option trace-function-calls doesn't seem to do anything with flakes

[nrdxp]

Describe the bug

The nix.conf option trace-function-calls seems to have no affect in 2.11.2 when using nix-command and flakes.

Steps To Reproduce

• Set trace-function-calls to true in your nix.conf or on the cli
• build a package with nix build the --no-eval-cache flag to ensure eval isn't skipped
• Notice no extra output is printed either to stdout or in the nix daemon logs

Expected behavior

The way the option is described in the manual and in the stack-collapse.py, I would expect extra output on stderr.

nix-env --version output
nix-env (Nix) 2.11.2

Additional context

I want to use this option along with the stack-collapse.py for profiling. For now it looks like I can use flake-compat and nix-instantiate to work around this.

update
Unfortunately trying to generate a flamegraph of a NixOS system evalution resulted in an oom situation on my machine with 64Gb of RAM. x-ref: brendangregg/FlameGraph#58.

We may want to explore a more efficient way to generate these graphs in the future, as this isn't really a practical situation atm.

[quentinmit]

I'm not able to reproduce this bug. --trace-function-calls produces output for me when stderr is directed to a pipe. I was able to get a working flamegraph for a NixOS build using this shell line:

nix eval --trace-function-calls --raw .#nixosConfigurations.hostname.config.system.build.toplevel 3>&2 2>&1 1>&3 | (./stack-collapse.py | perl -nle '@_ = split " "; $total{$_[0]} += $_[1]; END { foreach my $x (keys %total) { print "$x $total{$x}"; }; }' | nix shell nixpkgs#flamegraph --command sh -c "flamegraph.pl > flamegraph.svg" )

The main thing I had to do is use the perl one-liner to preaggregate duplicate stacks; for some reason, my one-liner is able to do that much more efficiently than flamegraph.pl is. It took about 9 GB of RAM for flamegraph.pl to generate the SVG for a relatively small nixos configuration.

That said... I found it very hard to get anything meaningful out of the flamegraph, because most NixOS modules don't actually contain any function definitions (or maybe the top-level flake is getting accounted as "«none»" in the trace?), so all I see taking most of the time are a bunch of functions in <nixpkgs>/lib/modules.nix and the utilities they call in lib/attrsets.nix, lib/lists.nix, etc. It would be really nice if there were some way to trace individual thunks instead of just function calls, though that would probably make the trace orders of magnitude larger again.

[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/nix-flamegraph-or-profiling-tool/33333/6