1
0
Fork 0
mirror of https://github.com/NixOS/nix synced 2025-07-08 19:23:54 +02:00

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
This commit is contained in:
pennae 2023-12-22 18:19:53 +01:00
parent ee439734e9
commit 1fe66852ff
9 changed files with 75 additions and 34 deletions

View file

@ -0,0 +1,7 @@
---
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%.