Subsetting and disk I/O

From: Jesper Larsen <jesper.webmail_at_nyahnyahspammersnyahnyah>
Date: Wed, 25 Feb 2009 13:02:05 +0100

Hi PyNIO people,

I have written a web application presenting marine weather forecasts
using PyNIO:

I am presently using forecasts from NOAA (sorry that credits are not
yet presented along with the forecasts but on a separate page - I will
change that before it is out of beta). Last month I upgraded from the
1x1 degrees resolution GFS to the 0.5x0.5 degrees resolution setup (4
times as large). Since I am about to release an upgraded version of
the application I would also like to begin addressing an issue that
has haunted me for some time.

One of the features of my application is that you can extract
forecasts at specific points (look for Point Weather in the plot type
selector in the left menu) or along trajectories in space and time
(Route Weather). Since the upgrade I have however experienced problems
with these features (they simply take too long to perform - so that
the browser timeouts unless the GRIB records are already cached in
memory). My suspicion is that this is caused by disk I/O. Am I correct
in asserting that PyNIO will read in an entire GRIB record even if I
only request data from a single point in the record (it seems like
that from the memory usage)?

This will of course accumulate to a lot of records when I request a
timeseries at a single point. In fact I will read in ~260.000 times
more data than I need. Do you have any suggestions for avoiding this
(other than making sure that the entire array of GRIB records are
already cached in memory)?

Best regards,
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Received on Wed Feb 25 2009 - 05:02:05 MST

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