Description Oleg Besedin 2011-09-09 14:04:15 EDT

From preliminary look at 4.x performance tests we spend a sizable portion of time re-calculating dependencies when value in the context change.

This is "as expected", but something that stands out in profiler results is that we spend a good portion of time removing "old" links (which are two-way links) between eclipse context entries and computations (IContextFunctions and IRunAndTrack).

It is possible to reduce the work we spend on "invalidation" of links. The basic idea is to not to remove links between context elements and computations but rather:
 - mark the computation is "invalid" and ignore/overwrite it as we come across it again
 - keep non-active computations referenced via WeakReferences so that they can be garbage collected.

One other idea is that, in practice, in most cases we don't "reduce" the set of dependencies of a computation. I.e., if we had a dependency on A, B, and C, chances are that subsequent calculations will expand the set of elements we depend on, but rarely will contract it. 

Combining those ideas we can:
- replace our two-way links with one way links (we don't need to remove existing dependencies from the computation side)
- keep ValueComputation and their dependencies unless the definition of ICF chnages (we'll simply mark them as "dirty" and re-calculate lazily when asked)
- replace one-by-one removal of listeners with a bulk cleanup (for instance, when > 50% of listeners becomes marked as invalid).
- significantly simply code that links context values and computation.

I used an open&close performance test to gauge the effects of this change. On my computer this results in about 40% improvement in CPU time for that test.