pub fn max_by_sum_u64(rows: &[(&str, Vec<SnapshotField<'_>>)]) -> Option<usize>Expand description
Pick the candidate row whose fields sum to the largest u64 — the
“max-activity bss” heuristic generalized to N co-picked variables.
For each row, the picker as_u64-projects every field and
saturating_adds them; rows containing ANY field that fails to
project to u64 are dropped entirely (not partial-summed); among
the surviving rows, the one with the largest sum wins.
Pairs with super::view::Snapshot::live_vars_via for ratio /
fraction metrics: the row whose counters have accumulated the
most TOTAL events is the live scheduler instance, and selecting
that row guarantees all N returned fields come from the same
source map (no cross-bss-copy corruption).
§When this picks WRONG
Inherits every failure mode of max_by_counter_value (gauges,
sentinel-init counters like u64::MAX, immediately-post-swap
windows before the new instance has accumulated past the old’s
BSS-zero point), plus one sum-specific mode:
Any-non-u64-field row excluded. A single rodata or non-counter variable accidentally included in the name set silently makes every candidate row ineligible (every row would have that non-u64 field). Verify all N names project to u64 counters BEFORE composing the picker.
Sums use saturating_add: a row containing a sentinel
u64::MAX saturates to u64::MAX and wins all comparisons.
Treat that as a sentinel-mode failure (per max_by_counter_value’s
caveat).
Ties resolve to the LAST tied row (matches
Iterator::max_by_key semantics — same as
max_by_counter_value).