Basic CS-Operating System [TCS Placement]: Sample Questions 11 - 11 of 35

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Question 11

Operating System

Describe in Detail


What is meant by arm-stickiness?


  • If one or a few processes have a high access rate to data on one track of a storage disk, then they may monopolize the device by repeated requests to that track.
  • This generally happens with most common device scheduling algorithms (LIFO, SSTF, C-SCAN, etc) .
  • High-density multisurface disks are more likely to be affected by this than low density ones.
  • Arm stickiness is phenomena in I/O scheduling where the scheduling algorithm continues to service requests at or near the current sector and thus prevents any seeking.
  • The name derives from the arm, which controls the seek head, of the disk being stuck or sticky to a particular part of the platter.
  • In general, minimizing seeking is a positive attribute of I/O scheduling — indeed, minimizing seeking is the primary reason we have I/O schedulers.
  • But if the I/O scheduler aggressively favors requests at or near the current sector and those requests continue to come in, then the system will starve requests at other sectors.
  • Many I/O schedulers, including the basic algorithm we are all taught in OS class, called elevator or SCAN, suffer from some amount of arm stickiness.
  • There are several ways to mitigate this.
  • One is using dual queues as in the FSCAN algorithm.
  • In FSCAN, new requests are stored in a second queue and aren՚t serviced until the first queue is exhausted.
  • Then the queues flip. This prevents requests at the current sector from preventing seeking.
  • A second solution is to implement deadlines, as in Linux՚s deadline I/O scheduler.
  • The deadlines prevent requests from going unserved for a configurable period, forcing a seek once the deadline passes.

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