Why 64 bit? [LWN.net]

Why 64 bit?

Posted Sep 13, 2006 15:02 UTC (Wed) by chill633 (guest, #16013)
In reply to: Why 64 bit? by shemminger
Parent article: What you should (and shouldn't) expect from 64-bit Linux (Linux.com)

64-bit is faster just because it has more registers? Not quite.

My one serious test w/64-bit involved benchmarking Slackware 10.2 w/2.6 kernel against Slamd64 w/2.6 kernel. The one thing I was interested in was converting DVDs to Xvid (MPEG-4) using vobcopy and transcode.

The 32-bit Slackware version was 40-50% faster than the identical Slamd64 setup. By "identical" I mean the same computer (dual-boot), same DVD, same software installed (well, 32-bit in one case, 64-bit in the other).

I never could figure it out. Vobcopy was identical in speed, down to the second. Transcode, however, was a dog using the Slamd64 version.

My next test is going to be Gentoo, when I have some time.

Charles

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Why 64 bit?

Posted Sep 13, 2006 15:29 UTC (Wed) by ewan (subscriber, #5533) [Link]

I believe that transcode has some handwritten assembler for some of its
heavy numerical loops that's only used on supported x86-32 processors;
everything else, including both non x86 and x86-64 uses the equivalent
portable versions written in C.

Why 64 bit?

Posted Sep 13, 2006 15:33 UTC (Wed) by khim (subscriber, #9252) [Link]

Gosh. You've compared hand-optimized 32bit assembler code and generic 64bit code without SIMD commands. You found VERY surprising that optimized code which uses SIMD commands are faster then non-optimized code. Why ? Have you EVER seen situation where generic code is faster then hand-optimized code with SIMD instructions ?

Hint: if you are comparing DIFFERENT programs (and make no mistake: 32bit transcode and 64bit transoce are totally different beasts) all bets are off. Gentoo will not help there...

P.S. AFAIK 64bit SIMD implementation is in the works and there are something in CVS but it's not enabled in any builds right now...