According to the readme, Rust is supported, did anyone tried and noticed improvement? rui314/mold: Mold: A Modern Linker 🦠 https://github.com/rui314/mold

  • @[email protected]English
    2•11 months ago

    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.

    • @[email protected]English
      2•11 months 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.