[cfe-dev] Out of Memory Building Clang and LLVM

[David Blaikie via cfe-dev]

On Thu, Jun 20, 2019 at 3:26 PM Chris Bieneman via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> 13GB to link clang and 60+ GB build directory is consistent with my
> experience building LLVM+Clang on Linux using the GNU linker.
>
> Using lld will dramatically reduce memory usage during linking and will
> also radically speed up linking. As will reducing the number of LLVM
> targets (LLVM_TARGETS_TO_BUILD). The documentation on the GettingStarted
> page is pretty far out of date, but it is tough to give a reasonable
> estimate given that there are so many configuration values that could
> impact the size and system resources required for a build.
>
> Since it was brought up in another email, generally speaking users on OS X
> with ld64 don't encounter these kinds of issues. That is because ld64's
> algorithm for linking is radically different from the GNU linker which
> results in shorter link times and lower memory usage.
>

I imagine the other reason is the debug info model (those sort of numbers
of memory usage and build directory size sound like a Debug build to me) -
leaving the debug info in the object files - less work for the linker and
smaller linked binaries. You can get /some/ of that on Linux if you enable
Split DWARF (LLVM_USE_SPLIT_DWARF).


>
> -Chris
>
> On Jun 20, 2019, at 1:07 PM, Neil Nelson via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
> From https://llvm.org/docs/GettingStarted.html
>
> Just finished a clang build on Xubuntu 19.04 using cpp.
>
> cmake -G Ninja -DLLVM_ENABLE_PROJECTS="clang" ../llvm
>
> Memory usage hit a maximum at the link for bin/clang-9 at 13 gigabytes
> with ninja -j 1.
>
> Disk usage for the build directory was 61 gigabytes.
>
> My sense is that this kinds of usage is more than what a typical new user
> would expect. Perhaps the page above might be changed to reflect the
> expected result and with an easier route noted by Erich below.
> On 6/20/19 6:56 AM, Keane, Erich via cfe-dev wrote:
>
> Clang definitely hits a pathological case in LD that makes it do terrible
> things if you use use more than a process or two in DEBUG.
>
> However, if you use “gold” or “LLD” instead, it works fine.  Assuming you
> have LLD, you can do:
> -DLLVM_USE_LINKER=lld or –DLLVM_ENABLE_LLD=ON
>
> Otherwise you should have gold available, which you can do with:
> -DLLVM_USE_LINKER=gold
>
> I can only run about 3-4 links at a time with LD and can do –j30 with no
> problems with Gold.
>
> *From:* cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org
> <cfe-dev-bounces at lists.llvm.org>] *On Behalf Of *James Courtier-Dutton
> via cfe-dev
> *Sent:* Thursday, June 20, 2019 3:27 AM
> *To:* nick <xerofoify at gmail.com> <xerofoify at gmail.com>
> *Cc:* cfe-dev at lists.llvm.org Developers <cfe-dev at lists.llvm.org>
> <cfe-dev at lists.llvm.org>
> *Subject:* Re: [cfe-dev] Out of Memory Building Clang and LLVM
>
>
>
> On Wed, 19 Jun 2019 at 23:36, nick via cfe-dev <cfe-dev at lists.llvm.org>
> wrote:
>
> Greetings,
>
> Seems I'm running out of memory on a host system trying to build llvm,
> tooling and clang. It was configured
> as:
> cmake -DLLVM_ENABLE_PROJECTS=clang -G "Unix Makefiles" ../llvm
>
> and build with:
>
> make -j4 using gcc as the host compiler, version 8.
>
> The dmesg I'm getting is related to the linker and not sure if the linker
> has a bug or it's normal
> to expect 3GB or more used for linking clang. Doesn't seem to be but not
> sure.
>
> dmesg output:
> [186517.023397] Out of memory: Kill process 2143 (ld) score 286 or
> sacrifice child
> [186517.023403] Killed process 2143 (ld) total-vm:4622188kB,
> anon-rss:3202188kB, file-rss:4kB, shmem-rss:0kB
>
>
> HI,
>
> I have a 16 Core/32 Thread system with 32GB RAM, and it can have
> difficulty building CLANG/LLVM.
> The link stage definitely needs to be with make -j1.
>
> That being said, if you force a "Release" version, and not the "Debug"
> version, much less RAM is needed.
>
> Kind Regards
>
> James
>
>
>
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