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

[James Y Knight via llvm-dev]

https://www.blackhat.com/docs/us-16/materials/us-16-Yason-Windows-10-Segment-Heap-Internals-wp.pdf
seems to be the paper that goes with the sides I linked before. It says
that there's some sort of adaptive mechanism that allocates per-CPU
"affinity slot" if it detects lots of lock contention. Which seems like it
*ought* to have good multithreaded behavior.

I see in your initial email that the sample backtrace is in "free", not
allocate. Is that just an example, or is "free" where effectively all the
contention is? If the latter, I wonder if we're hitting some pathological
edge-case...e.g. allocating on one thread, and then freeing on different
threads, or something along those lines.


On Thu, Jul 2, 2020, 11:56 PM Alexandre Ganea <alexandre.ganea at ubisoft.com>
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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>
>
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