The above process repeats until all particles have been emitted and absorbed.
There are several important factors to observe in order to achieve good
- The vector (or parallel) lengths should be kept sufficiently long so as
to yield good efficiency. Note that, as particles pass the hierarchical
tests (i.e., traverse the ``sieves''), the vector length tends to decrease.
To ensure good overall efficiency, the shortest vector length should be
long enough to obtain good efficiency. A suitable vector length is usually
determined empirically and depends upon: (1) the input parameters (geometry
and material properties) and (2) architecture.
- It is important to maintain long vector lengths by re-filling the vectors
with new emission as particles ``die off.''
- A ``tail'' will inevitably exist at the end of this process, as the
particles remaining after the last emission will need to be traced through
enough events for them all to ``die off.'' Zhong and Kalos 
have identified this as the ``straggler'' problem. We are
fortunate that it exists only at the end of our problem. In some physical
situations, stragglers exist during each event step, requiring
synchronization between events.