1. 1

2.  Introduction
 Why Buddy System
 About Buddy System
 Types of Buddy System
 Implementation in Linux
 Pros and Cons
 Conclusion

3.  According to Donald Knuth, the buddy system was
invented in 1963 by Harry Markowitz, who won the
1990 Nobel Memorial Prize in Economics.
 It was first described by Kenneth C.
Knowlton(published 1965).
 Now a days Linux uses the buddy system to manage
allocation of memory, possibly because it is allocating
many structures which are already powers of two, like
frames.

4.  The buddy memory allocation technique is a memory
allocation algorithm that divides memory into partitions to
try to satisfy a memory request as suitably as possible.
 This system makes use of splitting memory into halves to
try to give a best-fit.
 Compared to the more complex memory allocation
techniques that some modern operating systems use, buddy
memory allocation is relatively easy to implement.
 It supports limited but efficient splitting and coalescing of
memory blocks.

5.  A fixed partitioning scheme limits the number of
active processes and may use space inefficiently if
there is a poor match between available partition size
and process size
 A dynamic partitioning scheme is more complex to
maintain and includes the overhead of compaction.
 An interesting compromise of fixed and dynamic
partitioning is the buddy system.

6.  The buddy system(binary) allows a single allocation
block to be split, to form two blocks half the size of the
parent block. These two blocks are known as 'buddies'.
 Part of the definition of a 'buddy' is that the buddy of
block B must be the same size as B, and must be adjacent
in memory (so that it is possible to merge them later).
 The other important property of buddies, stems from the
fact that in the buddy system, every block is at an address
in memory which is exactly divisible by its size.
 So all the 16-byte blocks are at addresses which are
multiples of 16; all the 64K blocks are at addresses which
are multiples of 64K... and so on.

7.  There are number of buddy systems, proposed by
researcher, which are capable of reducing
execution time and increase memory utilization.
Four Types of Buddy System
 Binary buddy system
 Fibonacci buddy system
 Weighted buddy system
 Tertiary buddy system

8.  These three Buddy Systems are similar in the design of
the algorithm, the major difference is the sizes of the
memory blocks.
 It also differs in memory utilization and execution
time.
 In some situations, one buddy system looks good, may
not be good in other situation.
 It simply lies on the requests for memory which causes
external and internal fragmentation higher at some
situations.

9.  In binary buddy system the memory block of 2m is
into two equal parts of 2m-1.
 It satisfies the following recurrence relation
Li = Li-1+ Li-1
8
4 4
2 2 2 2

10.  The memory consists of a collection of blocks of
consecutive memory, each of which is a power of two
in size.
 Each block is marked either occupied or
free, depending on whether it is allocated to the user.
 For each block we also know its size .
 The system provides two operations for supporting
dynamic memory allocation:
 1. Allocate (2k): Finds a free block of size 2k, marks it
as occupied, and returns a pointer to it.
 2. Deallocate (B): Marks the previously allocated block
B as free and may merge it with others to form a larger
free block.

11.  The buddy system maintains a list of the free blocks of
each size (called a free list), so that it is easy to find a
block of the desired size, if one is available.
 If no block of the requested size is available, Allocate
searches for the first nonempty list for blocks of at
least the size requested.
 In either case, a block is removed from the free list.
 This process of finding a large enough free block will
indeed be the most difficult operation for us to perform
quickly.

12.  If the found block is larger than the requested size, say
2k instead of the desired 2i, then the block is split in
half, making two blocks of size 2k−1.
 If this is still too large (k − 1 > i),then one of the
blocks of size 2k−1 is split in half.
 This process is repeated until we have blocks of size
2k−1, 2k−2, . . . , 2i+1, 2i, and 2i.
 Then one of the blocks of size 2i is marked as occupied
and returned to the user.
 The others are added to the appropriate free lists.
 Each block B1 was created by splitting another block
into two halves, call them B1 (Buddy of B2) and
B2(Buddy of B1).

13.  Now when a block is deallocated, the buddy system checks
whether the block can be merged with any others or more
precisely whether we can undo any splits that were
performed to make this block.
 The merging process checks whether the buddy of a
deallocated block is also free, in which case the two blocks are
merged;
 then it checks whether the buddy of the resulting block is also
free, in which case they are merged; and so on.

14.  Thus it is crucial for performance purposes to
know, given a block address, the size of the block and
whether it is occupied.
 This is usually done by storing a block header in the
first few bits of the block.
 More precisely, we use headers in which the first bit is
the occupied bit , and the remaining bits specify the
size of the block.
 Eg) To determine whether the buddy of a block is
free, we compute the buddy’s address, look at the first
bit at this address, and also check that the two sizes
match.

15. Example:
Let us consider 1-Mbyte of memory is allocated using
Buddy System. Show the Binary tree form and list form for the
following :
Request 100k(A)
Request 240k(B)
Request 64k(C)
Request 256k(D)
Release B
Release A
Request 75k
Release C
Release E
Release D.

16. 1MB
(A)100 28 128 256 512
(A)100 28 128 (B)240 16 512
(A)100 28 (C)64 64 (B)240 16 512
(A)100 28 (C)64 64 (B)240 16 (D)256 256
(A)100 28 (C)64 64 256 (D)256 256
128 (C)64 64 256 (D)256 256
(E)75 53 (C)64 64 256 (D)256 256
(E)75 53 128 256 (D)256 256
512 (D)256 256
1MB

17. Unused
memory
Used
memory
A=128 C=64 64 256 D=256 256

18.  Hirschberg taking Knuth's suggestion has designed a Fibonacci
buddy system with block sizes which are Fibonacci numbers.
 It satisfies the following recurrence relation :
Li=Li-1 + Li-2.
 0, 1,1, 2, 3, 5, 8,13, 21, 34, 55, 144, 233,377, 610,
987, 1597, 2582…
610
377 233
233 144 144 89

19. The address calculation for the binary and weighted
buddy systems is straight forward, but the original
procedure for the Fibonacci buddy system was either
limited to a small, fixed number of block sizes or a
time consuming computation.
Problem:
Show the results of the following sequence in a
figure using Fibonacci Buddy system:
Request 60 (A); Request 135 (B); Request 70 (C);
Return A; Request 20 (D);Return B; Return C;
Return D.

20. 377 Byte Block 377(144+233)
Request 60 (A) 55 A= 89 233
Request 135 (B) 55 A=89 89 B=144
Request 70 (C) 55 A=89 C=89 B=144
Return A 144 C=89 B=144
Request 60 (D) 55 D=89 C=89 B=144
Return B 55 D=89 C=89 144
Return D 144 C=89 144
Return C 377

21. 377
144 + 233
(55+89)+ (89+144)
128K
64K
55 D=89 C=89 144

22.  In weighted buddy system the memory block of size
2k+2 is split in 3.(2k ) and 2k sized blocks.
 Further 3.(2k) is split in two 2k+1 and 2k sized blocks.
64
48 16
32 16 12 4
24 8 12 4 8 4 3 1

23.  In tertiary buddy system blocks of size 2k is split into
blocks of three different sizes .
 These blocks in turn are split into two different ways
depending on the size of the blocks to be split .
 Blocks of size 2k are split into three blocks of sizes
2k-1 ,3.2k-3 and 2k-3 .
 Blocks of size 3. 2k-3 are split into the blocks of sizes
2k-2 and 2k-3 .
 It decreases the amount of internal fragmentation by
allowing more block sizes .

24.  L i = Li –1 + Li -3 + Lβ(i)
◦ Where β is any function over positive integer with βi
<i.
64
32
24
16 12 4
8
16 8
8 6 2 8 4 4 3 1

25.  In LINUX, Buddy System in Contiguous Page Frame Allocation.
 All page frames are grouped into 10 lists of blocks that
containing groups of 1, 2, 4, 8, 16, 32, 64, 128, 256, and 512
contiguous page frames, respectively
 The address of the first page frame of a block is a multiple of the
group size
 For eg, a request for 128 then
 First checks for a free block in the 128 list
 If no free block, it then looks in the 256 list for a free block
 If it finds a block, the kernel allocates 128 of the 256 page
frames and puts the remaining 128 into the 128 list
 If no block it looks at the next larger list, allocating it and
dividing the block similarly
 If no block can be allocated an error is reported

26.  Less external fragmentation.
 Search for a block of the right size is cheaper
than, best fit because we need only find the first
available block on the block list for blocks of size 2k;
 Merging adjacent free blocks is easy.
 In buddy systems, the cost to allocate and free a
block of memory is low compared to that of best-fit
or first-fit algorithms.

27.  It allows internal fragmentation.
 For example, a request for 515k will require a block
of size 1024k. In consequence, such an approach
gives a waste of 509 k.
 Splitting and merging adjacent areas is a recurrent
operation and thus very unpredictable and inefficient.
 The another drawback of the buddy system is the
time required to fragment and coalesce blocks.