What is `outputting sysimage file` actually doing?

Internals & Design
1

it strikes me as odd that the longest step of building julia (like, from source with make) is ‘outputting’, which I think of as just writing something already in memory to disk. this step is pretty consistently the longest step, which makes me think it’s doing something other than just outputting:

Sysimage built. Summary:
Base ────────  82.924502 seconds 66.0224%
Stdlibs ─────  42.676037 seconds 33.9776%
Total ─────── 125.600575 seconds
    LINK usr/lib/julia/sysbase.so
    JULIA usr/lib/julia/sys-o.a
Collecting precompile statements
Executing precompile statements
Executed 400 precompile statements
Precompilation complete. Summary:
Total ─────── 455.682340 seconds
Outputting sysimage file...
Output ────── 823.662552 seconds
    LINK usr/lib/julia/sys.so
    JULIA stdlib/release.image

# or if there's an error:
Sysimage built. Summary:
Base ────────  83.128810 seconds 67.201%
Stdlibs ─────  40.572814 seconds 32.7989%
Total ─────── 123.701723 seconds
    LINK usr/lib/julia/sysbase.so
    JULIA usr/lib/julia/sys-o.a
Collecting precompile statements
Executing precompile statements
Executed 432 precompile statements
Precompilation complete. Summary:
Total ───────  19.288197 seconds
Outputting sysimage file...
Output ────── 195.348901 seconds
    LINK usr/lib/julia/sys.so
    JULIA stdlib/release.image

what’s it actually doing in the output step?

2

It’s compiling the basic system image (sys.dll in your build). The compilation engine (if you’re building from scratch) at this point is now LLVM and Julia’s own compilation machinery.

3

The other part of the answer is it is probably a lot slower than it should be.

4

Outputting is probably the wrong word, yes. It comes from the fact that the flag to generate this is --output-o. “compiling” or “generating” might be better.