this post was submitted on 26 Jul 2023
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Rust Programming

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Did you try Mold? (lemmy.ml)
submitted 1 year ago by [email protected] to c/[email protected]
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According to the readme, Rust is supported, did anyone tried and noticed improvement? rui314/mold: Mold: A Modern Linker ๐Ÿฆ  https://github.com/rui314/mold

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[โ€“] [email protected] 2 points 1 year ago* (last edited 1 year ago) (1 children)

Okay. I updated mold to v2.0.0. Added "-Z", "time-passes" to get link times, ran cargo with --timings to get CPU utilization graphs. Tested on two projects of mine (the one from yesterday is "X").

Link times are picked as the best from 3-4 runs, changing only white space on main.rs.

lto="fat" lld mold
project X (cu=1) 105.923 106.380
Project X (cu=8) 103.512 103.513
Project S (cu=1) 94.290 94.969
Project S (cu=8) 100.118 100.449

Observations (lto="fat"): As expected, not a lot of utilization of multi-core. Using codegen-units larger than 1 may even cause a regression in link time. Choice of linker between lld and mold appears to be of no significance.

lto="thin" lld mold
project X (cu=1) 46.596 47.118
Project X (cu=8) 34.167 33.839
Project X (cu=16) 36.296 36.621
Project S (cu=1) 41.817 41.404
Project S (cu=8) 32.062 32.162
Project S (cu=16) 35.780 36.074

Observations (lto="thin"): Here, we see parallel LLVM_lto_optimize runs kicking in. Testing with codegen-units=16 was also done. In that case, the number of parallel LLVM_lto_optimize runs was so big, the synchronization overhead caused a regression running that test on a humble workstation powered by an Intel i7-7700K processor (4 physical, 8 logical cores only). The results will probably look different running this test case (cu=16) in a more powerful setup. But still, the choice of linker between lld and mold appears to be of no significance.

lto=false lld mold
project X (cu=1) 29.160 29.231
Project X (cu=8) 8.130 8.293
Project X (cu=16) 7.076 6.953
Project S (cu=1) 11.996 12.069
Project S (cu=8) 4.418 4.462
Project S (cu=16) 4.357 4.455

Observations (lto=false): Here, codegen-units becomes the dominant factor with no heavy LLVM_lto_optimize runs involved. Going above codegen-units=8 does not hurt link time. Still, the choice of linker between lld and mold appears to be of no significance.

lto="off" lld mold
project X (cu=1) 29.109 29.201
Project X (cu=8) 5.896 6.117
Project X (cu=16) 3.479 3.637
Project S (cu=1) 11.732 11.742
Project S (cu=8) 2.354 2.355
Project S (cu=16) 1.517 1.499

Observations (lto="off"): Same observations as lto=false. Still, the choice of linker between lld and mold appears to be of no significance.

Debug builds link in <.4 seconds.

[โ€“] [email protected] 2 points 1 year ago* (last edited 1 year ago)

Debug builds link in <.4 seconds.

With such a small program I expected fixed costs to dominate. Not surprising there is no or almost no difference. You really have to go to cases where linking takes 10s of seconds to see scaling difference, even between ld.bfd and ld.gold.

I did those sort of measurements for my work at the time (a few years ago, before mold was a thing). I have not had the cause or opportunity to measure lld or mold however. Maybe it isn't faster than lld (certainly it seems so for small projects), but I don't think these result say anything useful about larger programs.

The best option is not to take the word of others (myself included) however, but measure on your own application and see which is the best option in your case.

If you however do want to measure linking something big, look at something like Chromium. That isn't rust code though. Not sure what a suitably large rust project would be.