Introduction

Here are some techniques to make wgrib2 work faster.

Stop using JPEG2000 and PNG compression

Complex packing (c1, c2 and c3) are much faster. For fields with undefined grid points, complex packing produces smaller files. For global fields, the optimum complex packing is typically 20% larger than jpeg2000 compression. Converting is easy: wgrib2 IN.grb -set_grib_type c2 -grib_out OUT.grb

When you make complex packed files, do not uses a bitmap to store undefined values, use the alternative method. If you use the bitmap, the resulting files are larger and will not use the parallelized decoder in wgrib2. Note: wgrib2 uses the alternative method when creating complex packed files.

When using complex packing, the best complex packing (c1, c2, c3) depends on the field. Multitasking scripts are avaiable to change the packing to the optimum complex packing.

Minimize the decoding

Decoding the grid point values is an expensive operation especially for jpeb2000 compressed files.

Decodes all records:   wgrib2 IN.grb -if "HGT:200 mb:" -csv csv.txt
Decodes one record:    wgrib2 IN.grb -match "HGT:200 mb:" -csv csv.txt

Decodes 3N records:    wgrib2 IN.grb -lon 0 10 >lon1.txt
                       wgrib2 IN.grb -lon 0 20 >lon2.txt
                       wgrib2 IN.grb -lon 0 30 >lon2.txt
Decodes  N records:    wgrib2 IN.grb -lon 0 10 -lon 0 20 -lon 0 20  >lons.txt

note: N is limited by the number of command line arguements (5000 v1.9.8) and
the allowable length of the command line (system dependent).

Don't use the wgrib2 | grep | wgrib2 -i syntax

The "wgrib2 | grep | wgrib2" systax can be replaced by the -match/-not options in about 99% of the cases. You are replacing 2 reads of the file by one and the running of 3 processes by one.

Do alot on one line

wgrib2 IN.grb -if ":(UGRD|VGRD):" -grib wind.grb -if ":TMP:" -grib tmp.grb \
  -if ":RH:" -grib rh.grb -if ":PRATE:" -grib prate.grb -if ":(10|2) m above ground:" \
  -grib surface.grb -if ":400 mb:" -grib 400mb.grb

wgrib PGB.grb -new_grid_winds earth -set_grib_type jpeg \
  -new_grid ncep grid 1 pgb1.grb  \
  -new_grid ncep grid 2 pgb2.grb  \
  -new_grid ncep grid 3 pgb3.grb  \
  -new_grid ncep grid 221 pgb221.grb \
  -netcdf pgb.nc

Use -bin instead of -ieee

-bin writes/reads the grid values using the native floating point format.
-ieee writes/reads the grid values using the ieee format (big-endian is default).
-ieee uses software to convert from an arbitrary native format to ieee.

Compile wgrib2 with OpenMP Enabled

More cores are better for large grids. For large grids, use complex packing without bitmaps. The decoder is parallelized for this case and runs 3x faster for 3 km global grids on a 6-core machine.

Compile wgrib2 without OpenMP Enabled

For small grids, OpenMP can slow the program. You can compile with OpenMP turned off or with "OMP_NUM_THREADS" set to zero.

Use multple cores

See: here and over there

Use pipes instead of temporary files

Wgrib2 can read and write through pipes. This is both a programming convenience as well as faster.

cat gfs*201201*grb2 | wgrib2 - -match ":HGT:500 mb:" -grib Z500.grb