preemption

A flat median with a broken tail means one in a hundred requests is hitting something the others do not. There are five usual suspects, and the order you rule them out in is the answer.

Static quotas waste half the fleet and a free-for-all starves the small teams. The four-layer policy (guaranteed quota, borrowing, fair-share ordering, preemption) with the pooling arithmetic that justifies it, the fair-share ratio worked by hand, and the five ways teams game it.

Spot is 60% cheaper until the preemptions eat the discount. The expected-loss formula (waste per preemption = half the checkpoint interval plus restart), the break-even worked at three cadences, the checkpoint cost that pushes back the other way, and the gang arithmetic that says which jobs must never be on spot.

A shared pool recovers the GPUs inference holds for its peaks, but a preempted training gang takes minutes to give them back and an SLO breaks in seconds. The utilization of each design, the reclaim-time arithmetic against the traffic ramp, and the floor-plus-borrow split most fleets land on.

A 100,000-entry queue cannot be scanned every cycle, a big gang cannot wait forever behind small jobs, and a preemption must not discard an hour of work. The loop in order (admit, order, place, reserve, backfill, preempt), the queue-wait arithmetic, and the checkpoint-aware victim rule.

A GPU credit system is an accounting ledger with a scheduler attached: who may spend, how fast, what happens when the account is empty and what happens when everyone spends at once. The data model, the debit path, the fairness rule and the arithmetic that keeps 4,096 GPUs busy without starving a team.

A hundred thousand jobs is a queue you cannot scan per tick, a preemption policy that has to know when each victim last checkpointed, and a state machine with one transition that most designs get wrong. The data model, the tick loop, the checkpoint-aware eviction and the arithmetic behind each.

A research cluster is shared by teams whose managers bought it and by researchers who want it now. A quota tree with guarantees and borrowing, a fair-share formula that decays so last week's usage does not cost you today, preemption that is cheap because checkpoints are, and the incentives that make hoarding lose.

A heap orders the queue and the interesting logic is elsewhere: which running jobs may be evicted, how many, and what happens to them. The victim-selection rule that avoids evicting more than necessary, the equal-priority case that must not preempt, and the starvation the design creates if nothing ages.

Admission, a step that grows every sequence, and preemption under memory pressure. The loop is short and it has a failure that only appears when the cache is tight: in a measured run it preempted seventeen times and completed nothing, because every preempted sequence restarted from zero.