amd64 slower than i386 on identical AMD 64 system?

JoaoBR joao at
Tue Mar 14 22:15:20 UTC 2006

On Tuesday 14 March 2006 13:26, Bill Moran wrote:

> We've been doing some tests here (on Dell Poweredge 2850) and haven't
> done extensive tweaking (have tried different -O or any other compile
> flags)

that is good I think because even if it could give certain benefit in certain 
cases the base system should run as is. Special configs can be necessary for 
special application but should not be necessary to get a standard setup 
suitable for what it was designed

> So far, our conclusion is that running amd64 binaries on an amd64
> kernel is slower than ia32 binaries on an ia32 kernel.  We're
> comparing identical 2850 hardware, both kernels built with SMP
> (although there seem to be some issues related to running SMP on
> amd64)

I can confirm this too
SMP amd64s are having constant crashes when running >2GB and <4GB of RAM.
In order not getting anything wrong I am talking about X2-SMP mono-chip-MBs
this is not happening on dual-chip-MB with two separate processors.
I run the same hardware as UP-amd64 and it never crashes
Since this crashes are more frequent with IPI_PREEMPTION I have now some 
servers under test running without PREEMPTION at all and appearently the 
crashes are gone
Overall the amd64-SMP kernels running on X2 processors are extermly sensitive 
to non polling NICs and are crashing often. The overall performance also is 
Soon I change this cards into polling ones, seems XL is best, I do not have 
crashes anymore. 
Funny that single 64bit AMDs are running fine with non polling NICs even when 
running a SMP enabled kernel. Soon I put back the X2 ... boom.

> We've been using ubench and pgbench (since these will be PostgreSQL
> servers) to test.  We're seeing that the 64b stuff runs just a bit
> slower.  We're also seeing that the amd64 doesn't seem to scale up
> to using more than one processor, but that's an issue under investigation
> (see other thread on this list)

this I can not confirm, I get SMP X2-amds with ULE and 4BSD running on both 
cpus, same for dual-chip-MBs
But I can not say anything about PGSQL at all
My servers are cache servers in first place and I have some web and mail 
server running amd64 and the cpu scheduling seems to work well. Overall I 
have the impression that the ULE scheduler is giving better performance on a 
machine with more than 2MB/s going through


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