Bug in ncdf_control, /nofill

["Mark Rivers" <rivers(at)cars.uchicago.edu>]

Folks,

I have discovered a bug in the netCDF routine NCDF_CONTROL.  The /NOFILL
keyword does not work.

The on-line help says that one should use /FILL to enable prefilling the
disk file with numbers and /NOFILL to disable this, improving performance.
I have found that /NOFILL behaves exactly the same as /FILL or FILL=1.
However, if I use FILL=0 then the performance DOES improve.  The use of
FILL=0 is not documented, and this is what /NOFILL should do.

Here are the performance numbers to prove my assertion.  These come from
running the appended test program. Here are the results running IDL 5.4 on a
Windows NT system to a network disk.
IDL> test_write, 200, /nofill
Time to initialize file:       7.3290000
Time to write data:       5.2340000
Speed:       1.2145816 MB/sec
IDL> test_write, 200, fill=1
Time to initialize file:       8.0310000
Time to write data:       4.2970001
Speed:       1.2377343 MB/sec
IDL> test_write, 200, fill=0
Time to initialize file:      0.26600003
Time to write data:       5.2970001
Speed:       2.7429065 MB/sec

These results show that the time for /NOFILL is the same as for FILL=1,
while the time to initialize the file for FILL=0 is much shorter.  /NOFILL
should be the same as FILL=0, not FILL=1.  I have found that the performance
gain in using FILL=0 (which should be /NOFILL) is a factor of 2.5, so it is
very significant.

Mark Rivers
CARS
Building 434A
Argonne National Laboratory
Argonne, IL 60439
Phone:  630-252-0422
FAX:    630-252-0443
e-mail: rivers@cars.uchicago.edu



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end