Describe the Buddy system of memory allocation.
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The buddy memory allocation technique allocates memory in
powers of 2, i.e 2x, where x is an integer. Thus, the
programmer has to decide on, or to write code to obtain, the
upper limit of x. For instance, if the system had 2000K of
physical memory, the upper limit on x would be 10, since 210
(1024K) is the biggest allocatable block. This results in
making it impossible to allocate everything in as a single
chunk; the remaining 976K of memory would have to be taken
in smaller blocks.
After deciding on the upper limit (let's call the upper
limit u), the programmer has to decide on the lower limit,
i.e. the smallest memory block that can be allocated. This
lower limit is necessary so that the overhead of storing
used and free memory locations is minimized. If this lower
limit did not exist, and many programs request small blocks
of memory like 1K or 2K, the system would waste a lot of
space trying to remember which blocks are allocated and
unallocated. Typically this number would be a moderate
number (like 2, so that memory is allocated in 2² = 4K
blocks), small enough to minimize wasted space, but large
enough to avoid excessive overhead. Let's call this lower
limit l.
Now that we have our limits, let us see what happens when a
program makes requests for memory. Let's say in this system,
l = 6, which results in blocks 26 = 64K in size, and u = 10,
which results in a largest possible allocatable block, 210 =
1024K in size. The following shows a possible state of the
system after various memory requests.
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