[mesa-users] update on mesa platform dependency bugs: natural log function different on Mac 10.6.8 vs 10.8.5

John ZuHone jzuhone at gmail.com
Sun Nov 24 18:45:32 EST 2013


It seems to me that unless you're prepared to believe that all of the libraries that MESA depends on have been verified to that kind of accuracy across platforms, that you're not going to get that kind of precision. 

It might be reasonable to expect that from the math intrinsics from the compilers, but you're probably going to end up chasing differences in precision in the libraries.

John ZuHone
Laboratory for High-Energy Astrophysics
NASA/Goddard Space Flight Center
8800 Greenbelt Rd., Mail Code 662
Greenbelt, MD 20771
(w) 301-286-2531
(m) 781-708-5004
john.zuhone at nasa.gov
jzuhone at gmail.com

> On Nov 24, 2013, at 2:40 PM, Bill Paxton <paxton at kitp.ucsb.edu> wrote:
> 
> 
> Hi Roni,
> 
> Thanks for the input -- but I disagree!   
> It will be useful to explore these points in detail, since if you're right, then I'm in deep-deep trouble.
> 
> 
>> On Nov 24, 2013, at 1:09 AM, Roni Waldman wrote:
>> 
>> Hi Bill,
>> As you probably know, the accuracy of double precision real is 15 significant digits, so the difference you detect between the platforms is expected and unavoidable.
> 
> I disagree
> 
>> You can't expect getting the exact same result in a MESA run on different platforms.
> 
> I disagree
> 
>> The best you can do is strive to avoid crashes by checking on various platforms and fixing bugs that get caught that way.
> 
> I disagree
> 
> 
> 
> Double precision is a specific format of 64 bits, and I see no reason why the different platforms given identical 64 bit input shouldn't give exactly the same 64 bit output for a well defined math function like log any more than I'd be happy with different answers for multiply or divide -- the answer is known damn it, there is no question about what it should be once you pick a standard rule for round-off (IEEE standards e.g.).   It may be that the operation will take slightly longer to compute in order to get the correct 64 bit answer, and some folks may be willing to give up accuracy for speed, but I certainly am not when it comes to MESA.  
> 
> In the past I've myself been guilty of waving off such cross platform differences as expected and unavoidable.  But MESA has reached a point where I don't think that it is still a viable option -- I just can't hope to make the code robust and reliable if the results vary across platforms like they do now.    I need to be able to develop and debug on one platform and have high confidence that once it works on that one it will work precisely (bit-for-bit) the same on a reasonable set of other platforms too -- for example, all platforms supported by the mesasdk.
> 
> If that's impossible, I'd like to understand why it is at a deeper level than "we've tried, but we've never been able to make it work".
> 
> Cheers,
> Bill
> 
> 
> 
> 
> 
> 
> 
> 
> 
> 
> ------------------------------------------------------------------------------
> Shape the Mobile Experience: Free Subscription
> Software experts and developers: Be at the forefront of tech innovation.
> Intel(R) Software Adrenaline delivers strategic insight and game-changing 
> conversations that shape the rapidly evolving mobile landscape. Sign up now. 
> http://pubads.g.doubleclick.net/gampad/clk?id=63431311&iu=/4140/ostg.clktrk
> _______________________________________________
> mesa-users mailing list
> mesa-users at lists.sourceforge.net
> https://lists.sourceforge.net/lists/listinfo/mesa-users




More information about the Mesa-users mailing list