Re: Performance of computing cross derivations

[Christopher Baines]

Ludovic Courtès <ludo@gnu.org> writes:

> Christopher Baines <mail@cbaines.net> skribis:
>
>> I think you're right, while I send some other changes in #68266, I think
>> it's this change around make-rust-sysroot that has pretty much all the
>> effects on performance.
>>
>> I think the tens of thousands of duplicated packages from cross-base
>> that I was looking at are almost entirely coming from
>> make-rust-sysroot. As Ludo mentions in [1], maybe this has something to
>> do with use of cross- procedures in native-inputs, although I'm not sure
>> that moving those calls out of native-inputs is a correct thing to do.
>>
>> I don't know what the correct approach here is, but I think something
>> needs doing here to address the performance regression.
>
> I probably missed it in the thread: what commit caused the regression,
> and how can I test any changes?  I’m willing to help but I missed some
> of the context.

It's not a pure performance regression, more that in it's current form,
rust cross derivations are very expensive to compute. It's been this way
since cross-compiling was enabled in [1].

1: 
https://git.savannah.gnu.org/cgit/guix.git/patch/?id=e604972d9c697302691aeb22e9c50c933a1a3c72

I've been looking at data service slowness in processing revisions over
the last few weeks, and I think it's mostly down to this. Looking at the
revision prior to the change [2], computing all the derivations took
around 3 hours, which is ages, but still quick compared to the nearly 9
hours it took after this change [3].

2: https://data.guix.gnu.org/revision/58bbb38c5bd2e42aab9e9408d8c9d8da3409f178
3: https://data.guix.gnu.org/revision/c9e1a72cc27925484635ae01bc4de28bf232689d

Obviously having more derivations is good and that usually means more
work for the data service, but in this case it seems like things can be
sped up quite a bit.

For testing locally, I've been computing all the derivations for
i586-pc-gnu, but Efraim also posted a concise command to look at
computing some cross derivations for a subset of rust packages [4].

4: https://lists.gnu.org/archive/html/guix-devel/2024-01/msg00053.html

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