[MITgcm-support] Something very slow has happened to the pressure solver

[Christopher L. Wolfe]

Hello modelers,

I ran MITgcm on a Cray X1E (Phoenix at ORNL) about a year ago with  
good results. I recently got more time on this machine and downloaded  
the latest MITgcm checkpoint (59n), but was dismayed to find that this  
checkpoint is A LOT (like 5x) slower than the one I was originally  
using (58q_post). I ran freshly compiled versions of  
checkpoint_58q_post and checkpoint_59n with identical model  
configurations, compiler options, and CPP options (to the extent  
possible) and discovered that the pressure solver in 59n takes ten  
times (!) more CPU time on the X1E than the pressure solver from  
58q_post. No other routine seems to have acquired more than about 10%  
greater overhead. Interestingly, I have not noticed significant  
increases in the pressure solver overhead on IBM sp4 machines.

Before I start a flurry of diffs, does anyone have any idea what has  
has been done to the pressure solver over the past year that would  
have had such a significant impact on its performance on the Cray?

Thanks in advance,
Christopher

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Dr. Christopher L. Wolfe              	   858-534-4560
Physical Oceanography Research Division    OAR 357
Scripps Institution of Oceanography, UCSD  clwolfe at ucsd.edu
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