# Nix flamegraph or profiling tool?

**URL:** https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333
**Category:** Development
**Created:** [September 22, 2023, 10:57am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333 "2023-09-22T10:57:29Z")
**Posts on this page:** 15
**Page:** 1

<div class="post-metadata">

### Author: ![Yajo](https://discourse.nixos.org/user_avatar/discourse.nixos.org/yajo/32/5602_2.png) [@Yajo](https://discourse.nixos.org/u/Yajo)
#### Post date: [September 22, 2023, 10:57am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/1 "2023-09-22T10:57:30Z")

</div>

I’m having a big closure that takes too much to evaluate. And it has no IFDs. That’s weird.

I’m used to using [py-spy](https://github.com/benfred/py-spy) to generate flamegraph data that I can feed into [https://www.speedscope.app/](https://www.speedscope.app/) to inspect and solve Python apps performance bottlenecks.

Is there anything similar for Nix lang?

---

<div class="post-metadata">

### Author: ![Artturin](https://discourse.nixos.org/letter_avatar_proxy/v4/letter/a/3da27b/32.png) [@Artturin](https://discourse.nixos.org/u/Artturin)
#### Post date: [September 26, 2023, 12:24am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/2 "2023-09-26T00:24:59Z")

</div>

I have this zsh function

```sh
function nixFunctionCalls {
    local WORKDIR=$(mktemp -d /tmp/nix-fun-calls-XXXXX)
    nix-instantiate --trace-function-calls "$1" -A "$2" 2> $WORKDIR/nix-function-calls.trace 1>/dev/null
    ~/nixgits/nix/contrib/stack-collapse.py $WORKDIR/nix-function-calls.trace > $WORKDIR/nix-function-calls.folded
    nix-shell -p flamegraph --run "flamegraph.pl $WORKDIR/nix-function-calls.folded > $WORKDIR/nix-function-calls.svg"
    echo "$WORKDIR/nix-function-calls.svg"
}

```

I use it like so `nixFunctionCalls . bash | xargs firefox`

here’s the `.folded` file in speedscope

 ![image](https://discourse.nixos.org/uploads/default/original/2X/1/1046bf2fbebff9e16214a818e18e11ec20e80cc3.png)

---

<div class="post-metadata">

### Author: ![Yajo](https://discourse.nixos.org/user_avatar/discourse.nixos.org/yajo/32/5602_2.png) [@Yajo](https://discourse.nixos.org/u/Yajo)
#### Post date: [September 26, 2023, 9:53am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/3 "2023-09-26T09:53:36Z")

</div>

Is there a flakes alternative to `nix-instantiate`?

---

<div class="post-metadata">

### Author: ![Yajo](https://discourse.nixos.org/user_avatar/discourse.nixos.org/yajo/32/5602_2.png) [@Yajo](https://discourse.nixos.org/u/Yajo)
#### Post date: [September 26, 2023, 9:58am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/4 "2023-09-26T09:58:22Z")

</div>

I’m trying your solution and a problem I find is that my `.folded` file occupies about 59 GB, so I cannot upload it to speedscope, and trying to get a local flamegraph out of it blows my machine.

Is there any other more efficient way to get a flamegraph?

---

<div class="post-metadata">

### Author: ![Artturin](https://discourse.nixos.org/letter_avatar_proxy/v4/letter/a/3da27b/32.png) [@Artturin](https://discourse.nixos.org/u/Artturin)
#### Post date: [September 26, 2023, 10:11am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/5 "2023-09-26T10:11:29Z")

</div>

Mine for the nixpkgs bash is only a few mb

I dont know of other ways

---

<div class="post-metadata">

### Author: ![Artturin](https://discourse.nixos.org/letter_avatar_proxy/v4/letter/a/3da27b/32.png) [@Artturin](https://discourse.nixos.org/u/Artturin)
#### Post date: [September 26, 2023, 10:40am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/6 "2023-09-26T10:40:05Z")

</div>

From this reply  
[https://github.com/NixOS/nix/issues/7179#issuecomment-1678230028](https://github.com/NixOS/nix/issues/7179#issuecomment-1678230028)

`nix eval --trace-function-calls --raw ".#bash" 2>&1 | cat`

For some reason it has to be piped, weird bug.

---

<div class="post-metadata">

### Author: ![Yajo](https://discourse.nixos.org/user_avatar/discourse.nixos.org/yajo/32/5602_2.png) [@Yajo](https://discourse.nixos.org/u/Yajo)
#### Post date: [September 26, 2023, 11:03am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/7 "2023-09-26T11:03:43Z")

</div>

py-spy has the notion of samples per second. I think that’s how they produce a reasonably-sized dump.

It seems to me that nix just traces every single call, which can amount to many millions when using modules (I have them while using flake-parts and dream2nix in the same project).

It would be nice to be able to have a “samples per second” option for `--trace-function-calls`.

---

<div class="post-metadata">

### Author: ![Artturin](https://discourse.nixos.org/letter_avatar_proxy/v4/letter/a/3da27b/32.png) [@Artturin](https://discourse.nixos.org/u/Artturin)
#### Post date: [September 26, 2023, 10:52pm UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/8 "2023-09-26T22:52:51Z")

</div>

`NIX_SHOW_STATS=1 NIX_COUNT_CALLS=1` could also be helpful

---

<div class="post-metadata">

### Author: ![Yajo](https://discourse.nixos.org/user_avatar/discourse.nixos.org/yajo/32/5602_2.png) [@Yajo](https://discourse.nixos.org/u/Yajo)
#### Post date: [September 27, 2023, 8:06am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/9 "2023-09-27T08:06:00Z")

</div>

Both options seem to dump useful information, but it’s not very human-readable.

Is there any tool that generates some sort of graphic out of that data?

---

<div class="post-metadata">

### Author: ![illustris](https://discourse.nixos.org/user_avatar/discourse.nixos.org/illustris/32/3334_2.png) [@illustris](https://discourse.nixos.org/u/illustris)
#### Post date: [October 5, 2023, 11:10am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/10 "2023-10-05T11:10:59Z")

</div>

> [@Artturin](#):
>
> For some reason it has to be piped, weird bug.

```auto
00000000: 0d1b 5b4b 0d1b 5b4b 6675 6e63 7469 6f6e ..[K..[Kfunction
00000010: 2d74 7261 6365 2065 6e74 6572 6564 20c2 -trace entered .

```

Nix is putting control chars in stderr for some reason. After cleaning up the control characters and passing it through [this very jank AI-generated script](https://gist.github.com/illustris/cce6cf01374917aa0d896ee34de958d5), flamegraph.pl is able to generate svgs.

EDIT: Nevermind, the script doesn’t produce correct results. But it should be easy enough to do.

---

<div class="post-metadata">

### Author: ![tailrecursive](https://discourse.nixos.org/user_avatar/discourse.nixos.org/tailrecursive/32/1672_2.png) [@tailrecursive](https://discourse.nixos.org/u/tailrecursive)
#### Post date: [March 12, 2024, 6:43am UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/11 "2024-03-12T06:43:06Z")

</div>

Adapted to home-manager configuration, made it use `nix run` to support flakes and inferno for magnitudes faster svg generation:

```auto
 home.packages =
      let
        # https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333
        stackCollapse = pkgs.writeTextFile {
          name = "stack-collapse.py";
          destination = "/bin/stack-collapse.py";
          text = builtins.readFile (builtins.fetchurl
            {
              url = "https://raw.githubusercontent.com/NixOS/nix/master/contrib/stack-collapse.py";
              sha256 = "sha256:0mi9cf3nx7xjxcrvll1hlkhmxiikjn0w95akvwxs50q270pafbjw";
            });
          executable = true;
        };
        nixFunctionCalls = pkgs.writeShellApplication {
          name = "nixFunctionCalls";
          runtimeInputs = [stackCollapse pkgs.inferno];
          text = builtins.readFile ./nix-function-calls.sh;
          checkPhase = "";
        };
      in
      [nixFunctionCalls];

```

```auto
#!/usr/bin/env zsh

WORKDIR=$(mktemp -d)

nix eval -vvvvvvvvvvvvvvvvvvvv --raw --option trace-function-calls true $1 1>/dev/null 2> $WORKDIR/nix-function-calls.trace
stack-collapse.py $WORKDIR/nix-function-calls.trace > $WORKDIR/nix-function-calls.folded
inferno-flamegraph $WORKDIR/nix-function-calls.folded > $WORKDIR/nix-function-calls.svg
echo "$WORKDIR/nix-function-calls.svg"

```

---

<div class="post-metadata">

### Author: ![randomnixuser](https://discourse.nixos.org/letter_avatar_proxy/v4/letter/r/7c8e57/32.png) [@randomnixuser](https://discourse.nixos.org/u/randomnixuser)
#### Post date: [March 12, 2024, 1:54pm UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/12 "2024-03-12T13:54:34Z")

</div>

thanks a lot for posting it here, can you please explain to me why the pipe xargs command?

like in your example:

```bash
nixFunctionCalls . bash | xargs firefox

```

thanks!

---

<div class="post-metadata">

### Author: ![norpol](https://discourse.nixos.org/user_avatar/discourse.nixos.org/norpol/32/8764_2.png) [@norpol](https://discourse.nixos.org/u/norpol)
#### Post date: [March 12, 2024, 1:56pm UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/13 "2024-03-12T13:56:28Z")

</div>

> [@Artturin](#):
>
> ` echo "$WORKDIR/nix-function-calls.svg"`

If you pipe this to `xargs firefox` it will be the equivalent of `firefox ./$WORKDIR/nix-functiona-calls.svg`.

---

<div class="post-metadata">

### Author: ![norpol](https://discourse.nixos.org/user_avatar/discourse.nixos.org/norpol/32/8764_2.png) [@norpol](https://discourse.nixos.org/u/norpol)
#### Post date: [March 12, 2024, 2:02pm UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/14 "2024-03-12T14:02:58Z")

</div>

Some additional ongoing efforts/experiment for a Nix expr evaluation profiler using tracy:

> <https://github.com/NixOS/nix/pull/9967>
>
> This is a WIP profiler for the Nix code itself, not the cpp Nix evaluator. The g…oal is to provide some tooling to improve the Nixpkgs evaluation performance.
> 
> This is a WIP Nix profiler using \[Tracy\](https://github.com/wolfpld/tracy). The idea is to trace the evaluation of the expression atoms to figure out what is expensive. We are roughly tracing the thunk evaluation. Nix is a lazy language, I think this is what makes most sense.
> 
> \[Short demo video\](https://social.alternativebit.fr/media/4fee7e0b64e3d7fd05102e2d3b6789099ceb506c6cd9fe4398efd5aea2e13779.nix-tracy-demo-short)
> 
> \## TODO before Undrafting
> 
> This is not ready yet. We need to fix at least those two issues before undrafting this PR.
> 
> \*\*HELP WELCOME\*\* :)
> 
> \### Elephant in the room: tracy SEGFAULTS!!!!!!!!!
> 
> We have those weird "???" frames in Tracy. Hovering them with the mouse segfault the viewer.
> 
> Removing the line and the column in the position pretty printer makes them go away. Sadly, removing the column and position from the trace also makes the profiler useless :D
> 
> There's something very wrong in our client code, but I couldn't figure out what. For now. This is the first item on my TODO list.
> 
> \### Profiler performance
> 
> There's room for improvement. Random datapoint, with the profiler enabled:
> \`\`\`
> \[nix-shell:~/code-root/github.com/NixOS/nix\]$ time ./outputs/out/bin/nix-instantiate -A nixosTests.pleroma ../nixpkgs
> warning: you did not specify '--add-root'; the result might be removed by the garbage collector
> /nix/store/z6nqb4zkx59hqn00rpzpzq4fcp34y9pa-vm-test-run-pleroma.drv
> 
> real 1m41,835s
> user 2m51,728s
> sys 0m11,388s
> \`\`\`
> 
> Without the profiler:
> \`\`\`
> \[nix-shell:~/code-root/github.com/NixOS/nix\]$ time nix-instantiate -A nixosTests.pleroma ../nixpkgs
> warning: you did not specify '--add-root'; the result might be removed by the garbage collector
> /nix/store/z6nqb4zkx59hqn00rpzpzq4fcp34y9pa-vm-test-run-pleroma.drv
> 
> real 0m12,478s
> user 0m9,998s
> sys 0m1,431s
> \`\`\`
> 
> That's not a surprise, I did not make any effort here, each profiler macro is generating \*\*at least\*\* 3 allocations.
> 
> We should run perf on this and see what's wrong.
> 
> 
> \### Move out the trace macros from the eval call sites
> 
> See https://github.com/NixOS/nix/pull/9967#issuecomment-1933695607
> 
> \### Is this useful/refine design?
> 
> We should spend some time using this profiler. Can we spot issues with it? Can we remove some useless noise from the traces?
> 
> There's no magic trick here, we need to spend time using the tool to improve it.
> 
> \## How to test?
> 
> 1. \`nix build github:picnoir/nix/pic/tracy-profiler\`
> 2. Open tracy. \`nix-shell -p tracy\`. Then, either lauch the graphical profiler to watch the capture in real time (\`tracy\`), either record the run through the headless capture utility (\`capture -o capture.tracy\`).
> 3. Evaluate something with the \`nix-instantiate\` utility you built in step 2. EG. \`./result/bin/nix-instantiate -A nixosTests.pleroma path-to/nixpkgs/\`
> 4. Enjoy the \[operation game\](https://en.wikipedia.org/wiki/Operation\_(game)), avoid the \`???\` with your mouse to prevent segfaults. Scroll-wheel to zoom/unzoom, left click to move the trace.
> 
> 
> \# Priorities and Process
> 
> Add :+1: to \[pull requests you find important\](https://github.com/NixOS/nix/pulls?q=is%3Aopen+sort%3Areactions-%2B1-desc).
> 
> The Nix maintainer team uses a \[GitHub project board\](https://github.com/orgs/NixOS/projects/19) to \[schedule and track reviews\](https://github.com/NixOS/nix/tree/master/maintainers#project-board-protocol).

---

<div class="post-metadata">

### Author: ![ada](https://discourse.nixos.org/user_avatar/discourse.nixos.org/ada/32/29742_2.png) [@ada](https://discourse.nixos.org/u/ada)
#### Post date: [December 4, 2024, 9:31pm UTC](https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/15 "2024-12-04T21:31:30Z")

</div>

in case any of you are interested, I went ahead and cobbled together the scripts @tailrecursive posted into an easily accessible nix flake. You can find it here: [GitHub - crabdancing/nix-flamegraph: Various primitive scripts cobbled together to produce an easy flamegraph of nix eval](https://github.com/crabdancing/nix-flamegraph)