. Fragmentation is of no consequence if blocks are of fixed size

. Blocks can not be allocated even if space is available

In explicit allocation of fixed size blocks, internal fragmentation can occur, that is, the heap may consist of alternate blocks that are free and in use, as shown in the figure. The situation shown can occur if a program allocates five blocks and then de-allocates the second and the fourth, for example. Fragmentation is of no consequence if blocks are of fixed size, but if they are of variable size, a situation like this is a problem, because we could not allocate a block larger than any one of the free blocks, even though the space is available in principle. So, if variable- sized blocks are allocated, then internal fragmentation can be avoided, as we only allocate as much space as we need in a block. But this creates the problem of external fragmentation, where enough space is available in total for our requirements, but not enough space is available in continuous memory locations, as needed for a block of allocated memory. For example, consider another case where we need to allocate 400 bytes of data for the next request, and the available continuous regions of memory that we have are of sizes 300, 200 and 100 bytes. So we have a total of 600 bytes, which is more than what we need. But still we are unable to allocate the memory as we do not have enough contiguous storage. The amount of external fragmentation while allocating variable-sized blocks can become very high on using certain strategies for memory allocation. So we try to use certain strategies for memory allocation, so that we can minimize memory wastage due to external fragmentation. These strategies are discussed in the next few slides.