---
name: schunix-build-speed
description: The build is fast now (no-op ~2s); the four things that made it slow and the traps that will make it slow again
metadata: 
  node_type: memory
  type: project
  originSessionId: a665e14b-86bd-4b41-a228-51377f3e9225
  modified: 2026-08-04T17:27:56.099Z
---

**A no-op `make` went from 1m48s to ~2s on 2026-08-04**, and a full kernel
rebuild from ~110s to ~7s. Almost none of it was compilation — with only 2.4s
of CPU in that 1m48, it was I/O across WSL's `/mnt/d` boundary. Four causes,
all still worth not reintroducing:

- **`tools/mkfs` built the image on the file**, one `lseek` + one 512-byte
  `write` per block: 300,000 syscall pairs, 150,000 of them just to zero it.
  It now assembles the image in RAM and writes once. It also stops at the last
  non-zero block and `ftruncate`s, so ~2 MB is written instead of 147 MB.
- **The image was rebuilt on every invocation.** `sysroot-install` is phony, so
  everything depending on it always ran. There is now a `build/sysroot.stamp`
  whose mtime only advances when a `find`-based summary of the staged tree
  actually changes (the `cmp -s ... || mv` idiom — **do not add a `touch`, and
  note `touch -c` still updates an existing file's mtime**, which is exactly
  the bug I wrote and had to remove).
- **`ports/toybox` re-linked its 236 applets every build**, because port.mk
  declares `install` .PHONY for every port. It hangs off
  `$(BUILDDIR)/toybox.installed` now. This was the sneaky one: it bumped the
  mtime of 236 files in `sysroot/` and so invalidated the stamp above.
- **mkfs and mkschfs re-read every file to dedup it.** They now check
  `(st_dev, st_ino)` first — toybox's applets are real hard links, so 236 of
  them are settled by one `fstat` instead of ~500 MB of reads.

Two real bugs found in the Makefile while doing this:

- **`$(BUILD)/kernel/**/*.d` only matched one directory level.** make's wildcard
  has no recursive `**`. `arch/x86_64/` and `fs/schfs/` therefore had **no
  header dependency tracking at all** — editing `include/schfs.h` left stale
  objects that linked fine. It uses `$(shell find ...)` now.
- **`CFLAGS` is recursively expanded and had a `$(shell)` appended**, so the
  stack-protector probe re-ran on every expansion: one extra gcc per object
  file. Probed once into `NO_SSP :=` now.

`schunix.img` is no longer `.PHONY` (it is a real file), and `MAKEFLAGS +=
-j$(nproc)` is on by default — use `make -j1` for readable output from a
failing build.

Verify with `make schfs-check` plus [[schunix-verify-by-driving-shell]]; the
superblock checksum is printed by mkfs and should not change when only build
mechanics change.