4 ms·

The biggest reason is because it is multi-threaded. When building a program, the compilation step is parallelized (the build system invokes a compiler for each

by rui314 4y ago

The biggest reason is because it is multi-threaded. When building a program, the compilation step is parallelized (the build system invokes a compiler for each source file), but the final link step is not. So it is important to make the linker itself multi-threaded. But even without multi-threading, mold is still faster than other linkers. I can think of various reasons why, but I don't know which attributes how much. I believe the biggest contributor is its efficient data structure -- it is hard to make program faster by writing fast code, but it can naturally be achieved by designing efficient data structures. That said, it is hard to compare two or more programs to find out why one program is faster than the others unless their designs are similar.

glandium 4y ago
Note that recent lld is also multithreaded, and with tweaks, can be faster than mold at low core counts: https://bugzilla.mozilla.org/show_bug.cgi?id=1746462#c2 https://bugzilla.mozilla.org/show_bug.cgi?id=1746462#c2
- rui314 4y ago
  I wouldn't be surprised. In mold, if we have more than one choices to implement a feature, I always take the one that scales well for more cores even if it doesn't perform the best on low-core count machines. My assumption is that future machines will have more cores than we have today on average, so I'm optimizing mold for such computers.
  - petr_tik 4y ago
    > My assumption is that future machines will have more cores than we have today on average, so I'm optimizing mold for such computers. From the manpage of mold-1.3.0, I get the impression mold is designed to scale up to 32 cores, but not more. man mold | rg -C2 32 --threads --no-threads Use multiple threads. By default, mold uses as many threads as the number of cores or 32, whichever is the smallest. The reason why it is capped to 32 is because mold doesn't scale well beyond that point. To use only one thread, pass --no-threads or --thread-count=1. Is this correct or does the manpage need to be updated?
bertr4nd 4y ago
Could you comment on which data structures are most critical to mold’s performance, and what makes them so fast?
- bertr4nd 4y ago
  Ah, I overlooked this explanation at first: https://github.com/rui314/mold/blob/main/docs/design.md https://github.com/rui314/mold/blob/main/docs/design.md Thanks for the great write up as well as mold itself!

by rui314 4y ago

glandium 4y ago

Note that recent lld is also multithreaded, and with tweaks, can be faster than mold at low core counts: https://bugzilla.mozilla.org/show_bug.cgi?id=1746462#c2 https://bugzilla.mozilla.org/show_bug.cgi?id=1746462#c2

rui314 4y ago
I wouldn't be surprised. In mold, if we have more than one choices to implement a feature, I always take the one that scales well for more cores even if it doesn't perform the best on low-core count machines. My assumption is that future machines will have more cores than we have today on average, so I'm optimizing mold for such computers.
- petr_tik 4y ago
  > My assumption is that future machines will have more cores than we have today on average, so I'm optimizing mold for such computers. From the manpage of mold-1.3.0, I get the impression mold is designed to scale up to 32 cores, but not more. man mold | rg -C2 32 --threads --no-threads Use multiple threads. By default, mold uses as many threads as the number of cores or 32, whichever is the smallest. The reason why it is capped to 32 is because mold doesn't scale well beyond that point. To use only one thread, pass --no-threads or --thread-count=1. Is this correct or does the manpage need to be updated?

bertr4nd 4y ago

Could you comment on which data structures are most critical to mold’s performance, and what makes them so fast?

bertr4nd 4y ago
Ah, I overlooked this explanation at first: https://github.com/rui314/mold/blob/main/docs/design.md https://github.com/rui314/mold/blob/main/docs/design.md Thanks for the great write up as well as mold itself!