# Questions about JULIA\_CPU\_TARGET

**URL:** <https://discourse.julialang.org/t/questions-about-julia-cpu-target/125339>\
**Category:** General Usage\
**Created:** [January 29, 2025, 10:05am UTC](https://discourse.julialang.org/t/questions-about-julia-cpu-target/125339 "2025-01-29T10:05:29Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![jw3126](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jw3126/32/3086_2.png) [@jw3126](https://discourse.julialang.org/u/jw3126)\
**Post date:** [January 29, 2025, 10:05am UTC](https://discourse.julialang.org/t/questions-about-julia-cpu-target/125339/1 "2025-01-29T10:05:30Z")

</div>

I want to build a docker container that will run on Amazon EC2 M6a and M7a instances. These have Zen 3 / Zen 4 based AMD Epyc processors.  
Now the docker container should contain some already precompiled juila packages. For the precompile cache to be reusable on the cloud machines, I need to specify a CPU target.  
I came up with the following and want to check if I understand it correctly:

`JULIA_CPU_TARGET="generic;znver3,clone_all;znver4,base(1)"`

When julia looks for a precompile cache, it will roughly do the following:

- It looks at the first target, `generic`. Except for very exotic ISA this will match.  
However, before picking it, it will also look at the other target.
- Next target is znver3. This would match M6a (=Zen3) and M7a (=Zen4), since Zen4 is a superset of Zen3. It would not be a match for most intel cpus.
- Last target is znver4. This would be a match only for M7a (=Zen4).

So M6a (=Zen3) would pick up the znver3 target while M7a (=Zen4) would pick up the znver4 target.

Now for the flags `clone_all` and `base(1)`.

- `znver3,clone_all` means that every single function in the precompile cache will be duplicated with a version specialized for `znver3` instruction set.
- `znver4,base(1)"` means that for znver4 we don’t clone every single function. Instead, LLVM uses a heuristic to either decide to clone a function or fall back to the znver3 version. Why `znver3`? Because of `base(1)` where `1` is the zero-based index of `znver3` in the above.

Do I get this correctly so far?

Here is another example, that I think would not do what I want:  
`JULIA_CPU_TARGET="generic;znver4,clone_all;znver3,clone_all"`  
Here the Zen4 CPU would also pick up the znver3 because it is the rightmost. Do I get this correctly as well?

Also how to debug this? Can I ask julia or LLVM or some other tool which target was chosen when loading a precompile cache? Or why a certain target is not a match?

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<div class="post-metadata">

**Author:** ![giordano](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/giordano/32/2166_2.png) [@giordano](https://discourse.julialang.org/u/giordano)\
**Post date:** [January 29, 2025, 12:08pm UTC](https://discourse.julialang.org/t/questions-about-julia-cpu-target/125339/2 "2025-01-29T12:08:37Z")

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> [@jw3126](#):
>
> Also how to debug this? Can I ask julia or LLVM or some other tool which target was chosen when loading a precompile cache? Or why a certain target is not a match?

```julia
ENV["JULIA_DEBUG"] = "loading"

```

and then you can parse the `.ji` file corresponding to the shared library that had been loaded, e.g.:  
you see

```julia
┌ Debug: Loading object cache file /home/me/.julia/compiled/v1.11/Example/blah_blah.so [...]
└ @ Base loading.jl:1282

```

and then you run the command

```julia
Base.parse_image_targets(Base.parse_cache_header("/home/me/.julia/compiled/v1.11/Example/blah_blah.ji")[7])

```

For comparison, you can see what’s the current target with

```julia
Base.current_image_targets()

```

`JULIA_DEBUG="loading"` should be helpful to explain _why_ a certain pkgimage was rejected (e.g. compiled for incompatible target).

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<div class="post-metadata">

**Author:** ![jw3126](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jw3126/32/3086_2.png) [@jw3126](https://discourse.julialang.org/u/jw3126)\
**Post date:** [January 29, 2025, 12:20pm UTC](https://discourse.julialang.org/t/questions-about-julia-cpu-target/125339/3 "2025-01-29T12:20:31Z")

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@giordano awesome this is very useful!

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<div class="post-metadata">

**Author:** ![jw3126](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jw3126/32/3086_2.png) [@jw3126](https://discourse.julialang.org/u/jw3126)\
**Post date:** [January 29, 2025, 12:24pm UTC](https://discourse.julialang.org/t/questions-about-julia-cpu-target/125339/4 "2025-01-29T12:24:36Z")

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Given a cache file with multiple targets, can I find out which one was used? Especially if `Base.current_image_targets()` is not present in the file?

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<div class="post-metadata">

**Author:** ![giordano](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/giordano/32/2166_2.png) [@giordano](https://discourse.julialang.org/u/giordano)\
**Post date:** [January 29, 2025, 12:29pm UTC](https://discourse.julialang.org/t/questions-about-julia-cpu-target/125339/5 "2025-01-29T12:29:10Z")

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I don’t know, you’ll have to read [julia/base/loading.jl at f209eba244d55afbf7aeff298434deba4fcbe30a · JuliaLang/julia · GitHub](https://github.com/JuliaLang/julia/blob/f209eba244d55afbf7aeff298434deba4fcbe30a/base/loading.jl) and see if/where that decision is taken and perhaps add debug logging if not there already (and submit a PR so that everybody can benefit)