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Re: 980213: "Headers" in memory?

>To: address@hidden
>From: address@hidden (John Sheldon)
>Subject: "Headers" in memory?
>Organization: Geophysical Fluid Dynamics Laboratory/NOAA
>Keywords: 199802130304.UAA29873

Hi John,

> When I create a netCDF file, does the API keep the "header" information in
> memory?  I have been trying to tune an application here on our Cray T90
> that writes long files, but has a bunch of little calls for header-type
> info while writing the data portion.  I expected this to impact efficiency
> as the system would have to go back and reload a cache page with the header
> info every time I asked for something, but this doesn't seem to be
> happening.  (I've been using procstat and the "event" layer in the
> FFIOSPEC.)

Yes, opening a netCDF file causes all header information to be read into
memory, so subsequent `inquire' calls are very fast and require no I/O.
For once, we actually included this in the documentation, though only in
the individual inquire function entries rather than in the chapter on
performance, where you might expect to find it:

    No I/O is performed when these functions are called, since the
    required information is available in memory for each open netCDF
    dataset.

--Russ

_____________________________________________________________________

Russ Rew                                         UCAR Unidata Program
address@hidden                     http://www.unidata.ucar.edu