[gradsusr] Speed optimizations

[Jennifer M Adams]

On Jan 14, 2016, at 8:25 PM, Travis Wilson - NOAA Federal <travis.wilson at noaa.gov<mailto:travis.wilson at noaa.gov>> wrote:

Hi James,

I converted the file to a netcdf via sdfwrite and you are right.  The file opens almost instantly.  I wonder why grib files are so slow to open and if this could be avoided?
Please see my earlier email for an explanation.

It is worth noting that the netcdf file is about 3x larger even with using the -zip option.  Unfortunately since this is an archive, we can't just increase the data stored by a factor of 3.
Did you use the -flt option?
If file size is really a problem, you can archive the grib2 files but use the netcdf file for the real-time web server — overwrite the netcdf file every time a new radar file comes in.


I like your idea of the OPenDAP server, we may have to check into this if there is no other option.
I don’t think this is going to save you any time because under the hood of the opendap server, the I/O problems remain. You’re just adding overhead by putting it behind GDS, which takes extra time to write out the subsets in flat binary and then send them to the opendap client.
—Jennifer






On Thu, Jan 14, 2016 at 4:05 PM, James T. Potemra <jimp at hawaii.edu<mailto:jimp at hawaii.edu>> wrote:
Hi Travis,

This seems a little strange to me.  I did a quick test with one of our higher res data sets (global bathy) that is 21600 by 64800 (x,y) and it loads almost instantly in GrADS.  Of course generating a display takes a long time, and my test was done on a netCDF file.

This aside, could you run the data through an OPeNDAP server?  Even though it would be on the same machine, you could use the server to subset via the DAP call, rather than relying on GrADS to load the entire file.

Jim


On 1/14/16 12:54 PM, Travis Wilson - NOAA Federal wrote:
Hi,

At the NWS we have been using grads to create images from grib files on the fly.  Users use a front end interface to select a model (which is just a grib file on a server), field, and view, and submits a job.  Within 3-4 seconds the server is able to launch grads, open the grib file, and produce 50-100 images for the user to view.  This works extremely well for CONUS datasets with medium to coarse resolutions (up to 6km).
We have started using higher resolution grib files (1 km radar files are attached) and they are substantially slower.  The main problem comes from opening the file.  If a user selects radar for Northern California, it takes grads approximately 18 seconds just to open the 1km CONUS radar file (scanning descriptor file)..and then grads plots rather quickly.

A workaround we have been using is to subset the data on the fly (since we don’t know the area the user will select).  This example regrids the CONUS radar for northern California.

wgrib2 1kmradar_201512241600.grib2 -small_grib 233:243 36.8:42.8 1kmradar_201512241600.grib2.small

The small grib file will open in GrADS in about 1 second.  Regridding, and remaking the ctl and idx is substantially faster, but still requires a lot of time.  My question is, is there any way to speed up the scanning of the CONUS descriptor file without subset the grib file.  For example, grads would not scan the entire conus descriptor file when we just need to look at Northern California (or whatever portion of the US the users selects).

We appreciate all the help,
Travis



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