4 ms·

This is not true. Compile times are usually much better with modules. They also don't inhibit parallelism, but perhaps you are referring to this paper (http://w

This is not true. Compile times are usually much better with modules. They also don't inhibit parallelism, but perhaps you are referring to this paper (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p1441r1.pdf http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2019/p144...), which shows that, with compiler versions from 2019, it can indeed be slower to compile with modules if you have a large number of threads and the depth of the module dependency graph is large.

Kranar 6y ago
Yes that's the correct benchmark. Do you have evidence that the situation has changed? Last I checked it still remains the case that modules inhibit parallelism and hence result in slower builds in most practical work loads. But of course if you have evidence the contrary I'd be happy to see it.
- gsliepen 6y ago
  I don't know of any newer benchmarks. However, I'm reading the results differently I guess, because the results show that with 128 threads, modules become slower only when the DAG depth is higher than 29, and that's quite a large depth! It also looks like each source file used in the benchmark only imports other modules and declares 300 variables, but nothing else. Practical workloads will have more interesting stuff in the source files, so I would expect the impact of module loading to be less, so more can be done in parallel.
  - account4mypc 6y ago
    > with 128 threads, modules become slower only when the DAG depth is higher than 29 yeah, but the same graph never shows modules being faster... it only ever shows them being the same or slower. If I'm going to put in all that work, the result should be *faster*
volta83 6y ago
> This is not true. Compile times are usually much better with modules. What significantly improves compile-times is Pre-Compiled Headers (PCH), which most compilers have supported for decades. The study you mention, does not show data for them. Having ported one >1 million LOC C++ app to use modules in two compilers, the compile time improvement of modules over PCH was not distinguishable from noise. Modules have many advantages, like better encapsulation, etc. The main thing people want from them seems to be better compile times, which is the one thing they don't deliver, at least over the PCH solutions that have existed for decades, are already supported by all build systems, etc. Compared to modules, PCHs are "zero-effort" and deliver performance instantaneously.
- colomon 6y ago
  Off-topic, but is there a guide to best practices for portable pre-compiled headers out there somewhere? I'm under considerable pressure to add pre-compiled headers for Windows to my code, and it won't have any significant benefit for me unless I can also make it work on MacOS and Linux. So far my Googling has turned up little information for any platform other than Windows, and nothing that would suggest how to do it well for all three platforms. (Well, more to the point, Visual C++, clang, and g++.)
  - volta83 6y ago
    Does your project use CMake ? CMake supports these with all major compilers... I'll just google "<your build system> pre-compiled headers" and see if there is a flag or option that you can enabled. You will definetly need quite a bit of fine tuning for apps over 500k LOC or so, but if your app is under that, and you are splitting code between .h and .cpp files appropriately, just flipping a flag might get you 80% there. The speed ups you see people get from PCHs is like 20-30% faster compile-times. So they are more a "nice to have" feature than something that will solve your compile-time problems. If your app is structured in such a way that it takes 20 min to compile, this can cut it to 15 min at most, but that would probably still suck. If you want more, then you'd need to consider other solutions like distributed build caches (sccache, etc.).
  - 6y ago

Kranar 6y ago

Yes that's the correct benchmark. Do you have evidence that the situation has changed? Last I checked it still remains the case that modules inhibit parallelism and hence result in slower builds in most practical work loads. But of course if you have evidence the contrary I'd be happy to see it.

gsliepen 6y ago
I don't know of any newer benchmarks. However, I'm reading the results differently I guess, because the results show that with 128 threads, modules become slower only when the DAG depth is higher than 29, and that's quite a large depth! It also looks like each source file used in the benchmark only imports other modules and declares 300 variables, but nothing else. Practical workloads will have more interesting stuff in the source files, so I would expect the impact of module loading to be less, so more can be done in parallel.
- account4mypc 6y ago
  > with 128 threads, modules become slower only when the DAG depth is higher than 29 yeah, but the same graph never shows modules being faster... it only ever shows them being the same or slower. If I'm going to put in all that work, the result should be *faster*

volta83 6y ago

> This is not true. Compile times are usually much better with modules. What significantly improves compile-times is Pre-Compiled Headers (PCH), which most compilers have supported for decades. The study you mention, does not show data for them. Having ported one >1 million LOC C++ app to use modules in two compilers, the compile time improvement of modules over PCH was not distinguishable from noise. Modules have many advantages, like better encapsulation, etc. The main thing people want from them seems to be better compile times, which is the one thing they don't deliver, at least over the PCH solutions that have existed for decades, are already supported by all build systems, etc. Compared to modules, PCHs are "zero-effort" and deliver performance instantaneously.

colomon 6y ago
Off-topic, but is there a guide to best practices for portable pre-compiled headers out there somewhere? I'm under considerable pressure to add pre-compiled headers for Windows to my code, and it won't have any significant benefit for me unless I can also make it work on MacOS and Linux. So far my Googling has turned up little information for any platform other than Windows, and nothing that would suggest how to do it well for all three platforms. (Well, more to the point, Visual C++, clang, and g++.)
- volta83 6y ago
  Does your project use CMake ? CMake supports these with all major compilers... I'll just google "<your build system> pre-compiled headers" and see if there is a flag or option that you can enabled. You will definetly need quite a bit of fine tuning for apps over 500k LOC or so, but if your app is under that, and you are splitting code between .h and .cpp files appropriately, just flipping a flag might get you 80% there. The speed ups you see people get from PCHs is like 20-30% faster compile-times. So they are more a "nice to have" feature than something that will solve your compile-time problems. If your app is structured in such a way that it takes 20 min to compile, this can cut it to 15 min at most, but that would probably still suck. If you want more, then you'd need to consider other solutions like distributed build caches (sccache, etc.).
- 6y ago