[petsc-dev] Building PETSc on LLNL Lassen
jacob.fai at gmail.com
jacob.fai at gmail.com
Mon Dec 14 09:37:48 CST 2020
According to https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html :
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is currently not implemented and treated equal to -ffp-contract=off.
The default is -ffp-contract=fast.
We don't need to turn it on, although maybe the default is -ffp-contract only after certain versions of gcc.
Best regards,
Jacob Faibussowitsch
(Jacob Fai - booss - oh - vitch)
Cell: (312) 694-3391
| -----Original Message-----
| From: petsc-dev <petsc-dev-bounces at mcs.anl.gov> On Behalf Of Jed Brown
| Sent: Monday, December 14, 2020 09:30
| To: Pierre Jolivet <pierre at joliv.et>; Stefano Zampini
| <stefano.zampini at gmail.com>
| Cc: Petsc-dev <petsc-dev at mcs.anl.gov>
| Subject: Re: [petsc-dev] Building PETSc on LLNL Lassen
|
| Honestly, we should pass -ffp-contract=fast in our standard optimization
| options. I'm not aware of any sensitive fp [1] that needs strict evaluation in
| PETSc, and if it does, we can use:
|
| #pragma STDC FP_CONTRACT off
|
| [1] An example is result = a*a - a*a (where each value a comes from a
| separate source). With contraction, this may be evaluated as
|
| x = a * a; // simple multiply
| result = a * a - x; // fused multiply-add yields nonzero
|
| The FMA carries extra accuracy through the operation, rounding the final
| result. It's generally more accurate, but doesn't satisfy this symmetry, which
| might be expected in some circumstances (e.g., convergence tolerances
| when up against machine precision).
|
| Pierre Jolivet <pierre at joliv.et> writes:
|
| > Don’t know if this still applies, but here is what Jed said about this 10
| months ago (https://gitlab.com/petsc/petsc/-
| /merge_requests/2466#note_275686270 <https://gitlab.com/petsc/petsc/-
| /merge_requests/2466#note_275686270>):
| > “A minor issue is that -ffp-contract=off is implied by -std=c*, versus -ffp-
| contract=fast being default otherwise (with GCC).”
| >
| > Thanks,
| > Pierre
| >
| >> On 14 Dec 2020, at 9:35 AM, Stefano Zampini
| <stefano.zampini at gmail.com> wrote:
| >>
| >> While we are discussing this dialect stuff, do we still want to test for
| gnu++{11|14} extensions before testing for c++{11|14} ? We get warnings
| when using KOKKOS since it replaces gnu++14 with c++14. What is the added
| value of using gnu++{11|14}?
| >>
| >> Il giorno lun 14 dic 2020 alle ore 08:21 Barry Smith <bsmith at petsc.dev
| <mailto:bsmith at petsc.dev>> ha scritto:
| >>
| >> So two configure sins. Setting values running configure without
| >> logging them into configure.log and setting values without testing
| >> them with configure tests :-)
| >>
| >> We need to finally deal properly with setting -cbin in configure; just
| defaulting causes all kinds of undebuggable problems. Why can't we also just
| set it to the C++ compiler? Is it because it cannot handle the MPI c++
| compiler so it needs to be set to the underlying C++ compiler?
| >>
| >> Barry
| >>
| >> It is also strange that Jacob's configure.log seems to believe the IBM
| compiler is a GNU compiler (but because of poor logging configure does not
| log this information; fix coming in a MR soon).
| >>
| >>> On Dec 13, 2020, at 6:32 PM, Junchao Zhang <junchao.zhang at gmail.com
| <mailto:junchao.zhang at gmail.com>> wrote:
| >>>
| >>> Jacob,
| >>> Do you need to add 'CUDAFLAGS=-ccbin xlc++' to specify the host
| >>> compiler for CUDA? Note in cuda.py I added
| >>>
| >>> if self.compilers.cxxdialect in ['C++11','C++14']: #nvcc is a C++ compiler
| so it is always good to add -std=xxx. It is even crucial when using thrust
| complex (see MR 2822)
| >>> self.setCompilers.CUDAFLAGS += ' -std=' +
| >>> self.compilers.cxxdialect.lower()
| >>>
| >>> In your configure.log, there are
| >>> #define PETSC_HAVE_CXX_DIALECT_CXX11 1 #define
| >>> PETSC_HAVE_CXX_DIALECT_CXX14 1
| >>>
| >>> I guess without -ccbin, nvcc uses gcc by default and your gcc does not
| support C++14.
| >>>
| >>> --Junchao Zhang
| >>>
| >>>
| >>> On Sun, Dec 13, 2020 at 1:25 PM Jacob Faibussowitsch
| <jacob.fai at gmail.com <mailto:jacob.fai at gmail.com>> wrote:
| >>> Hello All,
| >>>
| >>> Does anyone have any experience building petsc with cuda support on
| Lassen? I’ve been having trouble building with ibm xl compilers + spectrum-
| mpi + nvcc. NVCC seems to not like -std=c++14 argument, complaining that
| its configured host compiler doesn’t support it, but compiling the following
| “test.cc <http://test.cc/>":
| >>>
| >>> #include <stdlib.h>
| >>>
| >>> int main(int argc, char **argv)
| >>> {
| >>> int i = 1;
| >>> i += argc;
| >>> return(i);
| >>> }
| >>>
| >>> With mpicc -std=c++14 test.cc <http://test.cc/> produces zero errors.
| >>> --------------------------------------------------------------------
| >>> ----
| >>>
| >>> Modules loaded:
| >>>
| >>> module load xl/2020.11.12-cuda-11.1.1
| >>> module load spectrum-mpi
| >>> module load cuda/11.1.1
| >>> module load python/3.8.2
| >>> module load cmake
| >>> module load valgrind
| >>> module load lapack
| >>>
| >>> My configure commands:
| >>>
| >>> ./configure --with-cc=mpicc --with-cxx=mpiCC --with-fc=mpifort
| >>> --with-cuda --with-debugging=1 PETSC_ARCH=arch-linux-c-debug
| >>>
| >>> The error:
| >>>
| >>> TESTING: findMPIInc from
| config.packages.MPI(config/BuildSystem/config/packages/MPI.py:636)
| **********************************************************
| *********************
| >>> UNABLE to CONFIGURE with GIVEN OPTIONS (see configure.log
| for details):
| >>> --------------------------------------------------------------------
| >>> ----------- Bad compiler flag:
| >>> -I/usr/tce/packages/spectrum-mpi/ibm/spectrum-mpi-rolling-release/in
| >>> clude
| >>>
| **********************************************************
| **********
| >>> ***********
| >>>
| >>> The actual configure.log error:
| >>>
| >>> Executing: nvcc -c -o
| >>> /var/tmp/petsc-2v0k4k61/config.setCompilers/conftest.o
| >>> -I/var/tmp/petsc-2v0k4\ k61/config.setCompilers
| >>> -I/var/tmp/petsc-2v0k4k61/config.types -g -std=c++14
| >>> -I/usr/tce/packages/s\ pectrum-mpi/ibm/spectrum-mpi-rolling-
| release/include -Wno-deprecated-gpu-targets /var/tmp/petsc-2v\
| 0k4k61/config.setCompilers/conftest.cu <http://conftest.cu/> Possible
| ERROR while running compiler:
| >>> stderr:
| >>> nvcc warning : The -std=c++14 flag is not supported with the
| >>> configured host compiler. Flag will be\ ignored.
| >>> Source:
| >>> #include "confdefs.h"
| >>> #include "conffix.h"
| >>>
| >>> int main() {
| >>> ;
| >>> return 0;
| >>> }
| >>> Rejecting compiler flag
| >>> -I/usr/tce/packages/spectrum-mpi/ibm/spectrum-mpi-rolling-
| release/include due to nvcc warning : The -std=c++14 flag is not supported
| with the configured host compiler. Flag will be ignored.
| >>>
| >>>
| >>> Best regards,
| >>>
| >>> Jacob Faibussowitsch
| >>> (Jacob Fai - booss - oh - vitch)
| >>> Cell: (312) 694-3391
| >>
| >>
| >>
| >> --
| >> Stefano
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