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Subject:

Re: Fortran 95 on the G5

From:

Drew McCormack <[log in to unmask]>

Reply-To:

Fortran 90 List <[log in to unmask]>

Date:

Tue, 16 Dec 2003 13:30:20 +0100

Content-Type:

text/plain

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Parts/Attachments

text/plain (96 lines)

On Dec 15, 2003, at 6:33 PM, Aleksandar Donev wrote:

> Hello,
>
> My department is getting a new cluster and they are thinking about
> what kinds
> of processors to use. Among the options is G5.
>
> I would appreciate any info about Fortran 95 on G5 platforms. As far
> as I
> know, Absoft, NAG and now IBM (still beta) support this chip on OS X.
> Experiences as well as any information on web-pages to look at for info
> (pricing, requirements, etc.) would be helpful.
>
"We write quantum chemistry software (ADF) for several platforms,
including OS X. We recently had access to a Dual 2Ghz G5, and did some
testing. Here is what I posted to the scitech list at Apple:

G4 466Mhz, 512Mb RAM, 1 processor
Absoft Compiler (-O)                          CPU=     9394.72
Elapsed=    11076.76
XLF Compiler (-O3 -qhot -qtune=g5 -qarch=g5)  CPU=     6003.03
Elapsed=     6284.52

G4 1Ghz, 2 processor
Absoft compiler, 2 processor with PVM         CPU=     2570.18
Elapsed=     2605.85

G5 2GHz, 512Mb RAM, 2 processor
Absoft Compiler (-O)                          CPU=     1963.01
Elapsed=     1968.87
XLF Compiler (-O3 -qhot -qtune=g5 -qarch=g5)  CPU=     1291.18
Elapsed=     1294.25
XLF Compiler (same flags), 2 processors, PVM  CPU=      739.54
Elapsed=      800.63

Dual Xeon PIV 2.4 GHz, 1 CPU
PGF90 Compiler                                CPU=     2091.60
Elapsed=     2193.39
IFC Intel compiler (-O3)                      CPU=     1143.88
Elapsed=     1272.71

Dual-CPU Pentium IV cluster, 2.4 GHz
IFC compiler, 1 processor                     CPU=     1273.81
Elapsed=      ???
IFC compiler, 2 processor                     CPU=      784.35
Elapsed=      ???

1.5 GHz Madison itanium2
IFC compiler, 1 processor                     CPU=      840.20
Elapsed=      886.14
IFC compiler, 2 processor                     CPU=      432.58
Elapsed=      460.48

1.5 GHz Itanium2
HP Compiler, 1 processor                      CPU=      586.78
Elapsed=      604.61

My conclusions:
The XLF compiler produces code that is around 30-40% faster than
Absoft, at least for ADF.

I am a bit disappointed by the G5's performance with ADF. The times for
the G5 are approximately in proportion to those for my old G4, once
clock speed has been accounted for. I was hoping that architecture
improvements in the G5, such as two FPUs, and that phenomenal frontside
bus, would be seen in the benchmarks, but ADF doesn't seem to gain much
from them.

The times for a 2GHz G5 are very similar to those of a 2.4GHz Pentium,
when using the Intel compiler on the Pentium (the portland group
compiler produces considerably slower code). The G5 does seem to scale
a bit better when run on two processors than the Pentium. I have no
benchmarks for a 3.2GHz Pentium, but assuming that performance scales
with clock speed, Intel still has the upper hand for running ADF. The
difference for other programs seems to be less than for ADF though.

Lastly, the new Itanium2 is an efficient and fast chip, but also more
expensive. The HP compiler seems to be uncharacteristically good ;-)"


Since writing these comments, I now think I know why the G5 is not
great for ADF. ADF is old code, and is basically written as a bit dot
product. The G5 has 2 FPUs, and two load/store units. A dot product
requires two load/stores per multiply, and so basically the G5 is only
using 1 FPU. This explains why it is not really any faster than the G4,
when for many codes it is.

So, if you code is well optimized, I think you will find the G5 to be
very competitive with the fastest pentiums of the day, even with half
the clock speed.

Drew McCormack
Theoretical Chemistry
Free University, Amsterdam

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