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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Thanks for the suggestion James, it reduces the commit by about ~900 MB (14,9 GB -> 14 GB).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Unfortunately it does not solve the performance problem. The heap is global to the application and thread-safe, so every malloc/free locks it, which evidently doesn’t scale. We could manually create
thread-local heaps, but I didn’t want to go there. Ultimately allocated blocks need to share ownership between threads, and at that point it’s like re-writing a new allocator. I suppose most non-Windows platforms already have lock-free thread-local arenas,
which probably explains why this issue has gone (mostly) unnoticed.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><b><span lang="FR">De :</span></b><span lang="FR"> James Y Knight <jyknight@google.com>
<br>
<b>Envoyé :</b> July 2, 2020 6:08 PM<br>
<b>À :</b> Alexandre Ganea <alexandre.ganea@ubisoft.com><br>
<b>Cc :</b> Clang Dev <cfe-dev@lists.llvm.org>; LLVM Dev <llvm-dev@lists.llvm.org><br>
<b>Objet :</b> Re: [cfe-dev] RFC: Replacing the default CRT allocator on Windows<o:p></o:p></span></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Have you tried Microsoft's new "segment heap" implementation? Only apps that opt-in get it at the moment. Reportedly edge and chromium are getting large memory savings from switching, but I haven't seen performance comparisons.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">If the performance is good, seems like that might be the simplest choice <o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><a href="https://docs.microsoft.com/en-us/windows/win32/sbscs/application-manifests#heaptype">https://docs.microsoft.com/en-us/windows/win32/sbscs/application-manifests#heaptype</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><a href="https://www.blackhat.com/docs/us-16/materials/us-16-Yason-Windows-10-Segment-Heap-Internals.pdf">https://www.blackhat.com/docs/us-16/materials/us-16-Yason-Windows-10-Segment-Heap-Internals.pdf</a><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Thu, Jul 2, 2020, 12:20 AM Alexandre Ganea via cfe-dev <<a href="mailto:cfe-dev@lists.llvm.org">cfe-dev@lists.llvm.org</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="FR-CA">Hello,</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span lang="FR-CA"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I was wondering how folks were feeling about replacing the default Windows CRT allocator in Clang, LLD and other LLVM tools possibly.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The CRT heap allocator on Windows doesn’t scale well on large core count machines. Any multi-threaded workload in LLVM that allocates often is impacted by this. As a result, link
times with ThinLTO are extremely slow on Windows. We’re observing performance inversely proportional to the number of cores. The more cores the machines has, the slower ThinLTO linking gets.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">We’ve replaced the CRT heap allocator by modern lock-free thread-cache allocators such as rpmalloc (unlicence), mimalloc (MIT licence) or snmalloc (MIT licence). The runtime performance
is an order of magnitude faster.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Time to link clang.exe with LLD and -flto on 36-core:<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Windows CRT heap allocator: 38 min 47 sec<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> mimalloc: 2 min 22 sec<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> rpmalloc: 2 min 15 sec<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> snmalloc: 2 min 19 sec<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">We’re running in production with a downstream fork of LLVM + rpmalloc for more than a year. However when cross-compiling some specific game platforms we’re using other downstream
forks of LLVM that we can’t change.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Two questions arise:<o:p></o:p></p>
<ol start="1" type="1">
<li class="m-664316201346764609msolistparagraph" style="mso-list:l0 level1 lfo1">
The licencing. Should we embed one of these allocators into the LLVM tree, or keep them separate out-of-the-tree?
<o:p></o:p></li><li class="m-664316201346764609msolistparagraph" style="mso-list:l0 level1 lfo1">
If the answer for above question is “yes”, given the tremendous performance speedup, should we embed one of these allocators into Clang/LLD builds by default? (on Windows only) Considering that Windows doesn’t have a LD_PRELOAD mechanism.<o:p></o:p></li></ol>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Please see demo patch here:
<span lang="FR-CA"><a href="https://reviews.llvm.org/D71786" target="_blank"><span lang="EN-CA">https://reviews.llvm.org/D71786</span></a></span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thank you in advance for the feedback!<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Alex.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
cfe-dev mailing list<br>
<a href="mailto:cfe-dev@lists.llvm.org" target="_blank">cfe-dev@lists.llvm.org</a><br>
<a href="https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev" target="_blank">https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev</a><o:p></o:p></p>
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