What can Rust do for astrophysics? • Hacker News Board

Re: What can Rust do for astrophysics?

#11

Post by berkut » Mon, Feb 13, 2017, 9:07 AM UTC

marblestation wrote: post #10

ktta wrote: post #3

It's crazy for me to see that C is much slower than rust. I'm almost sure that there's something wrong there. In table 1 Rust Fortran C Go 0m13.660s 0m14.640s 2m32.910s 4m26.240s They did have a note that C could be faster if language specific features could be used. Is rust really that performant compared to C? Or did they neglect to even bother checking C's performance?

We do not claim that Rust is so much performant than C, we just showed that Rust can be as fast as Fortran or C. And indeed, there must be something wrong with the C implementation. Pull requests with improvements are welcome: https://github.com/marblestation/benchmark-leapfrog

GCC produces almost 30% more instructions for the C version at -O3 level as opposed to -O2.

Clang produces practically the same between the two levels.

On OS X, so can't test if this actually makes a performance difference.

Re: What can Rust do for astrophysics?

#12

Post by walrus » Mon, Feb 13, 2017, 9:14 AM UTC

marblestation wrote: post #8

reitzensteinm wrote: post #4

Earlier quoted context omitted.

There's something wrong there. Either a corner case, or they are multithreading and vectorizing the Rust/Fortran but not the C. Which would be the equivalent of writing a comparison between a sledgehammer and jackhammer, without turning the latter on.

The code is available here: https://github.com/marblestation/benchmark-leapfrog It would be great if somebody have a fresh look at the C version, since I completely agree that it is not normal that its execution time is so far away from Fortran or Rust.

On my system:

  gcc -O3
  1m40.302s

  gcc -O3 -ffast-math
  0m5.110s

  rustc -C opt-level=3
  0m9.278s

gcc (GCC) 6.3.1 20170109

rustc 1.15.1 (021bd294c 2017-02-08)

Edit: Looking into this a bit more, I think 'rustc -C opt-level=3' is optimizing out the actual integration. If I put a println!("{}", x[0][0]) at the end, I end up with 1m43s. Not sure what 'gcc -O3 -ffast-math' is doing; I haven't looked at the disassembly.

Re: What can Rust do for astrophysics?

#13

Post by srwalker101 » Mon, Feb 13, 2017, 9:16 AM UTC

For those who are looking at doing data analysis with Rust in astrophysics, I have a crate fitsio (https://crates.io/crates/fitsio) which wraps cfitsio into rust, allowing the reading and writing of .fits files.

Re: What can Rust do for astrophysics?

#14

Post by marblestation » Mon, Feb 13, 2017, 9:18 AM UTC

walrus wrote: post #12

marblestation wrote: post #8

Earlier quoted context omitted.

The code is available here: https://github.com/marblestation/benchmark-leapfrog It would be great if somebody have a fresh look at the C version, since I completely agree that it is not normal that its execution time is so far away from Fortran or Rust.

On my system: gcc -O3 1m40.302s gcc -O3 -ffast-math 0m5.110s rustc -C opt-level=3 0m9.278s gcc (GCC) 6.3.1 20170109 rustc 1.15.1 (021bd294c 2017-02-08) Edit: Looking into this a bit more, I think 'rustc -C opt-level=3' is optimizing out the actual integration. If I put a println!("{}", x[0][0]) at the end, I end up with 1m43s. Not sure what 'gcc -O3 -ffast-math' is doing; I haven't looked at the disassembly.

Fantastic, thanks!!!

Re: What can Rust do for astrophysics?

#15

Post by arximboldi » Mon, Feb 13, 2017, 9:20 AM UTC

marblestation wrote: post #10

ktta wrote: post #3

It's crazy for me to see that C is much slower than rust. I'm almost sure that there's something wrong there. In table 1 Rust Fortran C Go 0m13.660s 0m14.640s 2m32.910s 4m26.240s They did have a note that C could be faster if language specific features could be used. Is rust really that performant compared to C? Or did they neglect to even bother checking C's performance?

We do not claim that Rust is so much performant than C, we just showed that Rust can be as fast as Fortran or C. And indeed, there must be something wrong with the C implementation. Pull requests with improvements are welcome: https://github.com/marblestation/benchmark-leapfrog

Interesting, the code looks more similar than I expected, yet the results are very different! Could it be that the difference is due to aliasing rules? It would be interesting to see what happens after adding a few restrict keywords here and there...

Re: What can Rust do for astrophysics?

#16

Post by panic » Mon, Feb 13, 2017, 9:24 AM UTC

ktta wrote: post #3

It's crazy for me to see that C is much slower than rust. I'm almost sure that there's something wrong there. In table 1 Rust Fortran C Go 0m13.660s 0m14.640s 2m32.910s 4m26.240s They did have a note that C could be faster if language specific features could be used. Is rust really that performant compared to C? Or did they neglect to even bother checking C's performance?

For me, clang -O3 optimizes the main function into this single loop:

    LBB3_1:
        addsd	%xmm1, %xmm0
        addsd	%xmm1, %xmm0
        ucomisd	%xmm0, %xmm2
        jae	LBB3_1

%xmm0 begins at 0, %xmm1 is 0.04, and %xmm2 is 3.6525E+8.

In other words, it's measuring how long it takes to count to 3.6525E+8 by adding 0.04 repeatedly. LLVM (which both Rust and clang use as a backend) optimizes away the actual work of the program.

GCC doesn't seem to optimize things out as aggressively, so that probably explains the difference: https://godbolt.org/g/lBRIIW

Re: What can Rust do for astrophysics?

#17

Post by coldtea » Mon, Feb 13, 2017, 9:26 AM UTC

ktta wrote: post #3

It's crazy for me to see that C is much slower than rust. I'm almost sure that there's something wrong there. In table 1 Rust Fortran C Go 0m13.660s 0m14.640s 2m32.910s 4m26.240s They did have a note that C could be faster if language specific features could be used. Is rust really that performant compared to C? Or did they neglect to even bother checking C's performance?

>It's crazy for me to see that C is much slower than rust.

It's not in the general case, but can be for particular code.

Unless the C program was written badly, the fact that Fortran is also faster seems to point to compiler optimizations not easy/possible in C (e.g. involving aliasing assurances).

Re: What can Rust do for astrophysics?

#18

Post by claudius » Mon, Feb 13, 2017, 9:29 AM UTC

berkut wrote: post #11

marblestation wrote: post #10

Earlier quoted context omitted.

We do not claim that Rust is so much performant than C, we just showed that Rust can be as fast as Fortran or C. And indeed, there must be something wrong with the C implementation. Pull requests with improvements are welcome: https://github.com/marblestation/benchmark-leapfrog

GCC produces almost 30% more instructions for the C version at -O3 level as opposed to -O2. Clang produces practically the same between the two levels. On OS X, so can't test if this actually makes a performance difference.

On my laptop (i7-5500U), I get the following timings (always with -march=native or -xHOST):

    GCC 6.3, -Ofast:      5.93s
    GCC 6.3, -O3:        93.06s
    GCC 6.3, -O2:       134.43s
    Clang 3.9, -O3:     346.92s
    Clang 3.9, -Ofast:    5.87s
    Intel 15.0, -O3:    110.84s
    Intel 15.0, -Ofast: 106.88s

The Intel compiler is relatively old, but mostly it seems to be a case of IEEE 754 conformance -- if enabled, things take longer, if disabled, they’re (probably) as fast in C as in Rust.

Re: What can Rust do for astrophysics?

#19

Post by simias » Mon, Feb 13, 2017, 9:30 AM UTC

walrus wrote: post #12

marblestation wrote: post #8

Earlier quoted context omitted.

The code is available here: https://github.com/marblestation/benchmark-leapfrog It would be great if somebody have a fresh look at the C version, since I completely agree that it is not normal that its execution time is so far away from Fortran or Rust.

On my system: gcc -O3 1m40.302s gcc -O3 -ffast-math 0m5.110s rustc -C opt-level=3 0m9.278s gcc (GCC) 6.3.1 20170109 rustc 1.15.1 (021bd294c 2017-02-08) Edit: Looking into this a bit more, I think 'rustc -C opt-level=3' is optimizing out the actual integration. If I put a println!("{}", x[0][0]) at the end, I end up with 1m43s. Not sure what 'gcc -O3 -ffast-math' is doing; I haven't looked at the disassembly.

-ffast-math might be a bit cheating here, for scientific work it's not always applicable. Unless rustc uses fast math by default? That would be weird.

It's surprising that gcc does such a poor job at -O3 though.

Re: What can Rust do for astrophysics?

#20

Post by Sean1708 » Mon, Feb 13, 2017, 9:49 AM UTC

cyber1 wrote: post #9

ktta wrote: post #3

It's crazy for me to see that C is much slower than rust. I'm almost sure that there's something wrong there. In table 1 Rust Fortran C Go 0m13.660s 0m14.640s 2m32.910s 4m26.240s They did have a note that C could be faster if language specific features could be used. Is rust really that performant compared to C? Or did they neglect to even bother checking C's performance?

I don't belive in this result! C slower than Rust in 11 times!!! Are they kidding?! http://benchmarksgame.alioth.debian.org/u64q/compare.php?lan... -- in this benchmarks C in most cases faster than Rust, but only in few they have the same performance.

Compiler optimisations can be very nuanced, especially in C. You might find that changing the code very slightly, or even just using a different combination of compiler options, can speed up the code a surprising amount.

Edit: Looks like using -Ofast instead of -O3 makes a big difference: https://news.ycombinator.com/item?id=13634119