Re: [Boost-users] [Boost.Array] surprising performance issue

From: Mike Marchywka (marchywka_at_[hidden])
Date: 2007-11-27 12:06:25

( sorry if the formatting is bad, I'm fighting the new hotmail editor that seems to think
graphics are more important then text....) 

> Can you share a test case?

And, the compiled code if it is not too difficult?
I don't know if I can contribute anything but I am curious to see what the compilers generate.
If there is a spurious flag or combination of flags, it may be obvious in the generated code.
( extra calls in inner loop or some other dumb thing).

I guess it is possible you could have a wierd case where padding makes things worse by
increasing cache miss rate or something but otherwise I wouldn't expect it to be a big
deal.

Thanks.

Mike Marchywka586 Saint James WalkMarietta GA 30067-7165404-788-1216 (C)<- leave message989-348-4796 (P)<- emergency onlymarchywka_at_hotmail.comNote: Hotmail is blocking my mom's entireISP claiming it is to reduce spam but probably to force users to use hotmail. Please DON'Tassume I am ignoring you and tryme on marchywka_at_[hidden] if no replyhere. Thanks.

----------------------------------------
> From: john_at_[hidden]
> To: boost-users_at_[hidden]
> Date: Tue, 27 Nov 2007 16:52:28 +0000
> Subject: Re: [Boost-users] [Boost.Array] surprising performance issue
> 
> Bryan Green wrote:
> > Hello,
> > I've discovered a situation where using boost::array results in a
> > 2-fold performance degradation over other array types, including C
> > arrays, tr1::array, and a simple handwritten array class.
> > While the tr1::array performs well, I was suprised to find that it
> > pads the array, so sizeof(tr1::array<3,float>) ==
> > sizeof(tr1::array<4,float>).  Is that a requirement of tr1::array?
> 
> No, that will be an effect of the compiler's structure alignment rules.
> 
> BTW tr1::array and boost::array are basically the same thing, so I'm 
> surprised that there's any difference at all.
> 
> > Anyway, the performance problem can be seen using Intel C++ (v9.1) on
> > the Itanium platform, using '-O3' optimization.
> >
> > I'm forced to take boost::array out of my computation's inner loop.
> > Is this a previously known issue?  Any ideas why this performance
> > discrepancy might exist?
> 
> Can you share a test case?
> 
> As others have already noted, defining NDEBUG is likely to have quite an 
> effect as well.
> 
> HTH, John. 
> 
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> Boost-users_at_[hidden]
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