libexpr: Actually cache line information in PosTable#13211
Merged
Ericson2314 merged 2 commits intoNixOS:masterfrom May 16, 2025
Merged
libexpr: Actually cache line information in PosTable#13211Ericson2314 merged 2 commits intoNixOS:masterfrom
Ericson2314 merged 2 commits intoNixOS:masterfrom
Conversation
For heavier objects it doesn't make sense to return a std::optional with the copy of the data, when it can be used by const reference.
Previous code had a sneaky bug due to which no caching
actually happened:
```cpp
auto linesForInput = (*lines)[origin->offset];
```
That should have been:
```cpp
auto & linesForInput = (*lines)[origin->offset];
```
See [1].
Now that it also makes sense to make the cache bound in side
in order not to memoize all the sources without freeing any memory.
The default cache size has been chosen somewhat arbitrarily to be ~64k
origins. For reference, 25.05 nixpkgs has ~50k .nix files.
Simple benchmark:
```nix
let
pkgs = import <nixpkgs> { };
in
builtins.foldl' (acc: el: acc + el.line) 0 (
builtins.genList (x: builtins.unsafeGetAttrPos "gcc" pkgs) 10000
)
```
(After)
```
$ hyperfine "result/bin/nix eval -f ./test.nix"
Benchmark 1: result/bin/nix eval -f ./test.nix
Time (mean ± σ): 292.7 ms ± 3.9 ms [User: 131.0 ms, System: 120.5 ms]
Range (min … max): 288.1 ms … 300.5 ms 10 runs
```
(Before)
```
hyperfine "nix eval -f ./test.nix"
Benchmark 1: nix eval -f ./test.nix
Time (mean ± σ): 666.7 ms ± 6.4 ms [User: 428.3 ms, System: 191.2 ms]
Range (min … max): 659.7 ms … 681.3 ms 10 runs
```
If the origin happens to be a `all-packages.nix` or similar in size then the
difference is much more dramatic.
[1]: https://www.github.com/lix-project/lix/commit/22e3f0e9875082be7f4eec8e3caeb134a7f1c05f
roberth
added
backports created
This was referenced Jul 30, 2025
mergify bot
added a commit
that referenced
this pull request
Jul 30, 2025
…3211 libexpr: Actually cache line information in PosTable (backport #13211)
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment
3 participants
Add this suggestion to a batch that can be applied as a single commit.This suggestion is invalid because no changes were made to the code.Suggestions cannot be applied while the pull request is closed.Suggestions cannot be applied while viewing a subset of changes.Only one suggestion per line can be applied in a batch.Add this suggestion to a batch that can be applied as a single commit.Applying suggestions on deleted lines is not supported.You must change the existing code in this line in order to create a valid suggestion.Outdated suggestions cannot be applied.This suggestion has been applied or marked resolved.Suggestions cannot be applied from pending reviews.Suggestions cannot be applied on multi-line comments.Suggestions cannot be applied while the pull request is queued to merge.Suggestion cannot be applied right now. Please check back later.
Motivation
Previous code had a sneaky bug due to which no caching
actually happened:
auto linesForInput = (*lines)[origin->offset];That should have been:
auto & linesForInput = (*lines)[origin->offset];See 1.
Now that it also makes sense to make the cache bound in size
in order not to memoize all the sources without freeing any memory.
The default cache size has been chosen somewhat arbitrarily to be ~64k
origins. For reference, 25.05 nixpkgs has ~50k .nix files.
Simple benchmark:
(After)
(Before)
If the origin happens to be a
all-packages.nixor similar in size then thedifference is much more dramatic.
Context
#12645 (comment)
@roberth As it turns out the
PosTablewas indeed supposed to do the caching, but didn't due to the bug I described above.Add 👍 to pull requests you find important.
The Nix maintainer team uses a GitHub project board to schedule and track reviews.