125. The Cost of the Default Budget

• Status: Measured; a cost accepted for safety
• Date: 2026-10-30

#Problem and Mechanism

A parse with a zero Budget used to run without a control. It now gets the default budget max(2^20, 1024 * n) for an input of n units, so a parse with no limit and no context runs the controlled form: the engine uses its metered closure graph or charging VM loop, and generated Go and TypeScript parsers spend into a counter. Setting Unlimited with no context keeps the uncontrolled form, which is the fast path measured before this change.

The default is resolved once per entry point. Resolving it costs one input length: a byte count for Bytes or an input of at most 1024 units, and one pass over the string to count code points otherwise. A stream, which has no length, starts at the floor and raises it to 1024 times the furthest position the parser has demanded; the raise is a comparison on each demand.

#Results

Parse time with default options, before and after, medians of seven interleaved rounds. The engine columns parse a 256 KiB JSON and XML, a YAML stream of similar size and 5000 CSV rows; the generated column is a Go parser generated with budget controls from the same grammar:

The ratio is the cost of the controlled form that a parse with a limit already paid. A caller who trusts the input and wants the earlier speed passes Unlimited.

A Document parse with default options also runs controlled. The controlled form records the examined extent of a result conservatively, so a reparse can reuse slightly fewer results: on the generated-parser Document corpus one case reuses 249 results where the uncontrolled form reuses 253, with the same results. Unlimited keeps the uncontrolled reuse.

#Headroom

The smallest budget that parses the JSON and XML inputs is about 8 units per input unit and the YAML input about 24 (see record 124), against a default of 1024 per unit.