Deadlocks & File Systems
Expert Answer & Key Takeaways
Understanding the dangerous state of Deadlocks, the Coffman conditions, and the basics of OS File Management and Disk Scheduling.
Deadlocks in Operating Systems
In a multiprogramming environment, processes compete for resources (like printers, files, memory). A Deadlock is a situation where a set of processes are blocked because each process is holding a resource and waiting for another resource acquired by some other process.
- Example: Process A has a lock on File 1 and needs File 2. Process B has a lock on File 2 and needs File 1. Neither can proceed. They will wait forever.
1. The Four Necessary Conditions for Deadlock
A deadlock can ONLY arise if ALL four of the following conditions (known as Coffman conditions) hold simultaneously in a system:
- Mutual Exclusion: At least one resource must be held in a non-shareable mode. (Only one process can use it at a time).
- Hold and Wait: A process is holding at least one resource and is waiting to acquire additional resources held by other processes.
- No Preemption: Resources cannot be forcibly taken away from a process. They must be released voluntarily by the process holding them.
- Circular Wait: There exists a circular chain of processes, where each process is waiting for a resource held by the next process in the chain.
2. Handling Deadlocks
Operating systems can deal with deadlocks in three ways:
- Deadlock Prevention/Avoidance: Ensure that the system never enters a deadlock state (e.g., Banker's Algorithm).
- Deadlock Detection and Recovery: Allow deadlocks to occur, detect them, and then take action (like killing one of the processes) to recover.
- Deadlock Ignorance (Ostrich Algorithm): Pretend that deadlocks never happen. This is actually used by most operating systems (including Windows and Linux) because deadlocks are rare, and the cost of constantly checking for them is too high.
3. File Systems
A File System is the method and data structure that an OS uses to keep track of files on a disk (HDD/SSD). It organizes data into files and directories (folders).
- File Allocation Methods: How does the OS place a file on the disk?
- Contiguous Allocation: The file is saved in a single, continuous block of disk space. (Fast reading, but causes fragmentation).
- Linked Allocation: Each block of the file contains a pointer to the next block. They can be scattered anywhere on the disk. (Solves fragmentation, but slow to read randomly).
- Indexed Allocation: A special 'Index Block' is created that contains all the pointers to the scattered blocks of the file.
4. Disk Scheduling Algorithms
Just like CPU scheduling, the OS must schedule which read/write requests to the hard disk should be processed first to minimize the physical movement of the disk's read/write head.
- FCFS (First-Come, First-Served): Processes requests in the order they arrive.
- SSTF (Shortest Seek Time First): Processes the request that is closest to the current position of the head.
- SCAN (Elevator Algorithm): The head moves from one end of the disk to the other, servicing requests along the way, then reverses direction.
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