BQ Slot

BigQuery doesn’t just “assign slots.” It uses a fairness scheduler.

? High-Level Model

Each reservation has:

• Slot pool

• Job queue

• Fairness allocator

Scheduler attempts to balance:

• Throughput

• Latency

• Resource isolation

? Slot Allocation Is Stage-Based

Query → broken into stages
Stage → requests slots
Scheduler grants slots dynamically

Slots are NOT statically bound to a query.

? Fair Share Model

If:

• Reservation = 1000 slots

• 2 queries running

Each initially gets ~500.

If one finishes:

• Other gets up to 1000.

? When Many Queries Compete

Example:

1000 slots
10 active queries

Scheduler might:

• Give each 100 slots initially

• Reallocate as stages finish

• Prioritize interactive over batch (if configured)

? Slot Contention Internally

Contention occurs when:

Requested slots > Available slots

Then:

1. Queries queue

2. Or stages get fewer slots

3. Runtime increases

? Hidden Behavior: Elastic Stage Scaling

Within a single query:

• Early scan stages may use many slots

• Later aggregation uses fewer

• Slots are recycled immediately

This maximizes cluster utilization.

? Why Long Queries Hurt Everyone

Because fairness scheduling:

• Can’t preempt mid-stage easily

• Heavy shuffle stages monopolize slots

• Increase queue time for others

Which is why workload isolation matters.