Making devenv start fast, and the whole nixpkgs with it

I'm sitting here next to Farid Zakaria at Tacosprint where we looked at the stat storm that has been haunting nixpkgs for a decade.

The Tacosprint table

devenv auto activation runs devenv hook-should-activate on every shell prompt to decide whether you've stepped into a project directory. It does almost nothing: discover the project, check the trust database, print a path. So its runtime is pure startup overhead, and it runs on every single prompt redraw.

$ time devenv hook-should-activate
/home/domen/dev/myproject
real    0m0.070s
...

70ms before a prompt, every prompt.

And this isn't devenv's tax to pay, it's nixpkgs'. Every program pays it before it runs a line of its own code: the dynamic loader has to find each shared library, and the way Nix scatters packages across the store makes that search slow. This is not news. The cost has been measured, written up, and partly fixed more than once, and yet it has sat in limbo for the better part of a decade with no general fix merged into nixpkgs.

Most of that is the dynamic loader looking for a shared object that is sitting right there in the store, just not in the first directory it tried. The loader knocks on 486 wrong doors before it finds the right ones, and almost all of it happens before main even starts.

That number is the whole game. Above ~30ms you have to bolt a caching layer on top of the hook; in single digit milliseconds you just run it on every prompt and throw the cache away.

And it scales with the closure: imagemagick's magick --version makes 1225 failing opens:

$ strace -f -e openat magick --version 2>&1 >/dev/null | grep '\.so' | grep -c ENOENT
1225

The community has been circling a real fix for years. This post walks through the problem, the approaches people have tried with their tradeoffs, and a more radical one we spiked for devenv to see if it was even possible: deleting the dynamic loader altogether by linking the whole program into one static binary.

The umbrella tracking issue for the general problem is NixOS/nixpkgs#481620.

Why Nix makes the loader work so hard

On a traditional distribution every shared library lives in a handful of global directories such as /usr/lib. The dynamic loader has a short, mostly cached search path, and ld.so.cache (built by ldconfig) turns soname lookups into a hash table hit.

Nix is different by design. Every package lives in its own /nix/store/<hash>-name/lib directory, and there is no global ld.so.cache for store libraries. To make a binary find its dependencies, Nix records a DT_RUNPATH in the ELF header that lists one directory per dependency. A program linked against fifty libraries gets a DT_RUNPATH with dozens of entries.

Now recall how glibc resolves a DT_NEEDED soname with DT_RUNPATH present: it walks every DT_RUNPATH directory in order, trying to open dir/soname in each, until one succeeds. So resolving N libraries against a path of M directories costs on the order of N times M openat() attempts, almost all of which fail. That is the stat storm.

It gets worse. For every directory it searches, glibc first probes the glibc-hwcaps subdirectories for your CPU (x86-64-v3, x86-64-v2, and so on), which adds roughly three more failing opens per directory on a modern machine. On a fast SSD with a warm cache none of this is noticeable. On a slow disk, a network filesystem, a cold cache, or a low power ARM board, it is the difference between snappy and sluggish, and it multiplies across every process a shell script spawns.

Concretely, the two workloads we traced most closely:

Workload	Loaded libraries	`DT_RUNPATH` dirs	Failing `.so` opens
`devenv version`	83	12 (leaf binary)	~486
`imagemagick magick --version`	91	35	~1225

The wider a binary's own DT_RUNPATH and the deeper its transitive graph, the worse the storm.

What a good fix has to preserve

The reason this problem has stayed open so long is that the obvious fixes break things people rely on. Any serious solution is judged against a checklist:

LD_LIBRARY_PATH override. NixOS injects the GPU driver by putting /run/opengl-driver/lib on LD_LIBRARY_PATH. If a fix stops that from winning, graphics break.
LD_PRELOAD. Interposers and shims must still load first.
The libGL / glvnd runtime swap. A program built against Mesa must be able to pick up the vendor driver at runtime.
Two libraries with the same soname. This is the heart of the Nix model: different parts of one closure can legitimately depend on different builds of the same soname, and resolution must stay per object.
dlopen. Plugins loaded at runtime are a related but separate problem.
Cross compilation. A fix that has to run the target loader cannot cross compile cleanly.
Disk and closure size. Whatever metadata you add ships in every NAR.
Maintenance burden. A glibc or loader patch has to be rebased onto every new glibc release, and patching glibc rebuilds the world.

No approach so far ticks every box. The interesting part is how each one chooses which boxes to give up.

Approach 1: freeze the resolution with absolute paths

The simplest idea: rewrite every DT_NEEDED entry from a bare soname like libfoo.so.1 to the absolute store path of the library it resolves to. glibc has a "slash short circuit": a DT_NEEDED containing a / is opened directly, skipping all search. No search means no storm, and not even the glibc-hwcaps probes happen.

This is well trodden ground:

Farid Zakaria's shrinkwrap and the nix-harden-needed tool do exactly this as external post processing. Shrinkwrap is described in the paper Mapping Out the HPC Dependency Chaos (Zakaria, Scogland, Gamblin, Maltzahn, 2022; arXiv:2211.05118), which measures the storm directly: an Emacs launch drops from 1823 stat/openat syscalls to 104, a 36 times speedup, and a 900 library MPI application starting across 2048 processes on NFS goes from 344.6s to 47.8s, 7.2 times faster. Those NFS numbers are the clearest evidence that this overhead, invisible on a warm local cache, becomes brutal on a network or cold filesystem.
patchelf PR #357 (--shrink-wrap, open since 2021) pulls all transitive DT_NEEDED up onto the top binary and rewrites them to absolute paths.
Spack has a similar bind feature in the HPC world.
Inside nixpkgs, this mechanism is already used ad hoc in dozens of packages.

The cost is steep on the checklist. Absolute paths lose the LD_LIBRARY_PATH override, so the glvnd driver swap stops working, and you need an exemption list for libc, the loader itself, the GL stack, and initrd. There is also no runtime fallback: if the pinned path is gone, the program does not start.

There is also a build time fork in the road here. To rewrite a soname to an absolute path you first have to resolve it, and there are two ways to do that: run the binary's own loader and record what glibc actually picks, or walk DT_RUNPATH statically and resolve it yourself. The first is exact but executes target code, so it cannot cross compile; the second cross compiles cleanly. The absolute path tooling only ever did the first, which is why it stays a manual, per package tool rather than a default. The static walk is the same technique the ELF note cache (approach 3) later builds on.

So absolute paths are the zero disk, maximum speed option, attractive for self contained leaf applications, but wrong as a default because of the override semantics.

Approach 2: the RUNPATH symlink farm

If the problem is that the loader searches many directories, give it one. The farm idea, floated early on by Linus Heckemann (see #24844), is: for each ELF, create a single directory of symlinks pointing at exactly the libraries that ELF needs, and set its DT_RUNPATH to that one directory.

The crucial detail is that the sonames in DT_NEEDED stay short. The farm only changes where they are found, not how. Because the farm lives in DT_RUNPATH, which the loader consults after LD_LIBRARY_PATH, every override keeps working. And it builds with nothing but stock patchelf --set-rpath and symlinks, with no glibc or patchelf fork, and never executes the target binary, so it cross compiles.

But keeping the sonames short is also where it breaks the Nix model. A farm directory is a flat namespace keyed by soname, so it can hold exactly one libfoo.so.1. When a closure legitimately pulls two different builds of the same soname (the case Nix exists to allow), the farm cannot represent both, and glibc's soname based dedup collapses them to whichever loads first. Absolute paths (approach 1) sidestep this because the store path becomes the key; the farm, which deliberately keeps the bare soname, cannot.

The remaining costs are store pollution and the hwcaps floor. Every ELF gains its own extra directory of symlinks, so the store fills up with farm directories that shadow the real libraries. And the farm collapses the per directory multiplier but not the per hwcaps multiplier: the loader still probes glibc-hwcaps inside the one farm directory. So it is a large constant factor win, not an asymptotic one.

How large depends entirely on how much of the graph you farm:

Farmed scope	Failing opens	Reduction
`imagemagick`, binary only (wide 35 dir `DT_RUNPATH`)	1225 → ~213	83%
`devenv`, leaf binary only (narrow 12 dir `DT_RUNPATH`)	486 → 392	19%
`devenv`, whole graph (every dep built with the hook)	486 → 88	82%

The two devenv rows are the lesson. Farming the leaf alone barely moves the needle because the storm there is dominated by the 83 libraries resolving each other, which a leaf only farm never touches. Only whole graph adoption reaches 82%, and the residual 88 are irreducible hwcaps probes rather than real library searches. So the farm pays off immediately when a package's own binary has a wide DT_RUNPATH, but needs whole graph adoption for closure heavy applications.

Approach 3: a per DSO resolution cache in an ELF note

This is the most ambitious approach and, on the checklist, the best. The idea, designed by pennae in #207893: have patchelf write a small PT_NOTE into each library that records, for each DT_NEEDED soname, where the loader should find it. A patched glibc reads that note during loading, between the LD_LIBRARY_PATH step and the DT_RUNPATH walk, and resolves the dependency straight from it.

Placing the read after LD_LIBRARY_PATH is what makes it safe: overrides, LD_PRELOAD, and the glvnd swap all keep winning, and soname based dedup is unchanged because the sonames stay short. Each cache entry is either an exact path, which is opened directly with no search and therefore no hwcaps probing, or a directory hint for the rare cases that cannot be resolved at build time ($ORIGIN relative entries, or directories that themselves contain a glibc-hwcaps tree).

This is the only approach that preserves every semantic, adds zero closure references, and eliminates the hwcaps floor as well. pennae's original benchmark showed an armv7 workload dropping from 44s to 29s (seconds, not ms, measured under strace -cf) with about 24000 fewer syscalls. In our own end to end test of a revived, cleaned up version, a note bearing binary resolved its dependency with zero failing search probes, versus the full storm for the same binary without the note, while the LD_LIBRARY_PATH override still took precedence.

The price is the heaviest of any approach. It needs two source changes: a glibc patch so the loader understands the note, and a patchelf change to write it. It is a staging mass rebuild, because patching glibc rebuilds the world. pennae's draft was closed for lack of a go or no go decision rather than any technical failure; the main worry raised was the long term maintenance of a glibc patch.

Approach 4: a Guix style per package ld.so.cache

Guix solves the same problem in production by shipping a per package ld.so.cache, the same binary format ldconfig produces, and having a patched loader consult it (written up in their Taming the 'stat' storm with a loader cache; #207061 proposed it for nixpkgs). It preserves LD_LIBRARY_PATH and is proven at scale, but building the cache needs ldconfig/ldd for the target architecture, which breaks cross compilation, and it hits buildEnv collisions and dlmopen namespace issues. The ELF note (approach 3) was in part a response: it reads DT_NEEDED and DT_RUNPATH statically and never runs a foreign binary, so it keeps the same LD_LIBRARY_PATH guarantee without those costs.

Approach 5: delete the loader with static linking

The four approaches above all make the loader's job easier. Static linking removes the loader instead. For devenv, a self contained CLI, we spiked it: building the whole closure through pkgsStatic (which means musl, since glibc doesn't support a complete static link) drops devenv version and hook-should-activate from about 70ms to about 16ms.

Build	Loaded libraries	startup
Baseline (all dynamic, glibc)	83	~70ms
Fully static (musl)	0	~16ms

This is not a nixpkgs fix and was never meant to be. Deleting the loader also deletes everything the loader does at runtime: loading plugins on demand, honouring driver and interposer overrides, swapping in the GPU vendor's GL stack. A lot of nixpkgs depends on that, so static linking can never be a general default. It works for devenv only because devenv is a self contained CLI that talks to Nix through its own linked in C API and needs none of it.

One thing surprised us: at 16ms, with the loader gone, devenv is still far above the ~2ms a static musl hello world starts in, the rest being execve mapping the image and devenv's own startup work. Even so, 16ms is fast enough for the shell hook to drop its per directory activation cache and just run the check every prompt.

What about macOS?

macOS uses a different loader, dyld, and the storm isn't there. Nix on Darwin already ships approach 1: every Mach-O records its dependencies as absolute store paths in LC_LOAD_DYLIB rather than bare sonames, and carries no LC_RPATH. So dyld opens each library directly on the first path it tries, and system frameworks come straight from the in memory dyld shared cache without touching disk. Where the glibc devenv made ~486 failing opens, the macOS one makes essentially none.

The startup cost macOS does have is specific to Nix. To decide whether to advertise x86_64-darwin as an extra platform, libstore forked a child running arch -arch x86_64 /usr/bin/true on startup, costing ~13ms on every Nix process on Apple silicon. The fix answers the same question with a stat of Rosetta 2's fixed install path in ~0.01ms (NixOS/nix#16067).

Side by side

Every column is framed as a property you want, so ✅ is always good and ❌ always a cost. Legend: ✅ yes · ⚠️ with caveats · ❌ no · ➖ not applicable. Caveats marked ⚠️ or worth a word are footnoted below.

Approach	No glibc fork	No patchelf change	Cheap on disk	Keeps `LD_LIBRARY_PATH` / glvnd	Keeps dup sonames	Kills hwcaps floor	Cross safe
Absolute `DT_NEEDED`	✅	✅ ^a	✅	❌	⚠️ ^b	✅	⚠️ ^c
RUNPATH symlink farm	✅	✅ ^d	❌ ^e	✅	❌	❌	✅
Per DSO ELF note	❌	❌	✅	✅	✅	✅	✅
Per package ld.so.cache	❌	✅	✅	✅	⚠️ ^f	✅	❌
Static linking (musl)	✅ ^g	✅	❌ ^h	❌	➖	✅	✅ ⁱ

^a stock patchelf --replace-needed · ^b breaks on the rare duplicate soname · ^c only the static-resolution variant cross compiles, and it is unbuilt · ^d stock patchelf --set-rpath · ^e every ELF gains its own symlink directory in the store · ^f buildEnv collisions · ^g uses musl, not glibc, so no glibc fork to maintain · ^h ~82MB binary · ⁱ via pkgsStatic

The final approach

Over the week at Tacosprint we revived the ELF note cache, cleaned it up, and got it built and tested end to end. After a decade in limbo it now works: the note writer, patchelf --build-resolution-cache (#647), shipped in patchelf 0.19.0, the first patchelf release since 0.18.0 in April 2023.

The last thing to land is nixpkgs#535735, which turns the note on across the whole package set. Because it patches glibc it has to go through staging, which rebuilds the world, so every binary in nixpkgs comes out the other side resolving its libraries straight from the note. That is also where it gets exercised at scale, and we're committed to fixing whatever shakes out as we go.

Once it has proven itself there, the longer term goal is to upstream the loader patch into glibc itself, so the fix isn't a nixpkgs carry but something every store based, nix style package manager, guix included, can rely on.