Non-Contiguous Allocation in Operating System
In computer science, memory allocation refers to the process of assigning a portion of memory to a program or process. Memory allocation can be classified into two types, namely, contiguous allocation and non-contiguous allocation. In this blog, we will focus on the latter, non-contiguous allocation.
Non-contiguous allocation is a method of allocating memory in which memory blocks are assigned to a process in a non-sequential manner. This means that the memory blocks assigned to a process may not be adjacent to each other, but they are still part of the same process. In contrast, contiguous allocation assigns a single, continuous block of memory to a process.
Non-contiguous allocation is implemented using a data structure called a Linked List. The linked list contains a series of memory blocks, each of which is assigned to a process. The linked list contains information about the size of each memory block and whether or not it is currently in use. The operating system uses this information to allocate memory to processes in a non-contiguous manner.
Non-contiguous allocation is necessary because memory fragmentation is a common issue in contiguous allocation. Fragmentation occurs when small blocks of memory are left unallocated after a process terminates. These small blocks cannot be used by any process, as they are too small to be assigned to a new process. This results in a waste of memory and reduces the overall efficiency of the system.
The main advantage of non-contiguous allocation is that it minimizes memory fragmentation, as memory blocks can be allocated to processes in a non-sequential manner. This results in a better utilization of memory and reduces the risk of memory waste. Additionally, non-contiguous allocation provides more flexibility, as the size of the memory blocks can be adjusted to meet the needs of the processes.
Another advantage of non-contiguous allocation is its ability to accommodate larger processes. In contiguous allocation, if a process is too large to fit into a single block of contiguous memory, it cannot be assigned to the process, even if there is enough free memory available in the system. This can result in a lack of memory for the process, leading to an error or crash. However, with non-contiguous allocation, the operating system can assign multiple memory blocks to the process, thereby accommodating larger processes.
Moreover, non-contiguous allocation provides the operating system with the ability to implement dynamic memory allocation, which is the process of allocating memory at runtime. This is in contrast to static memory allocation, in which memory is allocated during the compilation time. Dynamic memory allocation is useful for processes that require a varying amount of memory during their execution. For example, in a process that handles a large amount of data, the memory requirement may change frequently. With non-contiguous allocation, the operating system can allocate the required memory blocks during runtime, ensuring that the process has enough memory to perform its task.
Despite its advantages, non-contiguous allocation can lead to external fragmentation, which occurs when there is enough free memory available in the system, but it is scattered throughout the memory in small blocks, making it difficult to allocate to a single process. This can result in a waste of memory and reduce the overall efficiency of the system. To overcome this problem, various algorithms have been developed to manage external fragmentation, such as the First Fit, Best Fit, and Worst Fit algorithms. These algorithms determine the best way to allocate memory to a process by considering factors such as the size of the memory block, the location of the memory block, and the memory utilization.
In recent years, non-contiguous allocation has been enhanced with the development of virtual memory. Virtual memory is a technology that enables an operating system to simulate additional memory by temporarily transferring data from the RAM to the hard drive. This allows the operating system to allocate memory to processes even when the physical memory is full. With virtual memory, the operating system can allocate memory to processes in a non-contiguous manner, thereby avoiding the problems associated with memory fragmentation and external fragmentation.
However, non-contiguous allocation is not without its disadvantages. One of the main drawbacks of non-contiguous allocation is that it increases the overhead associated with memory management. This is because the linked list must be maintained, which requires additional resources and time. Additionally, non-contiguous allocation may lead to increased processing time, as the operating system must search the linked list to find a suitable block of memory for a process.
Despite these disadvantages, non-contiguous allocation is still widely used in operating systems. It is used in various operating systems such as Windows, Linux, and Unix. In Windows, non-contiguous allocation is implemented using the Virtual Memory Manager (VMM) that provides virtual memory to the processes. In Linux, non-contiguous allocation is implemented using the Memory Management Unit (MMU) that provides virtual memory to the processes.
In conclusion, non-contiguous allocation is a powerful technique in operating systems that allows for better memory utilization and reduces the risk of memory fragmentation. Although it comes with its own set of disadvantages, the benefits it provides make it a popular method for memory management in modern computers. Understanding non-contiguous allocation is essential for anyone looking to understand the inner workings of an operating system and how it manages memory.
