[copybara-service]

Scale background treatment candidate scan budget with heap size and cap collapses per pass.

Previously, HugePageUnbackedTrackerTreatment scanned at most 64 never-observed or stale split trackers per pass in occupancy order, which could starve lower-occupancy trackers on large heaps and repeatedly rescan the same uncollapsible trackers at the head of the filler lists.

Select eligible candidate trackers using an oldest-record-first buffer compacted with std::nth_element, placing never-observed trackers ahead of stale split records. Break equal-timestamp ties by treatment priority and list traversal order, and scale the per-pass scan budget linearly with filler heap size between 64 and 1,024 trackers so large heaps are swept within one record interval.

When applying treatments, cap userspace collapse attempts separately at 64 per pass in priority order while continuing to observe residency and subrelease unbacked, swapped, and stale free pages for all scanned split candidates.

Co-authored-by: Chris Kennelly ckennelly@google.com

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.