3 ms·

As ever, it depends, probably on whether your code is dominated by matrix-matrix linear algebra. BLIS DGEMM on my SKX workstation runs at ~88GF, or ~48 if I re

by gnufx 5y ago

As ever, it depends, probably on whether your code is dominated by matrix-matrix linear algebra. BLIS DGEMM on my SKX workstation runs at ~88GF, or ~48 if I restrict it to using the haswell configuration (somewhat different on a typical compute node). But yes, I'd rather have twice the cores and memory bandwidth with AVX2. For those that don't know: non-benchmark code usually doesn't get close to peak floating point performance, constrained by memory bandwidth and/or inter-node communication. Everyone is trying to get the compute performance from GPUs anyway.

kergonath 5y ago
Sure, there are gains if the code is right and the density of AVX instructions is high enough. In most cases that’s not really the case as these dense matrix multiplications are part of a larger algorithm, which includes things like time integration and some calculations that are not straightforward matrix products. The logic is also more complex. The state changes and reduced frequency caused by AVX-512 make the tradeoff difficult. And as you say, if you have to redesign the code you might as well do it for a GPU that does not have the same issues.
- gnufx 5y ago
  I agree it's difficult. Do people ever just turn off the AVX512 in their BLAS, FFT, etc. because of throttling? I've never seen that, but then I usually don't see people measure anything usefully, if they even know the effect. For varying mileage, I'm more familiar with, say, molecular dynamics codes, where Gromacs, for instance, has its own hand-tuned avx512 in places, and cp2k might use it at least for FFT, small matrices (libxsmm) and eigenfunctions (ELPA), as well as BLAS. Elsewhere, some R applications (on the desktop in this context) might hit BLAS heavily. I should have made the point that there's avx512 and avx512, for people who don't know. At least GCC and LLVM -march=native does half-width SIMD on SKX et al because of throttling. If you wonder where a factor of two went, -mprefer-vector-width=512 might help.

by gnufx 5y ago

kergonath 5y ago

Sure, there are gains if the code is right and the density of AVX instructions is high enough. In most cases that’s not really the case as these dense matrix multiplications are part of a larger algorithm, which includes things like time integration and some calculations that are not straightforward matrix products. The logic is also more complex. The state changes and reduced frequency caused by AVX-512 make the tradeoff difficult. And as you say, if you have to redesign the code you might as well do it for a GPU that does not have the same issues.

gnufx 5y ago
I agree it's difficult. Do people ever just turn off the AVX512 in their BLAS, FFT, etc. because of throttling? I've never seen that, but then I usually don't see people measure anything usefully, if they even know the effect. For varying mileage, I'm more familiar with, say, molecular dynamics codes, where Gromacs, for instance, has its own hand-tuned avx512 in places, and cp2k might use it at least for FFT, small matrices (libxsmm) and eigenfunctions (ELPA), as well as BLAS. Elsewhere, some R applications (on the desktop in this context) might hit BLAS heavily. I should have made the point that there's avx512 and avx512, for people who don't know. At least GCC and LLVM -march=native does half-width SIMD on SKX et al because of throttling. If you wonder where a factor of two went, -mprefer-vector-width=512 might help.