[Llvm-bgq-discuss] Status of OpenMP support in BG/Q

[Schlottke, Michael]

Since a couple of people asked me privately, here are the results for everyone. Please note however, that my previous statement on the compiler performance difference (clang being only 5% slower than IBM) does not hold up anymore: unfortunately I did have some old results in mind, but the numbers in this email are the most up to date ones as obtained by one of my students in the past two month. It now seems like clang is up to 20% slower than IBM's XL compiler, depending on the compiler settings:

ZFS/Finite Volume kernel (IBM/clang):
production: 971s / 955s (-1.6%)
extreme: 873s / 977s (+11.8%)

ZFS/Lattice Boltzmann kernel (IBM/clang):
production: 107s / 117s (+10%)
extreme: 97s / 118s) (+21%)

The baseline is always the IBM compiler. All results were obtained using a single thread on BG/Q "JUQUEEN" at FZ Juelich between Nov. '13 and Jan. '14. The measurements were taken from our inner loops (total inner loop time > 100s), without any substantial I/O taking place (I/O time is less than 0.1%). These are the compiler flags used for the two build types "production" and "extreme":

mpixlcxx/*: -qarch=qp -qtune=qp -qmaxmem=-1 -qreport -qlist -qlanglvl=variadictemplates
mpixlcxx/production:  -g -O2
mpixlcxx/extreme: -O5 -qstrict

mpibgclang++/*: -std=c++11 -stdlib=libc++  -O3 -DNDEBUG -mtune=native -fvectorize -fslp-vectorize
mpibgclang++/production:  -g
mpibgclang++/extreme:  -fstrict-aliasing -fslp-vectorize-aggressive -fno-rtti -fno-exceptions -fomit-frame-pointer

Of course these numbers are not a perfect measurement: only one sample is not enough, the compiler flags could use some tuning etc. However, they might offer an initial insight for others who are thinking about switching completely.

Regards,

Michael




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