[cfe-dev] [llvm-dev] RFC: Replacing the default CRT allocator on Windows

Tue Jul 7 10:43:05 PDT 2020

I couldn’t tell you offhand, and a quick google search does not turn up a comparison. However, one obvious advantage of the debug heap vs address sanitizer is that the debug heap Just Works out of the box with no configuration. I prefer to use the builtin tools as much as possible because I find that integrating a bunch of random external stuff tends to be brittle and have little sharp edges here and there. Address Sanitizer is certainly better than nothing, but Windows has a built in instrumented malloc that just works, and works well. Ideally it remains available.

Realistically, we used the system allocator, and it was found wanting. So now somebody is going to do the work to use a custom allocator. Since we had to change the allocator once, we may have to do it again. One size does not fit all, so we probably want different allocators on different platforms. Just using the system allocator should be a first-class option. Since supporting N custom allocators means N different build configurations, I’d like to see the windows allocator remain the default in some configuration to ensure that it doesn’t bitrot.

Thanks,
   Christopher Tetreault

From: Zachary Turner <zturner at roblox.com>
Sent: Tuesday, July 7, 2020 10:25 AM
To: Chris Tetreault <ctetreau at quicinc.com>
Cc: Alexandre Ganea <alexandre.ganea at ubisoft.com>; LLVM Dev <llvm-dev at lists.llvm.org>; cfe-dev at lists.llvm.org
Subject: [EXT] Re: [llvm-dev] [cfe-dev] RFC: Replacing the default CRT allocator on Windows

Note that ASAN support is present on Windows now.  Does the Debug CRT provide any features that are not better served by ASAN?

On Tue, Jul 7, 2020 at 9:44 AM Chris Tetreault via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
For release builds, I think this is fine. However for debug builds, the Windows allocator provides a lot of built-in functionality for debugging memory issues that I would be very sad to lose. Therefore, I would request that:

  1.  This be added as a configuration option to either select the new allocator or the windows allocator
  2.  The Windows allocator be used by default in debug builds

Ideally, since you’re doing this work, you’d implement it in such a way that it’s fairly easy for anybody to use whatever allocator they want when building LLVM (on any platform, not just windows), and it’s not just hardcoded to system allocator vs whatever allocator ends up getting added. However, as long as I can use the windows debug allocator I’m happy.

Thanks,
   Christopher Tetreault

From: cfe-dev <cfe-dev-bounces at lists.llvm.org<mailto:cfe-dev-bounces at lists.llvm.org>> On Behalf Of Alexandre Ganea via cfe-dev
Sent: Wednesday, July 1, 2020 9:20 PM
To: cfe-dev at lists.llvm.org<mailto:cfe-dev at lists.llvm.org>; LLVM Dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: [EXT] [cfe-dev] RFC: Replacing the default CRT allocator on Windows

Hello,

I was wondering how folks were feeling about replacing the default Windows CRT allocator in Clang, LLD and other LLVM tools possibly.

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.

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.

Time to link clang.exe with LLD and -flto on 36-core:
  Windows CRT heap allocator: 38 min 47 sec
  mimalloc: 2 min 22 sec
  rpmalloc: 2 min 15 sec
  snmalloc: 2 min 19 sec

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.

Two questions arise:

  1.  The licencing. Should we embed one of these allocators into the LLVM tree, or keep them separate out-of-the-tree?
  2.  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.

Please see demo patch here: https://reviews.llvm.org/D71786

Thank you in advance for the feedback!
Alex.

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