumer relationship between processes represented by shared-memory segment. • Shared-memory segment circular buffer and in / out pointers. • Producer process writes into circular buffer moving in pointer. • Consumer process reads from circular buffer moving out pointer. SILICON VALLEY UNIVERSITY CONFIDENTIAL

17. Processes • Interprocess Communication • Shared-Memory Systems • POSIX API for Shared Memory • Create shared-memory segment segment_id = shmget( segment_id, size, S_IRUSR | S_IWUSR ) segment_id = IPC_PRIVATE or ftok(“<filename>”, id) IPC_PRIVATE: New shared memory segment is created. size: In bytes of shared-memory segment. S_IRUSR | S_IWUSR: Read and write to shared-memory segment. • Access shared-memory segment shared_memory = (char *) shmat( segment_id, NULL, 0 ) shared_memory: Pointer to shared-memory segment. segment_id: Shared-memory segment identifier. NULL: Operating System selects where to attach the shared-memory. 0: Attach shared-memory in read / write mode. • Remove shared-memory segment shmdt ( shared_memory ) shared memory: Detach shared-memory region. SILICON VALLEY UNIVERSITY CONFIDENTIAL

18. Processes • Interprocess Communication • Message-Passing System • Operating System provides message-passing facility for cooperating processes. • Communication Link for send ( ) / receive ( ) operations: • How to refer to the process • Direct Communication • Explicitly name the recipient or sender establishes link automatically: • send ( <processID1>, message ), receive ( <processID2>, message ). • Symmetry addressing ( recipient and sender names must be known). • Asymmetry addressing ( Sender name known, recipient receives processID ). • send ( <processID1>, message ), receive ( <id>, message ). • Indirect Communication • Messages sent to and received from mailboxes or ports. • send ( <mailbox ID>, message ), receive ( <mailboxID>, message ) • Process has number of different links, each link corresponding to one mailbox. • Mailbox Process Owned: Dependency, unique owner. • Mailbox Operating System Owned: Independent, not attached to a process. SILICON VALLEY UNIVERSITY CONFIDENTIAL

19. Processes • Interprocess Communication • Message-Passing System • Communication Link for send ( ) / receive ( ) operations: • Synchronous Communications • Blocking Send: Sending process blocked until receive by process or mailbox. • Blocking Receive: Receiving process blocks until message received. • Asynchronous Communications • NonBlocking Send: Sending process not blocked. • NonBlocking Receive: Receiving process receives message or NULL. • Buffering • Messages exchanged reside in queue. • Zero Capacity: Queue has depth of zero. Sender must block until the recipient receives the message. • Bounded Capacity: Queue has n depth. Sender must block when the queue is full. • Unbounded Capacity: Queue has infinite depth. Sender never blocks. SILICON VALLEY UNIVERSITY CONFIDENTIAL

20. Processes • Client-Server System Communications • Sockets • Endpoints for communication. • Network IP address: port uniquely identifies connection • Port identifies the services to handle the request. • Port 23 = Telnet, Port 21 = FTP, Port 80 = HTTP. • Ports below 1024 are well-known for standard services. • Structure of message imposed by the client and server. Client: Web Browser Server: Web Server Dest=192.168.1.100 : 80 Src = 192.168.1.10 : 1625 Dest = 192.168.1.10 : 1625 Src = 192.168.1.100 : 80 SILICON VALLEY UNIVERSITY CONFIDENTIAL

21. Processes • Client-Server System Communications • Remote Procedure Calls ( RPC ) • Abstract procedure-call mechanism to use between computer systems over a network connection. • Network IP address: RPC port uniquely identifies RPC program • RPC Port: • Statically defined RPC port number. • A daemon (program) at fixed port number that returns the RPC port number. Client: Send command in RPC message. Server: Process command in RPC message. Dest=192.168.1.100 : <RPC port> Src = 192.168.1.10 : 1625 Dest = 192.168.1.10 : 1625 Src = 192.168.1.100 : <RPC port> SILICON VALLEY UNIVERSITY CONFIDENTIAL

22. Processes Client Message Server User calls Kernel to send RPC message to procedure X. • Client-Server System Communications Kernel sends message to MM to find port number. From: Client To: Server, Port: MM Re: Address for RPC X MM (MatchMaker) receives message, looks up answer. Kernel places port P in user RPC message. From: Server To: Client, Port: Kernel Re: RPC X Port: P Matchmaker replies to client with port P. Kernel sends RPC. From: Client To: Server, Port: P < contents > Daemon listening to port P receives message. Kernel receives reply, passes it to user. From: RPC, Port: P To: Client, Port: Kernel < output > Daemon processes request and process send output. SILICON VALLEY UNIVERSITY CONFIDENTIAL

23. Processes • Client-Server System Communications • Pipes • Conduit allowing two processes to communicate • Ordindary Pipes • Unidirectional, one-way communication. • Two-way communication require two pipes. • Parent-Child process relationship. • UNIX treats pipes as special files, requiring file descriptors. • pipe ( int fd [ ] ) command creates two file descriptors. • fd [ 0 ] is the read-end of the pipe. • fd p 1 ] is the write-end of the pipe. • Use standard read ( ) and write ( ) system calls. Parent fd(0) fd(1) Child fd(0) fd(1) Pipe SILICON VALLEY UNIVERSITY CONFIDENTIAL

24. Processes • Client-Server System Communications • Pipes • Named Pipes • Bidirectional, but only half-duplex is permitted. • No Parent-Child process relationship. • Named pipes continue to exist after processes exits. • UNIX treats pipes as special FIFO files, requiring file descriptors. • mkfifo ( char *pathname ) command creates file. • Use standard open ( ) to get file descriptor and then use read ( ) and write ( ) system calls. • Both sides of fifo file must be opened, else one side blocks on open until another process opens the fifo file. • Processes must reside on same computer system. • Windows bidirectional, full-duplex permitted. • Processes can reside on the same or different computer systems. • CreateNamedPipe ( ) creates the named pipe. • ConnectNamedPipe ( ) connects to named pipe. • ReadFile() and WriteFile() used to manipulate the named pipe. SILICON VALLEY UNIVERSITY CONFIDENTIAL

25. Processes • Summary • Process is a Unit of Work in the Operating System. • System and User Processes Execute Concurrently with Operating System Multiplexing CPU Between Processes. • Operating System Placed Processes in Queues. • Two major classes of queues: I/O Request Queue and Ready Queue. • Scheduling the Queues: • Long-Term Scheduler, Short-Term Scheduler, Medium-Term Scheduler. • Parent / Child Process • Executes Concurrent or Parent Waits for Child Terminate. • Inter-Process Communication • Shared Memory and Message Passing • Client-Server Systems • Sockets, Remote Procedure Calls (RPC), Pipes (Ordinary and Named) SILICON VALLEY UNIVERSITY CONFIDENTIAL