Nix/doc/manual/rl-next/env-size-reduction.md

---
synopsis: Reduce eval memory usage and wall time
prs: 9658
---

Reduce the size of the `Env` struct used in the evaluator by a pointer, or 8 bytes on most modern machines.
This reduces memory usage during eval by around 2% and wall time by around 3%.
reduce the size of Env by one pointer since `up` and `values` are both pointer-aligned the type field will also be pointer-aligned, wasting 48 bits of space on most machines. we can get away with removing the type field altogether by encoding some information into the `with` expr that created the env to begin with, reducing the GC load for the absolutely massive amount of single-entry envs we create for lambdas. this reduces memory usage of system eval by quite a bit (reducing heap size of our system eval from 8.4GB to 8.23GB) and gives similar savings in eval time. running `nix eval --raw --impure --expr 'with import <nixpkgs/nixos> {}; system'` before: Time (mean ± σ): 5.576 s ± 0.003 s [User: 5.197 s, System: 0.378 s] Range (min … max): 5.572 s … 5.581 s 10 runs after: Time (mean ± σ): 5.408 s ± 0.002 s [User: 5.019 s, System: 0.388 s] Range (min … max): 5.405 s … 5.411 s 10 runs 2023-12-22 18:19:53 +01:00			`---`
			`synopsis: Reduce eval memory usage and wall time`
			`prs: 9658`
			`---`

			Reduce the size of the `Env` struct used in the evaluator by a pointer, or 8 bytes on most modern machines.
			`This reduces memory usage during eval by around 2% and wall time by around 3%.`