- Reduce dependency on dmlc parsers and provide an interface for users to load data by themselves.
- Remove use of threaded iterator and IO queue.
- Remove `page_size`.
- Make sure the number of pages in memory is bounded.
- Make sure the cache can not be violated.
- Provide an interface for internal algorithms to process data asynchronously.
The role of ProxyDMatrix is going beyond what it was designed. Now it's used by both
QuantileDeviceDMatrix and inplace prediction. After the refactoring of sparse DMatrix it
will also be used for external memory. Renaming the C API to extract it from
QuantileDeviceDMatrix.
