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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Oz, and others,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Thanks for this data point. Two things:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l2 level1 lfo3"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Oz:</span></b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"> is there any chance you could boil out a test case that highlights
the regression most starkly? It is sad to know that GHC’s compilation time is increasing, but hard to tackle. A test case, preferably one that doesn’t rely on zillions of other libraries, increases the chances of something getting done by a factor of 10
or 100.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Do make a ticket for it as well. Otherwise it just gets lost.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l2 level1 lfo3"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Everyone</span></b><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">. It’s really hard to keep focus on keeping GHC’s compilation
time and space down. Everyone is usually focused on bugs and features. It would be incredibly helpful if someone, or a small group, could build a profiled version of GHC 7.8, 7.10, and HEAD, and track down what is happening. Usually there are multiple causes,
but a factor of 7.5 ought not to be hard to nail down.<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Please!<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Thanks.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Ozgun Ataman<br>
<b>Sent:</b> 05 April 2015 19:01<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Slower Compilation on GHC 7.8.4 (vs. 7.6.3)<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Hello,<o:p></o:p></p>
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<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Apologies upfront for the long post below, but I thought our experience on this particular point might be worth sharing. I present some observations first and then mention the particular problem around "cabal repl" facilitated development workflow. Any ideas/feedback
on the latter point would be much appreciated and please let me know if there are other metrics, info or output I can produce here for further investigation/clarification. <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
Recently at work, we upgraded a decent size project from GHC 7.6.3 to GHC 7.8.4 and as a result observed a very significant slowdown in compile times. Code changes were very minimal and should not be a factor. Any mistakes below are mine, but I can fairly confidently
say that there is a visible-to-the-eye slowdown in compile times.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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We made a few measurements to quantify the issue and here is the data:<o:p></o:p></p>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
This project has 220 direct modules (non-dependency) that are compiled with each "cabal build" and we always use sandboxes<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
Tests were performed on OS X, though we saw similar results on Linux<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
Single threaded (-j=1) wall-clock compile time has gone up by around 30% for the overall project<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
With O1 and -j=N, the overall wall-clock time is approximately the same (around 8 min), but the CPU time spent is a staggering 7.5X higher (!)<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
With O0 and -j=N the overall wall-clock time is 40% higher, but the CPU time spent is a staggering 7.5X higher (!)<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
With -O0, "cabal repl" load times have gone up by a staggering 2.5X (!)<o:p></o:p></li></ul>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level2 lfo1">
7.8.4: 3 min. 35 seconds<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level2 lfo1">
7.6.3: 1 min. 2 seconds<o:p></o:p></li></ul>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
Measuring compile times for individual modules, we see that those that are heavily loaded with lots of types, TH-facilitated type and typeclass derivations and those that contain large "blobs" of values directly at the top level now take much longer to compile:<o:p></o:p></li></ul>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level2 lfo1">
We have 10 modules that each take over 10 seconds<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level2 lfo1">
We have 3 modules that each take over 35 seconds<o:p></o:p></li></ul>
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<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
Sidenote observation: In general, parallel builds with -j appear to cause a very significant overhead under the "system" part of timing:<o:p></o:p></li></ul>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level2 lfo1">
Example with 7.6.3, O0: cabal build 140.57s user 13.25s system 100% cpu 2:33.70 total<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level2 lfo1">
Example with 7.8.4, O0: cabal build 507.83s user 655.43s system 549% cpu 3:31.59 total<o:p></o:p></li></ul>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">The main hurt here has been the infeasibility of using "cabal repl / ghci" fast-feedback style development (via emacs, vim, command-line, etc.), since:<o:p></o:p></p>
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<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
Unlike "cabal build", "cabal repl" re-compiles from scratch with every invocation ":l App.Foo.Bar", which itself is the necessary first step when starting to hack on a module (we can then use :r to somewhat help the situation)<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
The slowdown is about 2.5X as stated above: What used to take a minute now takes 3.5 minutes.<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
cabal repl does not seem to benefit from -j at all<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
cabal repl 7.8.4 appears to be hurt particularly by the heavier modules<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
The heavier modules are often at the top of the compile tree (types, derivations, etc) and are practically loaded on the critical path all the time<o:p></o:p></li></ul>
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<p class="MsoNormal">Let me know if I'm missing anything here and any/all feedback is much appreciated!<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Cheers,<br>
Oz<o:p></o:p></p>
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