llvm dev - Jan 2020 - LLVM build performance with LLVM

David,
Yes, I was indeed trying to build LLVM with a Debug build. I'll stop doing
that from now on.

I am on JSC's JURON machine which has 251 GB of memory on the login node,
that's more than sufficient
to do a build, I suppose, and the linker is LLD as LLVM has a CMake
variable to select the linker.

On Fri, Jan 3, 2020 at 11:02 AM David Blaikie <dblaikie at gmail.com>
wrote:
> Were you trying to use a Debug build to build LLVM? Yes, that would be
> very slow.
>
> If you mean you were using a release build of LLVM to build a Debug build
> of LLVM - yeah, that's generally going to be recommended. Did this get
> slower/change significantly in performance? Many people have trouble with
> building Debug builds (no matter the host compiler) especially if
they're
> using bfd-ld, since it's quite slow/uses a lot of memory. There are a
few
> other issues to do with memory usage (do you have less than about a GB of
> RAM per CPU? Then you'll probably hit swapping by default & have a
bad time
> - there are ways around that)
>
>
> On Thu, Jan 2, 2020 at 5:42 PM Itaru Kitayama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Hi,
>> When building LLVM, is it always recommended to do it with the
>> latest official release, currently, it is 9.0.1? I ask because when I
>> tried
>> it with a Debug build, it took an enormous amount of time on POWER8.
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
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On Thu, Jan 2, 2020 at 6:09 PM Itaru Kitayama <itaru.kitayama at
gmail.com>
wrote:
> David,
> Yes, I was indeed trying to build LLVM with a Debug build. I'll stop
doing
> that from now on.
>
I mean, you can - it's just going to be super slow & mostly not what you
want, unless you're trying to debug that building process. Usually you'd
want to isolate one particular test case, maybe even reduce it, before
running it with a debug build of LLVM which will be quite slow (because
it's not optimized at all).

> I am on JSC's JURON machine which has 251 GB of memory on the login
node,
> that's more than sufficient
> to do a build, I suppose, and the linker is LLD as LLVM has a CMake
> variable to select the linker.
>
Yep, sounds like you're just tripping over the fact that an
unoptimized/debug build of LLVM is very slow. *thumbs up*

>
> On Fri, Jan 3, 2020 at 11:02 AM David Blaikie <dblaikie at gmail.com>
wrote:
>
>> Were you trying to use a Debug build to build LLVM? Yes, that would be
>> very slow.
>>
>> If you mean you were using a release build of LLVM to build a Debug
build
>> of LLVM - yeah, that's generally going to be recommended. Did this
get
>> slower/change significantly in performance? Many people have trouble
with
>> building Debug builds (no matter the host compiler) especially if
they're
>> using bfd-ld, since it's quite slow/uses a lot of memory. There are
a few
>> other issues to do with memory usage (do you have less than about a GB
of
>> RAM per CPU? Then you'll probably hit swapping by default &
have a bad time
>> - there are ways around that)
>>
>>
>> On Thu, Jan 2, 2020 at 5:42 PM Itaru Kitayama via llvm-dev <
>> llvm-dev at lists.llvm.org> wrote:
>>
>>> Hi,
>>> When building LLVM, is it always recommended to do it with the
>>> latest official release, currently, it is 9.0.1? I ask because when
I
>>> tried
>>> it with a Debug build, it took an enormous amount of time on
POWER8.
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>>
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As I am facing the serious super slow down:
https://bugs.llvm.org/show_bug.cgi?id=44407 , I'd like to
learn more about how it can be minimized. When debugging an app, there are
times building it with a
Debug build LLVM can not be avoided, but a situation like 40 times time
increase (40 minutes) is hard
to work with.


On Fri, Jan 3, 2020 at 11:14 AM David Blaikie <dblaikie at gmail.com>
wrote:
>
>
> On Thu, Jan 2, 2020 at 6:09 PM Itaru Kitayama <itaru.kitayama at
gmail.com>
> wrote:
>
>> David,
>> Yes, I was indeed trying to build LLVM with a Debug build. I'll
stop
>> doing that from now on.
>>
>
> I mean, you can - it's just going to be super slow & mostly not
what you
> want, unless you're trying to debug that building process. Usually
you'd
> want to isolate one particular test case, maybe even reduce it, before
> running it with a debug build of LLVM which will be quite slow (because
> it's not optimized at all).
>
>
>> I am on JSC's JURON machine which has 251 GB of memory on the login
node,
>> that's more than sufficient
>> to do a build, I suppose, and the linker is LLD as LLVM has a CMake
>> variable to select the linker.
>>
>
> Yep, sounds like you're just tripping over the fact that an
> unoptimized/debug build of LLVM is very slow. *thumbs up*
>
>
>>
>> On Fri, Jan 3, 2020 at 11:02 AM David Blaikie <dblaikie at
gmail.com> wrote:
>>
>>> Were you trying to use a Debug build to build LLVM? Yes, that would
be
>>> very slow.
>>>
>>> If you mean you were using a release build of LLVM to build a Debug
>>> build of LLVM - yeah, that's generally going to be recommended.
Did this
>>> get slower/change significantly in performance? Many people have
trouble
>>> with building Debug builds (no matter the host compiler) especially
if
>>> they're using bfd-ld, since it's quite slow/uses a lot of
memory. There are
>>> a few other issues to do with memory usage (do you have less than
about a
>>> GB of RAM per CPU? Then you'll probably hit swapping by default
& have a
>>> bad time - there are ways around that)
>>>
>>>
>>> On Thu, Jan 2, 2020 at 5:42 PM Itaru Kitayama via llvm-dev <
>>> llvm-dev at lists.llvm.org> wrote:
>>>
>>>> Hi,
>>>> When building LLVM, is it always recommended to do it with the
>>>> latest official release, currently, it is 9.0.1? I ask because
when I
>>>> tried
>>>> it with a Debug build, it took an enormous amount of time on
POWER8.
>>>> _______________________________________________
>>>> LLVM Developers mailing list
>>>> llvm-dev at lists.llvm.org
>>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>
>>>
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