why output disgree with different processors assigned

[lixz@ieecas_cn]

I am running CCSM3.0 on PCcluster. But the output is surprise. I have two cases with T31_gx3v5 resolution, new01 and new02. In the new01, I have assigned processors to cpl, atm, lnd, ocn and ice with 2, 16, 2, 2 and 2, respectively. And in the new02, I have assigned processors to cpl, atm, lnd, ocn and ice with 2, 8, 2, 8 and 4, respectively. But the outputs are different. For U (zonal wind speed) , the value of new02 is 2 m/s higher than new01.
Does anyone have the same questions? Who can respond to me? Thx.

 

[rneale]

Hare,
What compiler are you using?
If you are using portland group compiler it is entirely possible that different answers could result from different processor combinations. The compiler has proved somewhat unstable for use with CAM and CCSM and if possible you should use a different one.

 

[lixz@ieecas_cn]

Hi, Rneale
My OS is redhat linux 7.3. Compiler is PGI which includes pgcc, pgCC, fortran, and etc. I don't know how to compare two cases output from different processors. And is the output creditable or compellent? What can I do?
Thank you very much!
Hare!

 

[kauff]

There is not a requirement for the CCSM code to produce identical results with different processor counts because to do so could severely reduce the speed of the code. Thus if you do two otherwise identical runs with different processor counts, the solutions may diverge. This could explain the differences you are seeing.