 K-ary trees to the rescue

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Ciro Santilli 34  Updated 2024-12-15  +Created 1970-01-01

The algorithmically minded will have noticed that paging requires associative array (like Java Map of Python dict()) abstract data structure where:

the keys are linear pages addresses, thus of integer type
the values are physical page addresses, also of integer type

The single level paging scheme uses a simple array implementation of the associative array:

the keys are the array index
this implementation is very fast in time
but it is too inefficient in memory

and in C pseudo-code it looks like this:

linear_address[0]      = physical_address_0
linear_address[1]      = physical_address_1
linear_address[2]      = physical_address_2
...
linear_address[2^20-1] = physical_address_N

But there another simple associative array implementation that overcomes the memory problem: an (unbalanced) k-ary tree.

A K-ary tree, is just like a binary tree, but with K children instead of 2.

Using a K-ary tree instead of an array implementation has the following trade-offs:

it uses way less memory
it is slower since we have to de-reference extra pointers

In C-pseudo code, a 2-level K-ary tree with K = 2^10 looks like this:

level0[0] = &level1_0[0]
    level1_0[0]      = physical_address_0_0
    level1_0[1]      = physical_address_0_1
    ...
    level1_0[2^10-1] = physical_address_0_N
level0[1] = &level1_1[0]
    level1_1[0]      = physical_address_1_0
    level1_1[1]      = physical_address_1_1
    ...
    level1_1[2^10-1] = physical_address_1_N
...
level0[N] = &level1_N[0]
    level1_N[0]      = physical_address_N_0
    level1_N[1]      = physical_address_N_1
    ...
    level1_N[2^10-1] = physical_address_N_N

and we have the following arrays:

one directory, which has 2^10 elements. Each element contains a pointer to a page table array.
up to 2^10 pagetable arrays. Each one has 2^10 4 byte page entries.

and it still contains 2^10 * 2^10 = 2^20 possible keys.

K-ary trees can save up a lot of space, because if we only have one key, then we only need the following arrays:

one directory with 2^10 entries
one pagetable at directory[0] with 2^10 entries
all other directory[i] are marked as invalid, don't point to anything, and we don't allocate pagetable for them at all

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