[llvm-dev] LLVM build performance with LLVM

Itaru Kitayama via llvm-dev
Sun, 05 Jan 2020 19:39:23 -0000

Yes, exactly.

On Sun, Jan 5, 2020 at 23:43 Mehdi AMINI <joker.eph at gmail.com> wrote:

>
>
> On Fri, Jan 3, 2020 at 8:58 PM Itaru Kitayama via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> I have just tried a Release build of LLVM but
>> with
>> LIBOMPTARGET_ENABLE_DEBUG enabled. The app build became super fast, but
I
>> can not obtain debugging information that I need at runtime.
>>
>
> The description for this flag seems to be: LIBOMPTARGET_ENABLE_DEBUG
> "Allow debug output with the environment variable
LIBOMPTARGET_DEBUG=1"
>
> Is this what you are looking for? Or are you looking into getting debug
> info (Dwarf) for the OpenMP runtime itself?
>
> --
> Mehdi
>
>
>
>>
>> On Sat, Jan 4, 2020 at 5:03 David Blaikie <dblaikie at gmail.com>
wrote:
>>
>>> I'm still confused by that - whether or not the LLVM you built
has debug
>>> info in it shouldn't at all change what goes into the binaries
that LLVM
>>> produces (modulo a few bugs, but nothing that'd produce drastic
performance
>>> swings). You mention in the bug that it's
>>> specifically LIBOMPTARGET_ENABLE_DEBUG that is slowing down your
>>> compilation time? Not the use of a Debug build of LLVM? Have you
tried a
>>> Release build of LLVM but with LIBOMPTARGET_ENABLE_DEBUG enabled?
>>>
>>> On Fri, Jan 3, 2020 at 11:58 AM Itaru Kitayama <itaru.kitayama
at gmail.com>
>>> wrote:
>>>
>>>> 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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