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

Alexandre Ganea via llvm-dev
Thu, 02 Jul 2020 04:20:20 -0000

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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