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Mid2

Communications in Client-Server Systems

Socket

  • low-level form of communication
  • only allow an unstructured stream of bytes to be exchanged between the communicating threads.

RPC

  • aka Remote Procedure Calls

  • higher level form of communication than Socket

  • transmit parameters which are marshaled

  • RPC abstracts procedure call between processes on networked systems.

  • issues must be dealt in RPC

    • Differences in data representation on the client and server machines
    • the semantics of a call (messages are acted on exactly once or at most once)
    • Communication between a server and a client

Pipes

Concurrency

  • two or more tasks can start, run and complete in overlapping time periods
  • for example multitasking on single-core machine

Parallelism

  • at least two threads are executing simultaneously
  • for example on a multi-core processor.

Lightweight process

  • LWP
  • M:M and Two-level models require communication to maintain the appropriate number of kernel threads allocated to the application
  • an intermediate data structure between the user thread and kernel thread

Thread

User-level thread Kernel-level thread
It is managed by thread library in user space. It is managed by kernel
Faster context switching Slower context switching
Whole process will be blocked if one of threads calls blocking system call. Whole process will not be blocked if one of threads calls blocking system call.
Better utilization on multiprocessors. No advantage on multiprocessors.

Scheduling Algorithm

FCFS

  • First-Come First-Served
  • must be non-preemptive
  • troublesome in time-sharing systems

SJF

  • Shortest-Job_First
  • frequently used in long-term scheduling.

RR

  • Round-Robin
  • for time-sharing systems

Convoy Effect

  • Short process behind long process
  • All the other processes wait for the one big process to get off the CPU
  • Results in lower CPU and device utilization

Affinity

  • Soft affinity

    • possible to migrate between processors

  • Hard affinity

    • run on a specify subset of processors