What is dynamic memory allocation? Explain first fit, best fit and worst fit
Dynamic memory allocation is when an executing program requests that the operating system give it a block of main memory. The program then uses this memory for some purpose. Usually the purpose is to add a node to a data structure. In object oriented languages, dynamic memory allocation is used to get the memory for a new object.
The memory comes from above the static part of the data segment. Programs may request memory and may also return previously dynamically allocated memory. Memory may be returned whenever it is no longer needed. Memory can be returned in any order without any relation to the order in which it was allocated. The heap may develop "holes" where previously allocated memory has been returned between blocks of memory still in use.
A new dynamic request for memory might return a range of addresses out of one of the holes. But it might not use up all the hole, so further dynamic requests might be satisfied out of the original hole.
First Fit:
In the first fit approach is to allocate the first free partition or hole large enough which can accommodate the process. It finishes after finding the first suitable free partition.
Advantage:
Fastest algorithm because it searches as little as possible.
Disadvantage:
The remaining unused memory areas left after allocation become waste if it is too smaller. Thus, request for larger memory requirement cannot be accomplished.
Best Fit:
The best fit deals with allocating the smallest free partition, which meets the requirement of the requesting process. This algorithm first searches the entire list of free partitions and considers the smallest hole that is adequate. It then tries to find a hole, which is close to actual process size needed.
Advantage
Memory utilization is much better than first fit as it searches the smallest free partition first available.
Disadvantage
It is slower and may even tend to fill up memory with tiny useless holes.
Worst fit:
In worst fit approach is to locate largest available free portion so that the portion left will be big enough to be useful. It is the reverse of best fit.
Advantage
Reduces the rate of production of small gaps.
Disadvantage
If a process requiring larger memory arrives at a later stage then it cannot be accommodated as the largest hole is already split and occupied.
