Question

Operating Systems Questions

1. What is the difference between a blocked state and a deadlock state?

2. How do you define the state of the operating system?

3. Differentiate between daemons and zombies.

4. What is the maximum number of process in a standard Linux installation? Why is the number of processes limited to this number? Can it be changed?

Answer 1

`Hey,

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1)

Deadlock occurs when one process is blocked and waiting for a second process to complete its work and release locks, while the second process at the same time is blocked and waiting for the first process to release the lock.

Block (or blocking lock) occurs when two processes need access to same piece of data concurrently so one process locks the data and the other one needs to wait for the other one to complete and release the lock. As soon as the first process is complete, the blocked process resumes operation. The blocking chain is like a queue: once the blocking process is complete, the next processes can continue. In a normal server environment, infrequent blocking locks are acceptable. But if blocking locks are common (rather than infrequent), there is probably some kind of design or query implementation problem and the blocking may simply be causing performance issues.In a deadlock situation, the processes are already blocking each other so there needs to be an external intervention to resolve the deadlock. For that reason, SQL Server has a deadlock detection and resolution mechanism where one process needs to be chosen as the “deadlock victim” and killed so that the other process can continue working. The victim process receives a very specific error message indicating that it was chosen as a deadlock victim and therefore the process can be restarted via code based on that error message.

2)

States of a process are as following:

• New (Create) – In this step, the process is about to be created but not yet created, it is the program which is present in secondary memory that will be picked up by OS to create the process.
• Ready – New -> Ready to run. After the creation of a process, the process enters the ready state i.e. the process is loaded into the main memory. The process here is ready to run and is waiting to get the CPU time for its execution. Processes that are ready for execution by the CPU are maintained in a queue for ready processes.
• Run – The process is chosen by CPU for execution and the instructions within the process are executed by any one of the available CPU cores.
• Blocked or wait – Whenever the process requests access to I/O or needs input from the user or needs access to a critical region(the lock for which is already acquired) it enters the blocked or wait state. The process continues to wait in the main memory and does not require CPU. Once the I/O operation is completed the process goes to the ready state.
• Terminated or completed – Process is killed as well as PCB is deleted.
• Suspend ready – Process that was initially in the ready state but were swapped out of main memory(refer Virtual Memory topic) and placed onto external storage by scheduler are said to be in suspend ready state. The process will transition back to ready state whenever the process is again brought onto the main memory.
• Suspend wait or suspend blocked – Similar to suspend ready but uses the process which was performing I/O operation and lack of main memory caused them to move to secondary memory.
  When work is finished it may go to suspend ready.

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