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

Tue Jul 11 19:10:07 PDT 2017

On Tue, Jul 11, 2017 at 2:21 PM, David Blaikie via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

>
>
> 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)
>

I vaugely looked at this at one point in the past, though I got side
tracked before I could try it out.

Basically, what it seemed like could be done was something like:

1. use a PRE_BUILD add_custom_command for a given add_library command to
build the pcm (
https://cmake.org/cmake/help/v3.0/command/add_custom_command.html)
2. use an INTERFACE target_compile_definitions to add the command line flag
that dependent code needs to add to the clang invocation (
https://cmake.org/cmake/help/v3.0/command/target_compile_definitions.html#command:target_compile_definitions
)
3. an IMPORTED target library could be used to model external system
dependencies, and you would have one such IMPORTED target for each
different libc or C++ standard library

Of course, to have it properly integrated into CMake, ideally add_library
would take a list of headers and do 1. and 2. by itself. CMake would need
to have built-in knowledge of 3. also.

-- Sean Silva

>
> * 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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>>
>
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