In computer science, a program or architecture is scalable if it doesn't totally vomit up a lung as its constituent parts become bigger. For a cleaner definition, see Wikipedia, source of all internet knowledge.
An ant is not scalable - if you made an ant 100 times larger in every dimension, its tiny legs would break under its new weight. (An ant is not scalable because its weight grows faster than its structural strength. Thus elephants are not built like ants.) Geeks: scalability is to computer science as marginal cost is to economics.
Before X-Plane 940, the apt.dat file was distinctly not scalable. The entire file was loaded into memory; as users created more and more taxiway lines and signs and details, we simply used more and more memory. This approach isn't very scalable because authors have the potential to grow the apt.dat file faster than our system requirements can increase.
X-Plane 940 fixes this by not loading the entire apt.dat file into memory. Instead, only essential airport information is loaded into memory, along with a note as to where in the file the airport lives. Whenever an airport actually has to be built into a 3-d mesh while you fly, we go back to the apt. file and load the rest of the data for the one airport we are building, use it, and throw it out. Since 3-d airport meshes are built on a second core, the cost of loading one airport off disk is pretty harmless.
The problem with this fix is that it introduced a new scalability problem. Consider:
- Meshes in 940 are built on as many CPU cores as you have - some users have 8!
- Each CPU core could be working on a different airport, depending on how many are nearby.
- Each airport has to load up the apt.dat file to get the extra airport data.
This is of course completely silly - there's no reason to load the whole apt.dat file to get one airport, and the fix that is going into beta 8 lets the airport loader surgically grab just one airport. Thus we will be scalable again, because adding more cores won't cause memory usage to go up.
Beta Users: Please keep an eye out for X-Plane running out of memory - if it starts to do so in the next beta it means that some part of this code change munged memory management. We're running stress tests on the sim now, but touching the low level memory nd file handling code late in beta isn't something I like to do.
* While loading 8 copies of the apt.dat file is wasteful of memory, it is not slow; X-Plane uses memory mapped file I/O, so reading a small part of a large file is very fast - just not very virtual-memory efficient.