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On 12-01-2013 01:38, Kennedy, Paul wrote:
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<div>Hi,</div>
<div>Yes, we are pretty sure we will see a significant benefit.
The processing algorithms are CPU bound not io bound. Our
digital terrain model interpolations often run for many hours (
we do them overnight) but the underlying file is only a few
gigabytes. If we split them into multiple files of tiles and run
each on a dedicated process the whole thing is quicker, but this
is messy and results in a stitching error. <br>
</div>
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<br>
Some many years ago when I had to do that type of operations due to
memory limitations the trick was to compute each tile larger than
needed. Let say 10% wider in each of the 4 sides (except the borders
of course). The extra zone will work as a boundary condition and is
stripped at the end. The stripped tiles could than be pasted
together to build the final mosaic. I did that with minimum
curvature (GMT) interpolation and the final 'gluing' resulted
perfect as it couldn't be noticed not even with shaded illumination.<br>
<br>
Joaquim<br>
<br>
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<div><br>
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<div>Another example is gdalwarp. It takes quite some time with a
large data set and would be. A good candidate for
parallelisation, as would gdaladdo. </div>
<div><br>
</div>
<div>I believe slower cores but more of them in pcs are the
future. My pc has 8 but they rarely get used to their
potential. <br>
<br>
I am certain there are some challenges here, that's why it is
interesting;)</div>
<div><br>
Regards
<div>pk</div>
</div>
<div><br>
On 11/01/2013, at 6:54 PM, "Even Rouault" <<a
moz-do-not-send="true"
href="mailto:even.rouault@mines-paris.org">even.rouault@mines-paris.org</a>>
wrote:<br>
<br>
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<title>Re: [gdal-dev] does gdal support multiple simultaneous
writers to raster</title>
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<p><font size="2">Hi,<br>
<br>
This is an intersting topic, with many "intersecting"
issues to deal with at<br>
different levels.<br>
<br>
First, are you confident that in the use cases you imagine
that I/O access won't<br>
be the limiting factor, in which case serialization of I/O
could be acceptable<br>
and this would just require an API with a dataset level
mutex.<br>
<br>
There are several places where parallel write should be
addressed :<br>
- The GDAL core mechanisms that deal with the block cache<br>
- Each GDAL driver where parallel write would be
supported. I guess that GDAL<br>
drivers should advertize a specific capability<br>
- The low-level library used by the driver. In the case of
GDAL, libtiff<br>
<br>
And finally, as Frank underlined, there are intrinsic
limitations due to the<br>
format itself. For a compressed TIFF, at some point, you
have to serialize the<br>
writing of the tile, because you cannot kown in advance
the size of the<br>
compressed data, or at least have some coordination of the
writers so that a<br>
"next offset available" is properly synchronized between
them. The compression<br>
itself could be serialized.<br>
<br>
I'm not sure however if what Jan mentionned, different
process, writing the same<br>
dataset is doable.<br>
<br>
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