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20020702: inefficiency of writing one value at a time

Erik,

> To: address@hidden
> cc: address@hidden
> From: Erik gilje <address@hidden>
> Subject: time dependency
> Organization: ?
> Keywords: 200207022053.g62Krlu01763 netCDF F90

The above message contained the following:

> Hi, I am running a time dependent piece of code, and I
> am trying to write to a netcdf data set inside a loop,
>  I had previously just saved the data in an array then
> had that read into a netcdf dataset, however, we are
> planning to us this on a much more complicated program
> in which it will be necessary to write to the data set
> in a loop.  However, when I write to the dataset ina
> loop the program takes much longer (I estimate over
> 1,000 times longer).  So I was wondering what the
> problem might be.  Here is the code :

I don't believe there is a problem.  Writing 1000 values one value
at a time takes longer than writing all 1000 values at once.

> 
> Heres where I declare the netcdf dataset:
> status = NF90_CREATE("newoutput.nc", NF90_SHARE, cid)
>    if (status /= nf90_noerr) call handle_err(status)
>       status = NF90_DEF_DIM(cid, "y", (ns*nt), y)
>       if (status /= nf90_noerr) call handle_err(status)
>       status = NF90_DEF_VAR(cid, "ex", NF90_INT, y, a)
>       if (status /= nf90_noerr) call handle_err(status)
>       status = NF90_DEF_VAR(cid, "elvn", NF90_INT, y, b)      
>       if (status /= nf90_noerr) call handle_err(status)
>       status = NF90_DEF_VAR(cid, "veln", NF90_INT, y, c)
>       if (status /= nf90_noerr) call handle_err(status)
>       
>    status = NF90_ENDDEF(cid)
>         if (status /= nf90_noerr) call
> handle_err(status)
> 
> status = NF90_CLOSE(cid)
> 
> 
> Loop code:
> 
> 
> DO i=1, ns
>         status= nf90_open("newoutput.nc", nf90_Write, cid)
>        if (status /= nf90_noerr) call handle_err(status)
>               status = NF90_put_var(cid, a, x(i), (/i+(k-1)*ns/))
>               if (status /= nf90_noerr) call > handle_err(status)
>               status = NF90_put_var(cid, b, elvn(i), (/i+(k-1)*ns/))
>               if (status /= nf90_noerr) call handle_err(status)
>               status = NF90_PUT_VAR(cid, c, veln(i), (/i+(k-1)*ns/))
>               if (status /= nf90_noerr) call handle_err(status)
>        status = NF90_CLOSE(cid)
>          if (status /= nf90_noerr) call handle_err(status)
>        
>       END DO
> 
> I hope you can offer me some guidance, thank you.
> 
>         erik

From the above code, I don't see why you don't write all of the "x",
"elvn", and "veln" arrays at one time.

There is also no reason to close and then reopen the dataset. And,
unless you're planning on sharing the dataset with another program in
real-time, use of the NF90_SHARE flag is not necessary and may slow
things down.

If you know that you're going to write every value, then you should turn
off prefilling in order to save time.

Regards,
Steve Emmerson   <http://www.unidata.ucar.edu>