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
    3•11 months ago

    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.

    • @[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.