amd64 slower than i386 on identical AMD 64 system?

ray at ray at
Tue Mar 14 11:49:32 UTC 2006

At 12:38 PM 3/14/2006 +0100, Alexander Konovalenko wrote:
| On Tuesday 14 March 2006 11:40, JoaoBR wrote:
| >
| > so where is your comparism? My point was that the same hardware is faster
| > running i386
| >
| > I experience this also on X2 machines but do not have two machines to
| > compare I have a X2-4400-SMP running amd64 and a X2-4200-SMP running i386
| > and it gives me the same numbers running ubench
| >
| I have experienced that -O3 and -ffast-math optimizations flags on AMD64 might 
| cause degrade in performance, meaning that -O2 is the fastest. When you 
| compile your ports what opt. flags do you use? Try to reinstall ubench with 
| different flags. Also code produced with gcc4.x is faster then system 
| compiler and has no degrade effect. Some time ago I was interested in fast 
| scientific computations and did some primitive benchmark tests 
| (
| I just wonder what will happen if you run ubench (compiled for i386) on AMD64, 
| will performance overcome amd64 ubench? 
| /Alexander Konovalenko

+2 cents mode on... :)

I'm just coming in on the tail end of the message (missed the previous stuff).
I recently did some benchmarks between AMD64 and i386 (version 5.4) on the same
hardware.  I initially saw that the i386 ran faster also.  However, after
searching a bit further, I discovered that I had made an error: the i386 kernel
has the SMP stuff compiled into the generic kernel by default, while the AMD64
(at least on 5.4) does not.  It has a separate kernel file called SMP (as I
recall), which adds the SMP line and then does an include for the rest of the
generic kernel config file (or something to that effect).  

Anyway, if you are testing back and forth, it's easy to forget that and end up
accidently testing an i386 with SMP against an AMD64 without SMP.  

Like I say, I'm coming in on the tail end of the thread, so I might be off base,
but it might be something worth double checking - just to be 100% sure.



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