llvm dev - Jan 2020 - LLVM build performance with LLVM

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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On Thu, Jan 2, 2020 at 10:55 PM Itaru Kitayama <itaru.kitayama at
gmail.com>
wrote:
> 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.
>
Not sure I follow - you shouldn't need a debug build of clang to debug an
application built with clang. You'd use a release build of clang to build
your application with debug info (by passing -g).

>
>
> 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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At least, to obtain enough information from libomptarget while running my
offloading app on GPU capable environment, I have to build it with an LLVM
which was built in Debug mode.

On Sat, Jan 4, 2020 at 4:34 David Blaikie <dblaikie at gmail.com> wrote:
>
>
> On Thu, Jan 2, 2020 at 10:55 PM Itaru Kitayama <itaru.kitayama at
gmail.com>
> wrote:
>
>> 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.
>>
>
> Not sure I follow - you shouldn't need a debug build of clang to debug
an
> application built with clang. You'd use a release build of clang to
build
> your application with debug info (by passing -g).
>
>
>>
>>
>> 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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