abdude824's blog

By abdude824, 13 months ago, In English

If you are struggling with the right track and a 10-min read for Operating systems from beginning to end, you're at the right place. Even I am not sure where to start, but we would figure it and if you are reading this, I have successfully completed these notes or at least am on right track.

Let's start our struggle for OS:

The Track

I found many resources to learn, let's just list all: (Galvin Concise PPTs)[https://www.os-book.com/OS9/slide-dir/index.html]: These were great but I felt that these are a little bit too much, so here are the chapters we would do:

  1. Processes
  2. Threads
  3. Process Synchronization
  4. CPU Scheduling Algorithms
  5. Deadlocks
  6. Main memory
  7. Virtual memory
  8. Virtual Machines

I am skipping the introduction of OS for now as it was not that important, this is going to be a fast article that works like a last night dose.

A simple introduction(May skip if you are already familiar)

Processes

Processes are a program in execution. It has its own memory which is divided into four parts: Text, Data, Heap, and Stack.

Brief explanation about these

Process, as the name suggests, have different states (just like making a joint). Here is a diagram which says it all:

Diagram

Process Control Block(PCB)

It is a data structure that stores all information about a process.

It contains these:

Process Scheduling Queues

This handles the order of execution of the process. We have different queues for different states. When the state of a process is changed, it is simply detached from the current queue and added to its new state's queue.

This says it all

Schedulers

There are three types of schedulers, long term, short term, and mid term schedulers.

Brief explanation

Interrupts and Context Switch

An interrupt is like a signal to stop the current process and allow the higher priority process(the process causing the interrupt) to execute. But what happens to the previous process which CPU was executing? It gets saved! When an interrupt occurs, the system needs to save the current context of the process currently running on the CPU so that it can restore that context. It's like a bookmark we have while reading novels. For example, if you are reading a book on data structures and algorithms and suddenly you realize that watching that Avengers 10 min scene would be far more interesting, you place a bookmark and return after 2 hours to resume the same.

The context is saved in the PCB

Also, please note that taking up a new process requires saving the context of the current process and restore the context of the incoming process. This is known as context switch.

Inter-Process Communication

Inter-Process Communication or IPC for short is used for transferring information from one process to another.

Why we need IPC?

We have studied PCBs. We know Process information is stored in PCBs. And we need some sort of a temporary variable to transfer information(like we do to swap two numbers). And we do have two types of mechanisms on this type of principle.

IPC different mechanisms

Scheduling Algorithms

Terminology

Let's start with algorithms.

First Come First Serve(FCFS)
Shortest Job First(SJS)
Round Robin Algorithm

Process synchronization

Processes categorized on basis of synchronization

Process synchronization problem arises in the case of Cooperative process also because resources are shared in Cooperative processes.

Race Condition

When more than one process is executing the same code or accessing the same memory or any shared variable in that condition there is a possibility that the output or the value of the shared variable is wrong so for that all the processes doing the race to say that my output is correct this condition known as a race condition. Several processes access and process the manipulations over the same data concurrently, then the outcome depends on the particular order in which the access takes place.

Example

Critical Section

A critical section is a code segment that can be accessed by only one process at a time. This code segment is common in many processes and if many processes run simultaneously, we would have a hard time finding the process containing the error, if it happens.

Any solution to the critical section must satisfy these rules.
Here is what critical section looks like

Different Solutions to synchronization problems

1) Disabling Interrupts

We know we can have a race condition if we have some interrupt in a preemptive scheduling algorithm(in a single processor system. In a multiprocessor system, we can have a race condition if two processes on different processors execute the critical section). A process can simply announce that it is entering a critical section and the processor should not interrupt it. Well, it works fine, doesn't it? NO, of course not. This can lead to a lot of problems. Firstly, it is not applicable for multiprocessor systems. Secondly, we cannot give a process the freedom to block the interrupts. It can go on indefinitely inside the critical section, disabling interrupts forever.

2) Locks (or Mutex)

Here we have a lock. We acquire the lock, run a process in the critical section and then release the lock. How it's different from Disabling Interrupts? Here only the process which wants to execute inside the critical section wait, all other processes can still interrupt.

There are two types of implementations:

  1. Software: Peterson solution for two processes and Bakery Algorithm for multiple processes.

  2. Hardware: Generally used, because it's faster. We have test and lock instructions and much more.

Peterson's Solution

As we saw earlier, we need a solution for the critical section of code, as it can lead to anomalies. This solution should satisfy the rules we mentioned before.

Simple Psuedo code
How it works

3) Semaphores

Semaphore is nothing but an integer variable, and this can be changed by using only these two operations:

  1. Wait: It is like a decrement operation.
wait(S){
   while(S<=0);
   S--;
} 
  1. Signal:
Signal(S){
   S++
} 

Semaphores can be counting or binary(0 or 1).

Binary Semaphores are generally called mutex locks as they provide mutual exclusion.

Counting Semaphores are used to control access to a given resource for multiple processes.

Use of semaphores in handling critical section of N processes
How does this work?

Busy Waiting problem's solution

problem with semaphores
How we achieve busy waiting problem

4) Monitors

Deadlocks

Perfect example for deadlock
Okay, a serious example for deadlock
Conditions for a deadlock

Methods for deadlock prevention

  1. Deadlock Prevention or Avoidance: We have to prevent any of the four conditions mentioned above.
Let's see which one we can avoid.

2) Deadlock detection and recovery

3) Ignore Deadlock and Reboot the system

Example of this prevention

Problems were some processes along with resources and you have to find the order of execution or which process to remove to avoid deadlocks can be solved by these deadlock resolving techniques. We will be discussing some problems also

Banker's algorithm for avoiding Deadlocks

See the below table: Here initially, we had total resources of type A=10, B=5, C=7. (See it as A printers, B keyboards, C CPUs)

alt text

Here is the terminology used here:

  1. Allocation: Resources already allocated by the system

  2. Max Need: Need of resources by processes. (although we saw in the deadlock avoidance that pre-computing how many resources are needed by a process is not possible but still, if we somehow know the maximum need)

  3. Available: Resources available at a particular time. Here initially we had 10 types of A resources but at the point, we came, the system had already allocated some resources leaving us the mentioned resources.

  4. Remaining Need: We already have allocated some resources. The remaining need is just the maximum need-Allocated.

There can be two things here: detection and sequence of processes. We have to output a sequence of processes where deadlock not happens or say it's impossible.

Now, it's very simple from here. We have the remaining need for every process. We just now have to iterate sequentially and see which process we can execute. If it's executable, we can just execute it and we have the allocated resources of that particular process and the process is executed completely. If we come in a situation where we can't execute any process, we welcome the deadlock.

I will be making a Codeforces problem also on it. You can visualize this as a completely greedy algorithm. But as far as the practical application of this algorithm goes, as we saw previously also, we cannot have the max need of processes. We don't know which process wants what resource and for how long and thus, it's not that practical.

Threads

We saw in types of OS(in the introduction at the start of the blog) that we have multithreading as well. We have multiple threads running in an interleaved fashion inside a process. The foremost advantage is the increased responsiveness of the CPU. A thread is the smallest unit assignable to the CPU.

Interesting thing about threads
Advantages and Disadvantages of threads

Types of Threads

  1. User Level Thread: A process is creating multiple threads and the kernel is not at all aware of these threads being created. Here we don't need to call the OS and there are no system calls involved making it much faster for context switching. But we have both advantages and certain disadvantages as well. We get fast context switching, and very fast creation and termination time because of no involvement of kernel. But if even a single threads make a call that waits, then all of the other threads in the process have to wait for that call to complete. This is because the call would be made to kernel obviously, and kernel considers the thread as the whole process and would not entertain another call of the same process thread.

  2. Kernel Level Threads: Kernel knows and manages the threads. OS kernel provides system calls to create and manage threads. Kernel Level threads require the involvement of the kernel, which means that you have to do a MOR switch(Google it), you need to go from User space to the kernel space, and then you need to do the switch. And then we have to schedule some other thread as well.

Difference between the two

Feature User Level Thread Kernel Level Threads
Management In User space In kernel space
Context Switching Fast Slow
Blocking One thread might block all other threads A thread block itself only
Multicore or Multiprocessor Cannot take advantage of multicore system. Only concurrent execution on single processor Take full advantage of the multicore system.
Creation/Termination Fast Slow

Mapping of User threads to Kernel threads

  1. One to One: Most common way, used in common Operating Systems like windows. Every user thread is mapped to one kernel thread. Very similar to a pure kernel thread. They are slower than many to one but we can use multicore systems efficiently here.
  1. Many to One: Many users are mapped to one kernel thread. They can be seen as pure user threads. They do provide fast context switching along with fast creation and termination but we cannot utilize multicore systems and all the threads can be blocked by one thread making a system call.

  2. Many to Many: Very rarely used. Many user threads are mapped to many kernel threads.

Memory Management

 
 
 
 
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13 months ago, # |
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Thanks for this! However i think if these all can be provided in pdf form /similar it would be of much more help.

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    13 months ago, # ^ |
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    Actually I love the spoiler function of codesforces(

    This one

    )

    This enables efficient reading and I love to revise with it afterwards.

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13 months ago, # |
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thanks, next DBMS please.

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    13 months ago, # ^ |
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    Would also like to read some notes on the history of the Seleucid Empire.

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      13 months ago, # ^ |
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      ROFL

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      13 months ago, # ^ |
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      Yes, please can you share some? Also of the Greek, Macedonian and the Achamenid empires, if you have. Thank you.

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      13 months ago, # ^ |
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      would post on national geographic blog...

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      13 months ago, # ^ |
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      World history (ed. Zhukov) in 10 volumes, volume 2, page 247

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    13 months ago, # ^ |
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    Actually, I have already done DBMS in the past, and DBMS is easier and there are already many structured blogs for that. Making another one would be replicating them only. I couldn't find structured blogs for OS with which I could do a last day revision.

    Thanks

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13 months ago, # |
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It would be great if you could share a bit about CN as well. Thanks for this though.

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    13 months ago, # ^ |
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    Would do in my placement season, when I actually have to learn about computer networks also.

    Thanks.

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13 months ago, # |
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Got the real-world scenario of deadlock from Perfect example of deadlock. lol

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13 months ago, # |
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please provide for dbms and cn also

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13 months ago, # |
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What is this gag diagram?

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    13 months ago, # ^ |
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    I will upload these, it's just the timeline of processes.

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13 months ago, # |
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Thanks for the blog. Helped a lot for last time revision.

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13 months ago, # |
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kamaal

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13 months ago, # |
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Amazing notes. Can be helpful for all the non-CSE students preparing for interviews.

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13 months ago, # |
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thanks for this. I bookmarked the URL. Super easy to revise and learn. Thank you :)