Write short notes on:

  1. IPC in Linux
  2. Disk access

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 IPC in Linux

Inter-Process Communication (IPC) refers to a mechanism, where the operating systems allow various processes to communicate with each other. This involves synchronizing their actions and managing shared data. On modern systems, IPCs form the web that binds together each process within a large scale software architecture.

Let me explain in an example. suppose you are working in a company and you are the team lead where you are responsible for a total project or it is nothing but a job for you which you have to complete it. If you will only working on this project, obviously it will take time for you to complete and also you will be tired of working continuously and also you can’t do perfectly because of no rest. So to complete it you will create a team with some of your team members and divide this work into different sub-modules and each engineer will be responsible for a particular work. which will be easy, efficient, productive, etc.

If we will say in simple, in the above example each work given by each people is nothing but a process. How a human is having its own brain to make a decision as per the direction and memory to store the information. Whenever his boss will ask for anything he can reply with the answer like this a Process is having its own control and memory with execution capability.

Disk Access Time:
Disk Access Time is defined as the total time required by the computer to process a read/write request and then retrieve the required data from the disk storage.

Disk Access Time is divided into 2 parts:

  1. Access Time
  2. Data Transfer Time
Disk Access Time = Access Time + Data Transfer Time

1. Access Time:
Access Time is defined as the setup time before the actual data transfer takes place.
For example, the read/write head is on track 1 but we need to read data from another track or segment. Thus, the read/write head will move to the data block location before the actual transfer can take place. This delay is called Access Time.

Access Time is calculated by summation of the following:

(a). Seek Time
(b). Rotational Latency
(c). Command Processing Time
(d). Settle Time

These are explained as following below in brief.

  • (a). Seek Time –
    It is the time required by the read/write head to move from the current track to the requested track.

    Seek Time 
    = (Number of tracks/cylinders crossed) * (Time to cross one track/cylinder)
  • (b). Rotational Latency –
    It is the time required by the read/write head to move from the current sector to the requested sector.

    Rotational Latency 
    = (Angle by which disk is rotated) / (Angular Frequency)
  • (c). Command Processing Time –
    It is the time required by the disk device to process the command and establish a connection between the various components of the disk device to read/write data. It is due to the internal circuitry.
  • (d). Settle Time –
    Settle Time is the time required by reading/write head to stop vibrating.

Note: Command Processing Time and Settle Time are not normally mentioned in the numerical questions. We take them as zero.

2. Data Transfer Time:
Data Transfer Time is defined as the time required to transfer data between the system and the disk.
Data Transfer Time is of two types:

(a). Internal Transfer Rate
(b). External Transfer Rate

These are explained as following below in brief.

  • (a). Internal Transfer Rate –
    It is defined as the time required to move data between the disk surface and the hard disk cache.
  • (b). External Transfer Rate –
    It is defined as the time required to move data between the hard disk cache and the system.
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