Memory Allocation

• One of the simplest methods for allocating memory is to divide memory into several fixed-sized partitions. Each partition may contain exactly one process.
• Thus, the degree of multiprogramming is bound by the number of partitions. In this multiple-partition method,
  • When a partition is free, a process is selected from the input queue and is loaded into the free partition.
  • When the process terminates, the partition becomes available for another process.
• This method is no longer in use.
• The method described next is a generalization of the fixed-partition scheme (called MVT); it is used primarily in batch environments. In the fixed-partition scheme,
  • The OS keeps a table indicating which parts of memory are available and which are occupied.
  • Initially, all memory is available for user processes and is considered one large block of available memory, a hole.
  • When a process arrives and needs memory, we search for a hole large enough for this process.
  • If we find one, we allocate only as much memory as is needed, keeping the rest available to satisfy future requests.
• At any given time, we have a list of available block sizes and the input queue. The OS can order the input queue according to a scheduling algorithm.
• When a process terminates, it releases its block of memory, which is then placed back in the set of holes. If the new hole is adjacent to other holes, these adjacent holes are merged to form one larger hole.
• This procedure is a particular instance of the general dynamic storage-allocation problem, which concerns how to satisfy a request of size $n$ from a list of free holes. There are many solutions to this problem.
  • First fit. Allocate the first hole that is big enough. Searching can start either at the beginning of the set of holes or where the previous first-fit search ended. We can stop searching as soon as we find a free hole that is large enough.
  • Best fit. Allocate the smallest hole that is big enough. We must search the entire list, unless the list is ordered by size. This strategy produces the smallest leftover hole.
  • Worst fit. Allocate the largest hole. Again, we must search the entire list, unless it is sorted by size. This strategy produces the largest leftover hole, which may be more useful than the smaller leftover hole from a best-fit approach.
• Simulations have shown that both first fit and best fit are better than worst fit in terms of decreasing time and storage utilization.
• Neither first fit nor best fit is clearly better than the other in terms of storage utilization, but first fit is generally faster.