this post was submitted on 26 Jul 2023
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Did you try Mold? (lemmy.ml)
submitted 1 year ago by Gargari@lemmy.ml to c/rust@lemmy.ml
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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€“] MoSal@lemm.ee 6 points 1 year ago (1 children)

codegen-units=1, debug=true, varying lto

lto = "fat"

Flags Clean build time Pre-strip size Post-strip size
(default) 2:31 90.8207MiB 7.3374MiB
["-Z", "gcc-ld=lld"] 2:31 91.9731MiB 7.3332MiB
linker = "clang" 2:32 90.8207MiB 7.3375MiB
linker = "clang"; fuse-ld="mold" 2:31 92.1107MiB 7.3334MiB

lto = "thin"

Flags Clean build time Pre-strip size Post-strip size
(default) 1:33 96.9630MiB 8.1695MiB
["-Z", "gcc-ld=lld"] 1:32 98.3889MiB 8.1777MiB
linker = "clang" 1:33 96.9631MiB 8.1695MiB
linker = "clang"; fuse-ld="mold" 1:32 98.6903MiB 8.1797MiB

lto = false

Flags Clean build time Pre-strip size Post-strip size
(default) 1:32 113.5656MiB 8.0601MiB
["-Z", "gcc-ld=lld"] 1:30 115.1210MiB 8.1122MiB
linker = "clang" 1:32 113.5656MiB 8.0602MiB
linker = "clang"; fuse-ld="mold" 1:31 115.4679MiB 8.0663MiB

lto = "off"

Flags Clean build time Pre-strip size Post-strip size
(default) 1:33 113.5666MiB 8.0601MiB
["-Z", "gcc-ld=lld"] 1:31 115.1231MiB 8.1122MiB
linker = "clang" 1:32 113.5667MiB 8.0602MiB
linker = "clang"; fuse-ld="mold" 1:31 115.4697MiB 8.0662MiB

codegen-units=8, debug=true, varying lto

lto = "fat"

Flags Clean build time Pre-strip size Post-strip size
(default) 2:21 104.9842MiB 7.6304MiB
["-Z", "gcc-ld=lld"] 2:19 106.1436MiB 7.6264MiB
linker = "clang" 2:21 104.9882MiB 7.6344MiB
linker = "clang"; fuse-ld="mold" 2:19 106.2864MiB 7.6325MiB

lto = "thin"

Flags Clean build time Pre-strip size Post-strip size
(default) 1:12 134.1112MiB 9.0445MiB
["-Z", "gcc-ld=lld"] 1:09 136.1897MiB 9.0660MiB
linker = "clang" 1:12 134.1113MiB 9.0446MiB
linker = "clang"; fuse-ld="mold" 1:09 136.4466MiB 9.0494MiB

lto = false

Flags Clean build time Pre-strip size Post-strip size
(default) 1:14 158.1049MiB 9.0328MiB
["-Z", "gcc-ld=lld"] 1:11 159.9998MiB 9.1129MiB
linker = "clang" 1:14 158.1050MiB 9.0328MiB
linker = "clang"; fuse-ld="mold" 1:12 160.3123MiB 9.0428MiB

lto = "off"

Flags Clean build time Pre-strip size Post-strip size
(default) 0:57 145.9463MiB 9.4586MiB
["-Z", "gcc-ld=lld"] 0:54 148.6021MiB 9.6001MiB
linker = "clang" 0:57 145.9464MiB 9.4587MiB
linker = "clang"; fuse-ld="mold" 0:55 148.8842MiB 9.4668MiB

mold appears to be similar but not faster than lld.

With the caveat that this is not a proper benchmark since:

  • I didn't measure link time alone.
  • I didn't bother running each case multiple times picking the fastest run (since I perceived the differences to be insignificant).

And a side note, lto = false appears to be practically useless.

[โ€“] Vorpal@lemmyrs.org 3 points 1 year ago (1 children)

  1. With a total build time of less than 2 minutes, my guess is that link time is fairly small. At work we have a c++ project that takes around 40 minutes to build. Only in the incremental case does link time dominate (upwards of 10 seconds with gold, haven't tried lld or mold).

  2. My understanding is that mold supposedly has more scalable data structures and algorithms (better complexity). Thus for small links there likely will be little difference. So you need to measure it on your actual use case to see if it makes a difference.

  3. mold supposedly can take more advantage of multi core. How many cores did you run on? Again this will likely not show for small links, since there is also overhead in splitting work across threads.

[โ€“] MoSal@lemm.ee 1 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€“] Vorpal@lemmyrs.org 1 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.