Evaluation result is not cached for multiple nix subcommands #925

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opened 2025-07-21 12:58:37 +00:00 by gepbird · 2 comments

gepbird commented

2025-07-21 12:58:37 +00:00

Describe the bug

Evaluating a derivation with flakes doesn't populate the eval cache.

If you have a problem with a specific package or NixOS,
you probably want to file an issue at https://github.com/NixOS/nixpkgs/issues.

Steps To Reproduce

(Clear your ~/.cache/nix/eval-cache-v5 if the following steps are already cached somehow)
Run time nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox
Run the previous command again
Notice that for the second command, Lix evaluates the derivation again which is slow:

~ ❯ time nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox
«derivation /nix/store/15slf7h63nnf7pa0dqnln56p3qj8x131-firefox-140.0.4.drv»
nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox  3.30s user 0.62s system 77% cpu 5.048 total
~ ❯ time nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox
«derivation /nix/store/15slf7h63nnf7pa0dqnln56p3qj8x131-firefox-140.0.4.drv»
nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox  3.41s user 0.60s system 77% cpu 5.153 total

Expected behavior

Running the second command should be much quicker

`nix --version` output

nix (Lix, like Nix) 2.93.2
System type: x86_64-linux
Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux
Features: gc, signed-caches
System configuration file: /etc/nix/nix.conf
User configuration files: /home/gep/.config/nix/nix.conf:/etc/xdg/nix/nix.conf:/home/gep/.nix-profile/etc/xdg/nix/nix.conf:/nix/profile/etc/xdg/nix/nix.conf:/home/gep/.local/state/nix/profile/etc/xdg/nix/nix.conf:/etc/profiles/per-user/gep/etc/xdg/nix/nix.conf:/nix/var/nix/profiles/default/etc/xdg/nix/nix.conf:/run/current-system/sw/etc/xdg/nix/nix.conf
Store directory: /nix/store
State directory: /nix/var/nix
Data directory: /nix/store/pknhjvaknz0vc04zkjrw0hwld41z1s9r-lix-2.93.2/share

Additional context

I'm getting varying results with nix eval (some data gets written to the eval cache db) nix shell (only an empty table gets created in eval cache db) and nix build (seems to work well, but only for some flakes, maybe it's about pinning?)

My main problem I'm trying to solve, is evaluating our server configuration on a slower Raspberry Pi 4 takes 3-4 minutes. This configuration uses some flakes that steal ~70% of the eval time, and I'd like them to be cached. First, I want to understand why is Lix not caching even a single flake output with nix eval.

I've attached the output of nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox -vvvv.

## Describe the bug Evaluating a derivation with flakes doesn't populate the eval cache. If you have a problem with a specific package or NixOS, you probably want to file an issue at https://github.com/NixOS/nixpkgs/issues. ## Steps To Reproduce 0. (Clear your `~/.cache/nix/eval-cache-v5` if the following steps are already cached somehow) 1. Run `time nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox` 2. Run the previous command again 3. Notice that for the second command, Lix evaluates the derivation again which is slow: ```console ~ ❯ time nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox «derivation /nix/store/15slf7h63nnf7pa0dqnln56p3qj8x131-firefox-140.0.4.drv» nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox 3.30s user 0.62s system 77% cpu 5.048 total ~ ❯ time nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox «derivation /nix/store/15slf7h63nnf7pa0dqnln56p3qj8x131-firefox-140.0.4.drv» nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox 3.41s user 0.60s system 77% cpu 5.153 total ``` ## Expected behavior Running the second command should be much quicker ## `nix --version` output ```console nix (Lix, like Nix) 2.93.2 System type: x86_64-linux Additional system types: i686-linux, x86_64-v1-linux, x86_64-v2-linux, x86_64-v3-linux Features: gc, signed-caches System configuration file: /etc/nix/nix.conf User configuration files: /home/gep/.config/nix/nix.conf:/etc/xdg/nix/nix.conf:/home/gep/.nix-profile/etc/xdg/nix/nix.conf:/nix/profile/etc/xdg/nix/nix.conf:/home/gep/.local/state/nix/profile/etc/xdg/nix/nix.conf:/etc/profiles/per-user/gep/etc/xdg/nix/nix.conf:/nix/var/nix/profiles/default/etc/xdg/nix/nix.conf:/run/current-system/sw/etc/xdg/nix/nix.conf Store directory: /nix/store State directory: /nix/var/nix Data directory: /nix/store/pknhjvaknz0vc04zkjrw0hwld41z1s9r-lix-2.93.2/share ``` ## Additional context I'm getting varying results with `nix eval` (some data gets written to the eval cache db) `nix shell` (only an empty table gets created in eval cache db) and `nix build` (seems to work well, but only for some flakes, maybe it's about pinning?) My main problem I'm trying to solve, is evaluating our server [configuration](https://github.com/tchfoo/raspi-dotfiles) on a slower Raspberry Pi 4 takes 3-4 minutes. This configuration uses some flakes that steal ~70% of the eval time, and I'd like them to be cached. First, I want to understand *why* is Lix not caching even a single flake output with `nix eval`. I've attached the output of `nix eval nixpkgs/e364990b50adfbb8c32c1316903652f37834538c#firefox -vvvv`.

nix-eval-vvvv.txt

8.2 MiB

gepbird added the

bug

label

2025-07-21 12:58:37 +00:00

gepbird changed title from ~~Nothing is saved to eval-cache databases~~ to Evaluation result is not cached for multiple nix subcommands

2025-07-21 13:04:24 +00:00

pennae commented

2025-07-21 14:16:46 +00:00

Owner

we do have plans to fix all of this, but it's a long way out. caching shells the way we cache built installables is in theory possible, doing it for eval in general is not feasible because the cache drops far too much data.

the eval cache mechanism is indeed very badly busted, and it's possible that it cannot be fixed at all. your test case of evaluating eg firefox can probably be cached with enough reworking of the cache infrastructure, but due to how the system works today it is *completely impossible* to cache partial evaluation results in the way you're asking. the flake eval cache is applied at the very end of the process, it's an all-or-nothing deal. (yes, flakes are Absolutely Perfect In Every Single Imaginable Way like that.) we do have plans to fix all of this, but it's a *long* way out. caching shells the way we cache built installables is in *theory* possible, doing it for eval in general is not feasible because the cache drops far too much data.

gepbird commented

2025-07-21 17:18:56 +00:00

Author

Thanks for the explanation :)

Unfortunate that this is the state of flake eval caching, I think I'll try to implement a similar hack to https://github.com/DavHau/nix-eval-cache/blob/main/lib.nix, which puts the evaled values in a derivation which is cached. With this implementation, my problem was that it relied on pkgs to build that derivation in a way that nixpkgs had to be copied to the store on every eval. At least it cuts down all the eval time to the time of copying nixpkgs to the store, which is ~5 seconds on my drive.

Thanks for the explanation :) Unfortunate that this is the state of flake eval caching, I think I'll try to implement a similar hack to https://github.com/DavHau/nix-eval-cache/blob/main/lib.nix, which puts the evaled values in a derivation which is cached. With this implementation, my problem was that it relied on `pkgs` to build that derivation in a way that `nixpkgs` had to be copied to the store on every eval. At least it cuts down all the eval time to the time of copying nixpkgs to the store, which is ~5 seconds on my drive.