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

Mon Jul 6 08:52:26 PDT 2020

Let's make sure we're all working from the set set of assumptions.  There
are several layers of allocators in a Windows application, and I think it's
worth being explicit, since I've already seen comments referring to
multiple different "allocators."

The C++ allocators and new and delete operators are generally built on
malloc from the C run-time library.  malloc implementations typically rely
on process heaps (Win32 HeapAlloc) and/or go directly to virtual memory
(Win32 VirtualAlloc).  HeapAlloc itself uses VirtualAlloc.

C++ --> malloc (CRT) --> HeapAlloc (Win32) --> VirtualMalloc (Win32)
           |                                       ^
           +---------------------------------------+

This proposal is talking about replacing the malloc layer.

The "low-fragmentation heap" and "segment heap" are features of
process heaps (HeapAlloc).

Since those are in different layers, there's some orthogonality there.  If
your malloc implementation uses HeapAlloc, then tweaking process heap
features may affect performance assuming the bottleneck isn't in malloc
itself.  If you replace the malloc implementation with one that completely
bypasses the process heap by going directly to virtual memory, that's a
horse of a different color.

The only Windows app I worked on that switched malloc implementations was a
cross-platform app.  On every platform, tcmalloc was a win, *except *for
Windows.  We kept Windows on the default Microsoft implementation because
it performed better.  (The app was a multithreaded real-time graphics
simulation.  The number of threads was low, maybe 4 or 5.)

On Thu, Jul 2, 2020 at 8:57 PM Alexandre Ganea via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Thanks for the suggestion James, it reduces the commit by about ~900 MB
> (14,9 GB -> 14 GB).
>
>
>
> 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.
>
>
>
>
>
> *De :* James Y Knight <jyknight at google.com>
> *Envoyé :* July 2, 2020 6:08 PM
> *À :* Alexandre Ganea <alexandre.ganea at ubisoft.com>
> *Cc :* Clang Dev <cfe-dev at lists.llvm.org>; LLVM Dev <
> llvm-dev at lists.llvm.org>
> *Objet :* Re: [cfe-dev] RFC: Replacing the default CRT allocator on
> Windows
>
>
>
> 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.
>
>
>
> If the performance is good, seems like that might be the simplest choice
>
>
>
>
> https://docs.microsoft.com/en-us/windows/win32/sbscs/application-manifests#heaptype
>
>
>
>
> https://www.blackhat.com/docs/us-16/materials/us-16-Yason-Windows-10-Segment-Heap-Internals.pdf
>
>
>
> On Thu, Jul 2, 2020, 12:20 AM Alexandre Ganea via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> 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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