[Operating System] Process Management (2)

Process Management (2)

Scheduling Queues

Ready queue

• Processes in ready state
  • Requesting processor, all other resources allocated
• Ready queue (ready list)
• Process scheduling
  • Selecting a process from the ready list and dispatch it when the processor is available

I/O queue

• Processes in asleep state
  • Each requesting different resources
• Block list (block queue, I/O queue)
  • Separate list for each resource

Schedulers

Classification of schedulers

• Long-term scheduler (job scheduler)
  • Controls the multiprogramming degree
  • Selects a good process mix
    • I/O-bound processes and CPU-bound processes
  • Not used on some systems
• Medium-term scheduler
  • Swapping
• Short-term scheduler (process scheduler)
  • Considers several performance indices such as throughput and response time

Interrupts

: Unexpected (external) event

• Kinds of interrupts
  • I/O interrupt
  • Clock interrupt
  • Console interrupt
  • Machine check interrupt
  • Inter-process communication interrupt
  • Etc

Interrupt handling

• Interrupt hadling process

• Interrupt > Context saving > Interrupt handling > Interrupt service > Context restoring

Interrupts and Traps

• System Calls, Interrupts, and Exceptions

Context Switching

Context

• Set of information related to a process
  • User-level context
  • System-level context
  • Register context

Context switching

• Context saving
  • Saving the register context of a process when the process releases processor
• Context restoring
  • Reloading the register context of a process to continue execution
• Context switching
  • Saving the context of a process and restoring the context of another process

Information in PCB

• PID (Process Identification Number)
• Process state
• Scheduling information
  • Process priority, scheduling parameters, etc.
• Memory management information
  • Base/limit registers, page tables, segment tables, etc.
• I/O status information
  • List of I/O devices allocated, list of open files, etc.
• Accounting information
• Context save area
  • Space for saving register context of the process

Context switch time

• Pure overhead
• Highly dependent on hardware support
  • Some processors provide multiple sets of registers
    • Context switch by changing the pointer to the current register set
    • Sun UltraSPARC

• $P_A$ running > interrupt > [ Context saving > IH > ISR > Scheduling > Context restoring ] > $P_B$ running

IPC (Inter-Process Communication)

• Reasons for process cooperation
  • Information sharing
  • Computation speedup
  • Modularity
  • Convenience
• Schemes
  • Message passing and shared memory
  • In distributed systems
    • Message passing
    • RPC(Remote Procedure Call)
    • DSM(Distributed Shared Memory)

Message Passing

• Primitives
  • Direct/indirect communication
    • send(P, message), receive(Q, message)
    • send(mailbox, message), receive(mailbox, message)
• Issues
  • Synchronization mechanism
    • Blocking/non-blocking send/receive
  • Buffering mechanism
  • Etc

Shared Memory

• Primitives
  • Indirect communication
    • read(buf, message), write(message, buf)

RPC (Remote Procedure Call)

• A way to abstract the procedure call mechanism for use between systems with network connections
  • Allows a client to invoke a procedure on a remote host as it would invoke a procedure locally
  • Hides the communication-level details
• Usually built on top of the message passing framework
• Terminologies
  • Client stub, server stub
  • Marshalling, unmarshalling
  • Matchmaker (rendezvous)
  • XDR(eXternal Data Representation)

RMI (Remote Method Invocation)

• Java feature similar to RPC
  • Object-based
  • Allows a thread to invoke a method on a remote object
• Built on JVM
• Terminologies
  • Stub, skeleton
  • Parcel

LRPC (Lightweight RPC)

• Supports communication between two processes on the same machine
• Similar to standard RPC
  • Optimized for local communications

DSM (Distributed Shared Memory)

• Also called DSVM(Distributed Shared Virtual Memory)
• A shared-memory paradigm applied to loosely-coupled distributed memory systems
  • An abstraction that integrates the local memory of different machines in a network environment into a single logical entity shared by cooperating processes executing on multiple hosts
    • A software layer on top of the message-passing communication system to provide a shared-memory abstraction to the programmers (in loosely-coupled distributed systems)
  • Can be implemented either in the OS kernel or in runtime library routines with proper system kernel support

Summary

• Process concept
  • An entity that is registered to the kernel
  • Can request/allocate/release resources in the system
• Process state transition
• PCB
  • Area for maintaining information about each process
• Scheduling queues and schedulers
• Interrupt, context saving, context switching
• Inter-process communication
  • Message passing, RPC/RMI/LPC
  • Shared memory