static TIERED_POLICY: LazyLock<Arc<TieredPolicy>>Expand description
Process-wide policy::TieredPolicy singleton.
Why a singleton: every ResidentTicket keeps an Arc<dyn PagingPolicy>
pointing at the policy that decided to keep the column resident.
Replacing the global TieredPolicy would orphan in-flight tickets onto
the previous instance — they would credit a budget atomic that the new
policy can no longer see, draining the new pool monotonically until it
locks up on Page decisions. A persistent singleton with in-place
policy::TieredPolicy::reconfigure sidesteps the issue: all tickets,
past and present, share the same atomic.
Initialized eagerly with zero budget so metrics::register can read
it during compute startup, before any apply_tiered_config call. The
first config apply resizes the pool via reconfigure, which is the same
path operator-driven tunes take.