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

[NAKAMURA Takumi via llvm-dev]

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

* 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)

** 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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