[MITgcm-support] ifort optimization puzzle

Patrick Heimbach heimbach at MIT.EDU
Tue Mar 8 04:46:32 EST 2005

Hi Martin,

a potentially similar problem was reported (off list)
by a user from Norway. He identified problems when using 
optimization with ifort in conjunction with the mnc package.
To confirm, could you try using same setup without mnc,
and see whether problems persist?
(I will try to get more details).


Quoting Martin Losch <mlosch at awi-bremerhaven.de>:

> Hi,
> together with Michael Schodlok I have set up a 1D experiment: 1 column 
> of water (nx=ny=1, overlap=2, nr=30, dz=30*10). We specify zero net 
> surface heat flux (qnet = 0), but -50W/m^2 shortwave heat flux (warming 
> due to the sun), in order to drive kpp. This works all very nicely and 
> we were happy, the mean temperature was constant, as it should (why 
> didn't we stop there?). Then I suggested to move to a faster machine 
> (from a $%^&* SUN to a nice linux-box with ifort). We used the standard 
> ifort build-options file with paths to netcdf appended. But suddenly 
> the system was losing heat at 1200W/m^2. Turning off optimization (-O0) 
> or using g77 gave the old results (with constant mean temperature). We 
> clearly have an optimization problem here. How serious do you think 
> this is (I admit that we are using a somewhat pathological 
> configuration)?
> Martin
> PS. in sea.mit.edu:/data4/mlosch/test1d.tgz I have a case similar to 
> our original one, but with different initial conditions for T and S, 
> here the mean temperature decreases by 0.5degC in 10 days, 
> corresponding to approximately 690W/m^2 heat loss
> build_g77 has the Makefile for g77 (result in run00/mnc_0001)
> build_o0 for ifort -O0 (result in run00/mnc_0002)
> build_o1 for ifort -O1 (gives almost the same results as -O3, result in 
> run00/mnc_0003)
> data files etc. in run00
> I included the build option files I used, but I manipulated the 
> makefiles afterwards to change the optimization level for ifort.
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Patrick Heimbach   Massachusetts Institute of Technology
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