Switch droids to use PagedEntityContainer<DROID> as backing storage

[ManManson]

Aside from changing droids to be allocated via a seprated PagedEntityContainer<DROID>, this changeset also implements a couple of other important things:

• The number of elements per page in PagedEntityContainer can now be customized
  via MaxElementsPerPage template argument. The default value is 1024 elements.
  NOTE: The storage for droids uses 256 elements per page.
• PagedEntityContainer can now be configured not to reuse slots upon element erasure. In such case, the slot becomes invalid right after first use. The behavior is controlled by ReuseSlots template argument. The default value is true.
  NOTE: The storage for droids disable slot reuse feature, at least until we are sure that other memory-related and dependency-tracking issues between DROID instances are eliminated throughout the WZ code base.

This PR should not introduce any notable performance difference. Although, memory utilization patterns should become slightly better (e.g. there should be less memory fragmentation overall).

Signed-off-by: Pavel Solodovnikov [email protected]

[past-due]

I've pushed a commit to Clear the PagedEntityContainer<DROID> in objmemShutdown() (so memory usage doesn't just continue to grow even after shutting down one match and starting another).

However I still have a concern that, in a long-duration campaign play session for example (progressing through many levels of the campaign), we'll just keep allocating blocks and never freeing them.

I think it might be helpful for PagedEntityContainer to free a block of memory once all of its elements are invalidated, and allocate a new one (when ReuseSlots == false, of course). Maybe by calling allocate_storage(); reset_metadata(); on the page that's fully invalidated?
(Perhaps we can make that another template parameter, if needed, and enable it by default.)

(Only in debug development situations should we just accumulate memory and never free it, IMHO.)

[ManManson]

I think it might be helpful for PagedEntityContainer to free a block of memory once all of its elements are invalidated, and allocate a new one (when ReuseSlots == false, of course). Maybe by calling allocate_storage(); reset_metadata(); on the page that's fully invalidated? (Perhaps we can make that another template parameter, if needed, and enable it by default.)

(Only in debug development situations should we just accumulate memory and never free it, IMHO.)

Generally, I agree, the memory should be recycled in such cases. Although, technically, I have another proposition for how to implement this.

So, to keep the internal housekeeping as simple, as it was before, I think instead of re-initializing existing pages, better to put them into "deactivated" (or "discarded") state indefinitely (that is, free their internal storage) and just allocate new ones at the end.

There's even more to why I think we should just abandon older pages:

Theoretically, we can run into a situation which involves undesired iteration behavior.

Say, we have an iterator I pointing to element E1 at {X, Y} page index (which points into page Z). Suppose, after a while, page Z gets invalidated and later re-initialized, giving us another MaxElementsPerPage fresh indices in the container c.

After some more time, we allocate a new element E2 at {X, Y} page index and access the iterator I, which will access the E2 instead of E1. And this can be very wrong and dangerous for a bunch of reasons. So, this actually breaks the guarantees, which ReuseSlots=false gives us in terms of pointer safety.

I'll prepare a patch to implement this.

[past-due]

Generally, I agree, the memory should be recycled in such cases. Although, technically, I have another proposition for how to implement this.

So, to keep the internal housekeeping as simple, as it was before, I think instead of re-initializing existing pages, better to put them into "deactivated" (or "discarded") state indefinitely (that is, free their internal storage) and just allocate new ones at the end.

Great idea for ReuseSlots=false mode. 👍