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benchmark-leapfrog's Introduction

Simple N-Body with LeapFrog integrator

Implementation in C, Fortran, Go, Julia and Rust of a very simple N-Body simulator with 3 particles using a LeapFrog integrator. Presented in What can the programming language Rust do for astrophysics?, to appear in the Proceedings of the IAU Symposium 325 on Astroinformatics.

The code has evolved since publication, implementing several suggestion made by the Hacker News and Rust subreddit communities. The current times on a 1,6 GHz Intel Core i5 machine:

  • C: 2m53.504s
  • Fortran: 3m16.314s
  • Rust: 2m33.082s
  • Go: 4m10.233s
  • Julia: 3m59.223s

Output positions for the two particles after a one million year simulation:

[
    [0.00011259641961937496, 0.00011235312238407964, 0.00000982962683070781],
    [-2.986377308237493, 14588403.613911422, 1276320.0322206458]
]

The original article's conclusions are still valid for this simple user case, Rust can be as fast and precise as C or Fortran. Additionally, Rust characteristics ensures that scientific results are not affected by memory management issues and this is a great advantage for reliable scientific computation.

Compilation & execution

From each directory:

make clean
make
time target/optimized/leapfrog

benchmark-leapfrog's People

Contributors

asmunder avatar btracey avatar freeboson avatar jdstorm avatar kristjansson avatar marblestation avatar mikeinnes avatar sbinet avatar

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Watchers

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Forkers

kristjansson jdstorm btracey sbinet-staging asmunder manodeep noxbru kristofferc freeboson pkestene chessoli lmiq skphy leotot48

benchmark-leapfrog's Issues

Benchmarks should e.g. write output

As written, the compiled benchmarks don't measure the time taken for a n-body simulation. For example, the Fortran compiles (GNU Fortran (GCC) 4.8.5 20150623 (Red Hat 4.8.5-11)) to

0000000000400610 <main>:
  400610:       48 83 ec 08             sub    $0x8,%rsp
  400614:       e8 d7 ff ff ff          callq  4005f0 <_gfortran_set_args@plt>
  400619:       be 70 0b 40 00          mov    $0x400b70,%esi
  40061e:       bf 07 00 00 00          mov    $0x7,%edi
  400623:       e8 98 ff ff ff          callq  4005c0 <_gfortran_set_options@plt>
  400628:       66 0f 57 c0             xorpd  %xmm0,%xmm0
  40062c:       f2 0f 10 0d 64 05 00    movsd  0x564(%rip),%xmm1        # 400b98 <options.38.2155+0x28>
  400633:       00
  400634:       f2 0f 10 15 64 05 00    movsd  0x564(%rip),%xmm2        # 400ba0 <options.38.2155+0x30>
  40063b:       00
  40063c:       0f 1f 40 00             nopl   0x0(%rax)
  400640:       f2 0f 58 c1             addsd  %xmm1,%xmm0
  400644:       f2 0f 58 c1             addsd  %xmm1,%xmm0
  400648:       66 0f 2e d0             ucomisd %xmm0,%xmm2
  40064c:       73 f2                   jae    400640 <main+0x30>
  40064e:       31 c0                   xor    %eax,%eax
  400650:       48 83 c4 08             add    $0x8,%rsp
  400654:       c3                      retq
  400655:       0f 1f 00                nopl   (%rax)

The loop from 400640 to 40064c corresponds to the time = time + half_time_step lines only i.e. the entire n-body simulation is optimised away. For these benchmarks to be at all useful, you must ensure that the compiler is unable to prove that virtually all the work is unnecessary. The easiest way to do this is to e.g. write out the results at the end, and measure only the computation time if that's what's of interest to you.

Bug in C implementation.

benchmark-leapfrog/c/leapfrog.c

Lines 33 to 35 in 773792a

double dx = x[i][0] - x[j][0];
double dy = x[i][0] - x[j][0];
double dz = x[i][0] - x[j][0];

This isn't correct, right? Shouldn't it be:

            double dx = x[i][0] - x[j][0];
            double dy = x[i][1] - x[j][1];
            double dz = x[i][2] - x[j][2];

Rust isn't actually doing any work?

I'm not an expert on rust, but I added to the end: println!("{:?}",x); (to check the answer), to the end of main and now the runtime on my machine has gone from 11 seconds to more than 2 minutes. Still running.

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