LARGE-SCALE PARALLEL GRAPH-BASED SIMULATIONS - MATSim

In this section, two STL functions will be tested, namely, list and deque. Because of the
low performance they involve, in addition a user-defined data structure Ring is implemented
and tested.
List
The STL-list is a doubly linked list. With the STL-list, insertion anywhere is fast but
it provides only sequential access. Since random access is not needed, this should in theory
be a fast data structure for the purpose here. Unfortunately, the STL-list comes with high
overhead as explained below.
Deque
The STL-deque (double-ended queue) is similar in usage and syntax to the STL-vector.
It allows random access and inserts elements fast at either end. Therefore the STL-deque
is the data structure of choice when most insertions and deletions take place at the beginning
or at the end of a container. The STL-deque is different from the STL-vector in terms
of memory management. When resizing is necessary, the STL-deque allocates memory in
chunks as it grows, with room for a fixed number of elements in each reallocation. In other
words, the STL-deque uses a series of smaller blocks of memory. The STL-vector, on the
other hand, allocates its memory in a contiguous block, i.e., the STL-vector is represented
by one long block of memory.
Self-implemented Ring data structure
To overcome the inefficiencies of the STL-list and the STL-deque, a new data structure,
Ring is implemented. Ring is a circular vector. Removal is at the beginning and insertion
is at the end. Removing from the beginning of the STL-vector is a costly operation since
it includes the forward movement of all the remaining elements. In order to get rid of this
difficulty, supplementary pointers are used to keep track of the head and the tail of the data
structure. Hence, with this new structure, only head/tail pointers move back and forth, not the
elements as in the STL-vector container. The same pointers are also used for insertion.
Figure 3.11(a) shows how insertion takes place at the end of Ring structure. The supplementary
pointers head and tail are used to keep track of elements. The maximum size of the
structure is 8 in the example. When the size is 0, the head and the tail point to NIL. Then, an
object, O1, is requested to be inserted. A pointer to the object is placed in the first cell of the
structure. Both the head and the tail point to this cell (accordingly O1) after the insertion. Then
another object, O2, is to be inserted. Since the call is push back, it is placed at the end, i.e.,
the next cell after the last item (O1). The tail pointer is advanced. Now, the head points to O1
and the tail points to O2. The current size becomes 2.
Figure 3.11(b) illustrates deletion from the beginning, by using pop front. The structure
is full, i.e., the size is 8. The head points to the first element(O1) and the tail points the last
element (O8). After deletion, the tail remains the same, but the head is advanced to the next
object (O2) from O1. If further deletion is requested, the head is moved one more cell (to O3).
After two deletions, the size becomes 6.
It is important to note that this works because of the fixed maximum size. This corresponds
to the maximum number of vehicles on the link.
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