Currently, we use an `UnrefQueue` which supposedly aims to transfer destruction costs to another thread. The implications of this unusual pattern can not be well understood because some allocators might free resources more efficiently if they are freed by the same thread that allocated them. In addition, `UnrefQueue` complicates the validation of thread safety in our engine. Lastly, our current usage of `UnrefQueue` triggers `ref()`, `unref()` atomic operations as objects are passed into the queue. These operations could be more expensive than the actual destruction.
With this PR we thus remove `UnrefQueue`. We can expect these changes to have a minor impact at most because we free most resources elsewhere in `ResourceSystem::updateCache`.
We can take elsid's commit 605cb8d further by avoiding the terrain sync completely in most cases. Currently in changeCellGrid we wait for a new preloading task to ensure the getPagedRefnums for the new active cells have been filled in by object paging. This is usually not necessary because we have already completed a preload in the past containing these active cells. With this PR we remember what we preloaded and skip the terrain sync if it is not needed.
This reduces average time spent on in. 5 milliseconds as a base precision is
quite a lot considering that for 60 FPS frame time is 1000/16 = ~16.67 ms
when it's a cell loading frame and there is more important work to do rather
than sleeping.
A dry run takes about ~1.5ms. Even though it's all done in the worker thread, the locks used can stall loading operations that are about to happen in other threads, and just in general this CPU load is unnecessary.
Disable 'preload fast travel' by default.
Add 'min cache size' and 'max cache size' settings.
Split the 'cache expiry delay' into 'preload cell expiry delay' and 'cache expiry delay'.
