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modules_perf_P1441R0.html
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<title>Are modules fast?</title>
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<body class="article toc2 toc-left">
<div id="header">
<h1>Are modules fast?</h1>
<div class="details">
<span id="author" class="author">Rene Rivera</span><br>
<span id="email" class="email"><a href="mailto:[email protected]">[email protected]</a></span><br>
<span id="revnumber"> P1441R0,</span>
<span id="revdate">2019-01-21</span>
</div>
<div id="toc" class="toc2">
<div id="toctitle">Table of Contents</div>
<ul class="sectlevel1">
<li><a href="#_abstract">1. Abstract</a></li>
<li><a href="#_changes">2. Changes</a>
<ul class="sectlevel2">
<li><a href="#_r0_initial">2.1. R0 (Initial)</a></li>
</ul>
</li>
<li><a href="#_introduction">3. Introduction</a></li>
<li><a href="#_parallel_build">4. Parallel Build</a>
<ul class="sectlevel2">
<li><a href="#_method">4.1. Method</a>
<ul class="sectlevel3">
<li><a href="#_modular_source">4.1.1. Modular Source</a></li>
<li><a href="#_non_modular_source">4.1.2. Non-modular Source</a></li>
</ul>
</li>
<li><a href="#_limitations">4.2. Limitations</a></li>
<li><a href="#_results">4.3. Results</a></li>
</ul>
</li>
<li><a href="#_conclusion">5. Conclusion</a></li>
<li><a href="#_acknowledgements">6. Acknowledgements</a></li>
<li><a href="#_references">7. References</a></li>
</ul>
</div>
</div>
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<div id="preamble">
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<div class="hdlist">
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<tr>
<td class="hdlist1">
Document number
</td>
<td class="hdlist2">
<p>ISO/IEC/JTC1/SC22/WG21/P1441R0</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Date
</td>
<td class="hdlist2">
<p>2019-01-21</p>
</td>
</tr>
<tr>
<td class="hdlist1">
Reply-to
</td>
<td class="hdlist2">
<p>Rene Rivera, <a href="mailto:[email protected]">[email protected]</a></p>
</td>
</tr>
<tr>
<td class="hdlist1">
Audience
</td>
<td class="hdlist2">
<p>WG21</p>
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_abstract"><a class="anchor" href="#_abstract"></a>1. Abstract</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Measurements of the performance of building C++ modules and relevant comparisons.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_changes"><a class="anchor" href="#_changes"></a>2. Changes</h2>
<div class="sectionbody">
<div class="sect2">
<h3 id="_r0_initial"><a class="anchor" href="#_r0_initial"></a>2.1. R0 (Initial)</h3>
<div class="paragraph">
<p>Initial performance measurements for synthetic tests of gcc merged modules
implementation (rev 268043, 2019-01-17 10:58:47 -0600 (Thu, 17 Jan 2019)) .</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_introduction"><a class="anchor" href="#_introduction"></a>3. Introduction</h2>
<div class="sectionbody">
<div class="paragraph">
<p>One of the stated goals of the modules proposals was performance over existing
non-modular source specifically in terms of scalable builds. This paper aims to
answer the important question of wether that goal was achieved by current
modules implementations.</p>
</div>
<div class="paragraph">
<p>Building software does not happen in the solitude of the C++ compiler. It is
a careful orchestration of a collection of tools from the preprocessor,
compiler, linker, assembler, etc controlled by the build system. These have
all been optimized to deal with the current separate compilation model and
generally perform gargantuan feats. But as such it means that performance
measurement needs to take into account, to some degree, all those in a
controlled method to generate meaningful measurements.</p>
</div>
<div class="paragraph">
<p>To that end the measurements presented here are "synthetic". They are structured
such that they simulate and isolate certain components, like the build system,
to facilitate the extraction of relevance in the data. Such methods are
described below for individual measurements.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_parallel_build"><a class="anchor" href="#_parallel_build"></a>4. Parallel Build</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Although there are various aspects comprising performance scalability of key
interest is in seeing how the potential algorithmic savings of modules "caching"
compiler work at the cost of longer DAG build chains compares against the current,
almost unlimited, parallelized building of plain source compiles. This test aims
to measure the overall compile times of modular source across varied dependency
chain depth against equivalent non-modular source.</p>
</div>
<div class="sect2">
<h3 id="_method"><a class="anchor" href="#_method"></a>4.1. Method</h3>
<div class="paragraph">
<p>Overall we perform the basic task of compiling 300 C++ source files into
corresponding object files only. We perform the compile at different dependency
DAG chain depth counts. Each source depends, either as an <code>import</code> or
<code>#include</code>, on some number of sources (headers or modules) from all the
previous DAG levels. The overall time to compile all 300 files is taken
as the result of the measurements.</p>
</div>
<div class="sect3">
<h4 id="_modular_source"><a class="anchor" href="#_modular_source"></a>4.1.1. Modular Source</h4>
<div class="paragraph">
<p>For modular source this test simulates the behavior of a build system to
compile individual modular source files in parallel executions of the
compiler to the limit of the available CPU threads. It does so in
appropriate, but simulated, dependency DAG order; compiling each DAG
level as a group. The process goes as such:</p>
</div>
<div class="paragraph">
<p>1) Generate a synthetic, and simplified, DAG to describe the build. Where
all the sources in the level can be compiled in parallel.</p>
</div>
<div class="paragraph">
<p>2) Generate all source files similar to this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-c++ hljs" data-lang="c++">export module m148;
import m0;
import m15;
import m84;
namespace m148_ns
{
export int n = 0;
export int i1 = 1;
// ...
export int i300 = 300;
}</code></pre>
</div>
</div>
<div class="paragraph">
<p>Where the imports are randomly chosen from the set of all already compiled
modules (in previous DAG levels).</p>
</div>
<div class="paragraph">
<p>3) For each DAG level compile all the source files therein with a GCC invocation
similar to:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-bash hljs" data-lang="bash">g++ -fmodules-ts -c -O0 m148.cpp</code></pre>
</div>
</div>
</div>
<div class="sect3">
<h4 id="_non_modular_source"><a class="anchor" href="#_non_modular_source"></a>4.1.2. Non-modular Source</h4>
<div class="paragraph">
<p>For no-modular source this test simulates the behavior of a build system to
compile individual all source files in parallel executions of the
compiler to the limit of the available CPU threads. As the non-modular
sources only depend on already existing header source files all source
files can be attempted to be compiled at once. The process goes as such:</p>
</div>
<div class="paragraph">
<p>1) Generate a synthetic, and simplified, DAG to describe the build with
parity to the modular build. But which is not used in the build itself.</p>
</div>
<div class="paragraph">
<p>2) Generate all source header files similar to this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-c++ hljs" data-lang="c++">#ifndef H_GUARD_h148
#define H_GUARD_h148
#include "h77.hpp"
#include "h78.hpp"
#include "h92.hpp"
namespace h148_ns
{
int n = 0;
int i1 = 1;
// ...
int i300 = 300;
}
#endif</code></pre>
</div>
</div>
<div class="paragraph">
<p>Where the includes are randomly chosen from the set of all headers files in
previous DAG levels. Although not needed to limit to previous DAG levels
this is done to keep parity with the modular source in terms of statistical
complexity of the included source size.</p>
</div>
<div class="paragraph">
<p>3) Generate all source files similar to this:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-c++ hljs" data-lang="c++">#include "h148.hpp"</code></pre>
</div>
</div>
<div class="paragraph">
<p>4) Compile all source files as one group with a GCC invocation similar to:</p>
</div>
<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-bash hljs" data-lang="bash">g++ -c -O0 h148.cpp</code></pre>
</div>
</div>
</div>
</div>
<div class="sect2">
<h3 id="_limitations"><a class="anchor" href="#_limitations"></a>4.2. Limitations</h3>
<div class="paragraph">
<p>This test has some real-life limitations borne out of the software and hardware
used for testing. Some of these limitations where discovered through
experimentation, for example with internal compiler errors.</p>
</div>
<div class="paragraph">
<p>The current test hardware is an OSX laptop with an Intel 4 core and 8 thread CPU.
And as such the execution pool for compilation only attempts to execute 8
processes at once.</p>
</div>
<div class="paragraph">
<p>Being an experimental compiler the support for compiling modules is fragile and
placed severe limits on what the modular sources could contain. Most current
C++ construct fail to compile reliably, for example templates, and cause ICEs.
This is why the generated sources only contain <code>int</code> variable definitions.</p>
</div>
<div class="paragraph">
<p>Along that same line the current modular compiler also seems to have stability
limits on how many imports can be done. This is why only a maximum of three (3)
<code>import</code> statements are included in the source.</p>
</div>
<div class="paragraph">
<p>This is a simulation with "perfect" build knowledge as all dependency and source
information is known before building starts. Hence it is only a very rough
approximation of reality where build system have to deal with dependency
discovery while building and in the face of generated source files.</p>
</div>
<div style="page-break-before: always"></div>
</div>
<div class="sect2">
<h3 id="_results"><a class="anchor" href="#_results"></a>4.3. Results</h3>
<div id="parallel_build" style="height: 500px;"></div>
<script type="text/javascript">
var parallel_build_chart = echarts.init(document.getElementById('parallel_build'));
parallel_build_chart.setOption({
legend: {
data:['Non-Modular', 'Modular']
},
tooltip: {
trigger: 'axis',
axisPointer: {
type: 'shadow'
}
},
xAxis: [ { name: 'DAG Depth', type: 'category' } ],
yAxis: [ { name: 'Wall Clock Seconds', type: 'value' } ],
series: [
{
name: 'Non-Modular',
type: 'bar',
label: { normal: { show: true, position: 'top', rotate: 90, align: 'left', verticalAlign: 'middle' } },
encode: { x: 'dag_depth', y: 'headers' },
},
{
name: 'Modular',
type: 'bar',
label: { normal: { show: true, position: 'top', rotate: 90, align: 'left', verticalAlign: 'middle' } },
encode: { x: 'dag_depth', y: 'modules' },
},
],
dataset: { source:
[["dag_depth", "headers", "modules"],
[3, 4.20, 4.03],
[32, 17.67, 7.25],
[61, 19.99, 8.92],
[90, 20.23, 11.78],
[119, 20.94, 17.66],
[148, 18.66, 17.67],
[177, 18.71, 35.62],
[206, 18.58, 35.40],
[235, 19.00, 36.04],
[264, 16.80, 36.04],
[293, 18.89, 36.10],
]
}
});
</script>
<div class="paragraph">
<p>As we can see there is an initial performance advantage to modular source
over non-modular source. This advantage diminishes as the dependency depth
grows and hence the number of parallel modular source compiles decreases.
The performance of the non-modular stays about the same throughout as the
amount of header information per source doesn’t change substantially.
Whereas the modular build reaches a point where the number of parallel
builds are either one (1) or two (2) executions making the execution
essentially linear.</p>
</div>
<div class="paragraph">
<p>This indicates that as long as we can execute parallel modular compiles at
the same rate as non-modular compiles it appears that modular compiles have
a varied but perceptible performance advantage.</p>
</div>
<div class="paragraph">
<p>This also indicates would indicate that the larger the number of real
parallel builds possible the less advantage modular compiles have over
non-modular compiles. This would be something to investigate further with
more capable hardware.</p>
</div>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_conclusion"><a class="anchor" href="#_conclusion"></a>5. Conclusion</h2>
<div class="sectionbody">
<div class="paragraph">
<p>With the limitations of the sampling currently possible the best we can
conclude is that there are some possible gains in modular compilation
but that the gains might not be attainable for really large and complex
source code projects.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_acknowledgements"><a class="anchor" href="#_acknowledgements"></a>6. Acknowledgements</h2>
<div class="sectionbody">
<div class="paragraph">
<p>Thanks to Nathan Sidwell for the work to implement the latest merged modules
proposal in GCC. And the citizens of the Internet and more specifically the
contributors to StackOverflow for the hints that made it possible to
decipher how to get the GCC svn checkout build in OSX.</p>
</div>
</div>
</div>
<div class="sect1">
<h2 id="_references"><a class="anchor" href="#_references"></a>7. References</h2>
<div class="sectionbody">
<div class="paragraph">
<p>GCC cxx-modules implementation, Nathan Sidwell
<a href="https://gcc.gnu.org/wiki/cxx-modules">https://gcc.gnu.org/wiki/cxx-modules</a></p>
</div>
<div class="paragraph">
<p>C++ Tooling Stats, Rene Rivera
<a href="https://github.com/bfgroup/cpp_tooling_stats">https://github.com/bfgroup/cpp_tooling_stats</a></p>
</div>
</div>
</div>
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