Jiaming Yuan 54da4b3185
Cleanup to prepare for using mmap pointer in external memory. (#9317)
- Update SparseDMatrix comment.
- Use a pointer in the bitfield. We will replace the `std::vector<bool>` in `ColumnMatrix` with bitfield.
- Clean up the page source. The timer is removed as it's inaccurate once we swap the mmap pointer into the page.
2023-06-22 06:43:11 +08:00
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