[MITgcm-support] IO on NEC SX machines

[Martin Losch]

Hi there,

in my pathological configuration I have daily forcing with a 12h time  
step (for a spinup), so every other time step new forcing fields are  
read. This produces an overhead on any machine (I guess), but on our  
SX8R, I find this overhead to be very large: apparently in some of my  
runs 2/3rds of the time is spend in reading the forcing. I don't  
observe any significant differences between runs with and with  
useSingleCPUio = .true., one or more CPUs. I ran exactly the same  
configuration on our Cray XD1 on 18cpus and there is hardly any  
overhead (< 5%, regardless of useSingleCPUio=.true. or not).

I have talked to Jens-Olaf about this (maybe he can correct me, if I  
am mixing things up here), and it appears that the chunks that are  
read in mdsioreadfield are too small to take advantage of the  
otherwise fast GFS filesystem of the machine. This is my domain size  
(and forcing field size): 180x120 grid points and my forcing is in  
double precision so one record is 172800 bytes (correct?).

The obvious solution is to read forcing in larger chunks (e.g. per  
year) and keep it in memory, but that would require considerable  
recoding.

Is there any experience with this problem on other NEC SX machines,  
GFS file systems? Are there some hidden switches in the MITgcm that  
would help to improve this IO performance?

Martin