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.
Consider an example when two trains are coming toward each other on same track and there is only one track, none of the trains can move once they are in front of each other. Similar situation occurs in operating systems when there are two or more processes hold some resources and wait for resources held by other(s). For example, in the below diagram, Process 1 is holding Resource 1 and waiting for resource 2 which is acquired by process 2, and process 2 is waiting for resource 1.
How to avoid Deadlocks
Deadlocks can be avoided by avoiding at least one of the four conditions, because all this four conditions are required simultaneously to cause deadlock.
- Mutual Exclusion
Resources shared such as read-only files do not lead to deadlocks but resources, such as printers and tape drives, requires exclusive access by a single process.
- Hold and Wait
In this condition processes must be prevented from holding one or more resources while simultaneously waiting for one or more others.
- No Preemption
Preemption of process resource allocations can avoid the condition of deadlocks, where ever possible.
- Circular Wait
Circular wait can be avoided if we number all resources, and require that processes request resources only in strictly increasing(or decreasing) order.
Handling Deadlock
The above points focus on preventing deadlocks. But what to do once a deadlock has occurred. Following three strategies can be used to remove deadlock after its occurrence.
- Preemption
We can take a resource from one process and give it to other. This will resolve the deadlock situation, but sometimes it does causes problems.
- Rollback
In situations where deadlock is a real possibility, the system can periodically make a record of the state of each process and when deadlock occurs, roll everything back to the last checkpoint, and restart, but allocating resources differently so that deadlock does not occur.
- Kill one or more processes
This is the simplest way, but it works.
Memory Hierarchy
Design and its Characteristics
In the Computer
System Design, Memory Hierarchy is an enhancement to organize the memory such
that it can minimize the access time. The Memory Hierarchy was developed based
on a program behaviour known as locality of references. The figure below
clearly demonstrates the different levels of memory hierarchy:
This Memory
Hierarchy Design is divided into 2 main types:
External Memory or
Secondary Memory –
Comprising of Magnetic Disk, Optical Disk, Magnetic Tape i.e. peripheral
storage devices which are accessible by the processor via I/O Module.
Internal Memory or
Primary Memory –
Comprising of Main Memory, Cache Memory & CPU registers. This is directly
accessible by the processor.
We can infer the
following characteristics of Memory Hierarchy Design from above figure:
Capacity:
It is the global volume of information the memory can store. As we move from
top to bottom in the Hierarchy, the capacity increases.
Access Time:
It is the time interval between the read/write request and the availability of
the data. As we move from top to bottom in the Hierarchy, the access time increases.
Performance:
Earlier when the computer system was designed without Memory Hierarchy design,
the speed gap increases between the CPU registers and Main Memory due to large
difference in access time. This results in lower performance of the system and
thus, enhancement was required. This enhancement was made in the form of Memory
Hierarchy Design because of which the performance of the system increases. One
of the most significant ways to increase system performance is minimizing how
far down the memory hierarchy one has to go to manipulate data.
Cost per bit:
As we move from bottom to top in the Hierarchy, the cost per bit increases i.e.
Internal Memory is costlier than External Memory.
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