[gmx-developers] Segfault in 2021-beta1 enabling SVE and ArmIE emulator

[gilles at rist.or.jp]

Guido,

Strictly speaking, SVE gives developers and compilers the opportunity to 
build/generate vector length agnostic (VLA) binaries/libraries
whose performance almost as good as their fixed vector length 
counterpart. Such VLA programs can run (correctly) on any arm+sve 
processors,
regardless the vector length that is available at runtime.

Gromacs is highly optimized and assumes the vector length is known at 
cmake time, and this is assumed in several places of the code base.
So the most straightforward and efficient way of supporting SVE is to 
fix the SVE vector length at cmake time.

FWIW, I made a proof-of-concept of non bonded VLA kernels, and 
performance is not even close to current (fixed vector length)
non bonded kernels.

Cheers,

Gilles

----- Original Message -----
> Hi Gilles,
> 
> Thanks a lot, I tested again and it is working with ArmIE for a length 
of 512. I will close the Issue in GitLab.
> 
> SVE allows to set the vector length at run time, is there a particular 
reason for doing this at compile time?
> 
> Cheers, Guido. 
> 
> -----Original Message-----
> From: gromacs.org_gmx-developers-bounces at maillist.sys.kth.se [mailto:
gromacs.org_gmx-developers-bounces at maillist.sys.kth.se] On Behalf Of 
Gilles Gouaillardet
> Sent: Tuesday, October 27, 2020 10:05 AM
> To: gmx-developers at gromacs.org
> Subject: Re: [gmx-developers] Segfault in 2021-beta1 enabling SVE and 
ArmIE emulator
> 
> Guido,
> 
> 
> The SVE vector length must be fixed at cmake time, and the default 
value is 512.
> 
> In your case, you have to pass
> 
> -DGMX_SIMD_ARM_SVE_LENGTH=256
> 
> to your cmake command line.
> 
> 
> Cheers,
> 
> 
> Gilles
> 
> On 10/27/2020 5:58 PM, Guido Giuntoli wrote:
> >
> > Hi,
> >
> > I have run the nonbonded-benchmark using the ArmIE emulator.
> >
> > I compiled gmx_d (DOUBLE FP) with GCC 10.2 for SVE by using:
> >
> > *CC=gcc-10 CXX=g++-10 \*
> >
> > *cmake .. \*
> >
> > *        -DCMAKE_BUILD_TYPE=Release \*
> >
> > *-DCMAKE_C_FLAGS="-march=armv8-a+sve" \*
> >
> > *-DCMAKE_CXX_FLAGS="-march=armv8-a+sve" \*
> >
> > *        -DGMX_MPI=off \*
> >
> > *        -DGMX_GPU=off \*
> >
> > *        -DGMX_USE_OPENCL=off \*
> >
> > *        -DGMX_BUILD_MDRUN_ONLY=off \*
> >
> > *        -DGMX_DOUBLE=on \*
> >
> > *        -DGMX_SIMD=ARM_SVE \*
> >
> > *        -DBUILD_SHARED_LIBS=off \*
> >
> > *        -DGMX_FFT_LIBRARY=fftw3 \*
> >
> > *        -DGMX_INSTALL_NBLIB_API=OFF \*
> >
> > *        -DGMXAPI=OFF \*
> >
> > *        | tee cmake.log*
> >
> > I ran the benchmark with:
> >
> > *armie -msve-vector-bits=256 \*
> >
> > *        gmx_d nonbonded-benchmark \*
> >
> > *        -size 1 \*
> >
> > *        -nt 1 \*
> >
> > *        -simd 4xm \*
> >
> > *        -coulomb ewald \*
> >
> > *        -notable \*
> >
> > *        -combrule geometric \*
> >
> > *        -nohalflj \*
> >
> > *        -noenergy \*
> >
> > *        -noall \*
> >
> > *        -cutoff 1 \*
> >
> > *        -iter 5 \*
> >
> > *        -warmup 0 \*
> >
> > *        -nocycles*
> >
> > I get:
> >
> > *armie's SVESigHandler caught signal SIGSEGV from the process being 
> > emulated, passing it to the default signal handler.*
> >
> > *Guest process terminated by signal: 15152 Segmentation fault /home/.
> > /codes/gromacs/gromacs-2021-beta1/build-sve/bin/gmx_d*
> >
> > According to the last meeting, I am eager to know if this branch has 
> > passed the regression tests using SVE. BTW, are you using SVE-
enabled 
> > hardware or emulation for your development/tests?
> >
> > *Best regards | Mit freundlichen Grüßen*
> >
> > **
> >
> > *Guido Giuntoli***
> >
> > **
> >
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> >
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