Re: Quad Xeon vs. Dual Itanium

John Gibson wrote:
> Assuming similar memory and disk sub-systems, I am considering a Quad
> Xeon system vs. a Dual Itanium for PostgreSQL.  I believe that the
> PostgreSQL code is written for 32 bit and not optimized for the 64 bit
> Itanium cpu.  That makes me think that the Xeon system would be a better
> choice.

Unfortunately, both have issues.

With Itanium, you absolutely have to use the Intel compiler. GCC is
beyond unoptimized for Itanium; you should expect 1/3rd the performance
with Postgres of what the Intel compiler would do. On the otherhand,
I've heard a few caveats about the Intel compiler that a lot of the
tricks they use are 100% designed for the Spec benchmarks and could
break real world code. Depends on what your stomach is for risk taking
on how many of the optimization flags you are willing to turn on.

Quad Xeon has it's own problems. The shared bus makes the 2P to 4P
scaling a bit problematic. The more intensive you use the memory
subsystem (which probably is the case with database uses), the bigger
hit you will take. As an example, SpecIntRate's 2->4 scaling for the
XeonMP is 75% but the SpecFPRate (SpecFP uses much larger datasets)
scaling drops to a mere 31%. Which memory usage model your DB would fall
under unfortunately can only be tested by you. Also throw in the need
for PAE to go over 2GB (~1GB for caching under most OS's) and you could
see some performance penalties there versus a 64-bit server.

But looking at the straight SpecIntRate numbers though, a 4P Xeon MP
will be faster than an 2P Itanium. There's enough of a performance gap
where penalties for shared memory bus and PAE won't make enough of a
difference.

4P XeonMP 2.8GHz   47.4
4P Xeon 2GHz       34.7
2P Itanium2 1.5GHz 25.4

As for pricing, you'll have to look that up yourself. :) Personally, I'm
very fond of Opteron servers due to the combination of 64-bit + ondie
memory controller + point-to-point inter-cpu connect. As a point of
comparison, 2P-4P Opteron Spec scaling numbers are 87% ad 76%.