[cfe-dev] FYI: ENABLE_MODULES would make building faster

[David Blaikie via cfe-dev]

On Mon, Jul 10, 2017 at 2:44 AM NAKAMURA Takumi via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> I was testing efficiency with LLVM_ENABLE_MODULES to build clang/llvm tree.
>

Awesome - thanks for trying it out & gathering all this data!


>
> * Summary
>
> ** Efficiency of Modules increases as the degree of parallelism decreases.
> For example with -j8, Modules is 67% of elapsed time than no-modules.
>
> ** With higher parallelism, Modules is inefficient.
> For example with -j72, Modules is just 23 seconds faster than no-modules.
> Then, processor usage of Modules is about 55%.
> (Assuming (user+sys)/72 is ideal)
>

As Vasil mentioned, probably implicit modules.

I have some hope/aspirations of implementing explicit modules* support in
cmake & so it'll be interesting to compare how much more parallelism can be
achieved by that. If anyone else is interesting in doing/helping with this
work, I'd love any help - I've never touched cmake... so it'll be an
adventure. (I assume it'll need changes to cmake itself, but I could be
wrong)

* Explicit modules are used at Google & implemented in Clang (though only
accessible via cc1 at the moment) - where an explicit clang invocation must
be made by the build system to build a .pcm file, and then explicit
arguments given to a clang invocation of a file using those modules, etc.


>
> ** If each module(s) is not rebuilt, rebuilding is sufficiently efficient.
> For example with -j72 to remove just *.o, processor usage is 84%.
>
> * Random notes for improvements
> - Get rid of -DCLANG_ENABLE_(ARCMT|REWRITER|SATATIC_ANALYZER), =>
> clang-config.h
> - Propagate definitions in unittests to whole the tree.
>   Modules is sensitive of -D in command line.
> - Recognize CMake and Ninja to rebuild module cache.
>   IIRC, there was the discussion about Fortran modules.
> - Parse and issue "module rebuilder" from modules.cache in advance of
> building the tree.
>   Anyways, Ninja doesn't do anything while each compilation unit is
> waiting for module lock.
>
> I expect developers and users would be happier with Modules.
> Thanks,
> Takumi
>
>
> Below, building clang with "/usr/bin/time ninja -jN clang"
> Host compiler is clang with libc++ and lld, -Asserts
> The host is Xeon 36 cores, 72 logical processors.
>
> Columns are;
> N,user,system,elapsed,Ideal:(u+s)/N,(Ideal/elapsed)
>
> N, Number of jobs -jN
> user, user time (sec)
> system, system time (sec)
> elapsed, elapsed time (sec)
> Ideal:(u+s)/N, Ideal elapsed time w/o idle
> (Ideal/elapsed): Efficiency -- elapsed processor usage
>
> *ENABLE_MODULES=OFF
> 96,11959.10,413.57,184.52,128.882,69.8%
> 80,12000.47,411.62,184.67,155.151,84.0%
> 72,11952.46,407.66,184.98,171.668,92.8%
> 64,10970.09,375.14,189.08,177.269,93.8%
> 48,8716.43,310.69,198.75,188.065,94.6%
> 41,7651.71,274.48,202.32,193.322,95.6%
> 40,7496.75,270.23,205.38,194.175,94.5%
> 39,7377.94,266.18,206.45,196.003,94.9%
> 38,7227.33,259.33,206.22,197.017,95.5%
> 37,7068.51,254.84,207.64,197.928,95.3%
> 36,6914.62,250.31,208.13,199.026,95.6%
> 35,6815.70,247.86,210.31,201.816,96.0%
> 34,6728.49,244.93,214.57,205.101,95.6%
> 33,6608.13,239.37,216.54,207.500,95.8%
> 32,6585.52,235.59,221.93,213.160,96.0%
> 28,6502.79,231.50,248.85,240.510,96.6%
> 24,6451.13,230.06,289.14,278.383,96.3%
> 20,6386.95,225.27,342.18,330.611,96.6%
> 16,6183.61,222.80,411.88,400.401,97.2%
> 8,5558.17,205.07,728.88,720.405,98.8%
>
> *ENABLE_MODULES=ON
> 96,6396.47,330.73,169.28,70.075,41.4%
> 88,6249.93,329.12,160.22,74.762,46.7%
> 80,6259.91,322.27,163.59,82.277,50.3%
> 72,6092.58,315.70,161.55,89.004,55.1%
> 64,5727.81,297.64,168.78,94.148,55.8%
> 56,5421.81,283.95,168.71,101.889,60.4%
> 48,4896.81,260.07,171.05,107.435,62.8%
> 40,4375.71,235.90,177.60,115.290,64.9%
> 32,3959.32,214.67,188.10,130.437,69.3%
> 24,3892.54,206.40,230.70,170.789,74.0%
> 16,3690.52,201.41,294.12,243.246,82.7%
> 8,3298.95,185.68,488.59,435.579,89.2%
>
> *ENABLE_MODULES_ON building to remove just *.o
> 96,6898.51,347.36,120.62,75.478,62.6%
> 88,6908.61,345.52,121.14,82.433,68.0%
> 80,6823.66,338.48,118.72,89.527,75.4%
> 72,6819.25,339.82,118.30,99.432,84.1%
> 64,6311.53,310.03,120.06,103.462,86.2%
> 56,5729.12,287.76,123.73,107.444,86.8%
> 48,5108.16,264.21,127.25,111.924,88.0%
> 40,4449.20,231.17,131.42,117.009,89.0%
> 32,3933.69,205.94,142.74,129.363,90.6%
> 24,3844.17,201.83,181.55,168.583,92.9%
> 16,3669.73,193.59,251.15,241.458,96.1%
> 8,3225.63,178.68,434.85,425.539,97.9%
>
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