[gdal-dev] GDALRasterBand::RasterIO and FlushCache

[Frank Warmerdam]

David,

Note that the image cache will start discarding blocks on it's own when it
is full.  I think the default cache is about 64MB.    I don't think that
flushing the cache should be part of normal applications operations, though
if you are very tight on memory you might want to alter the default size.
 Note that the cached blocks are discarded when the dataset is closed.

I will update the example to free the buffer at the end.

Best regards,
Frank

On Fri, May 31, 2013 at 4:24 PM, David Strip <gdal at stripfamily.net> wrote:

>  I'm reading a fairly large (at least to me) geotif (about 2GB) a line at
> a time. I noticed that memory usage increases as I read the image. It
> appears that what's going on is that the driver hangs onto these lines of
> the image, even though I'm providing the same buffer for it to read into
> each time. At this point I noticed the FlushCache method. Calling this
> after each reading each scan line solves the problem of the growing memory
> usage.
>
> I assume this behavior of caching whatever is read is deliberate. I think
> it would be useful in the API examples to make this behavior clear and
> perhaps show the use of the FlushCache method.
> For example, the API Tutorial page shows this for how to read a line into
> a buffer:
>
> *Reading Raster Data
> *There are a few ways to read raster data, but the most common is via the
> GDALRasterBand::RasterIO() method. This method will automatically take care
> of data type conversion, up/down sampling and windowing. The following code
> will read the first scanline of data into a similarly sized buffer,
> converting it to floating point as part of the operation.
>
> In C++:
>
>         float *pafScanline;
>         int   nXSize = poBand->GetXSize();
>
>         pafScanline = (float *) CPLMalloc(sizeof(float)*nXSize);
>         poBand->RasterIO( GF_Read, 0, 0, nXSize, 1,
>                           pafScanline, nXSize, 1, GDT_Float32,
>                           0, 0 );
>
> The pafScanline buffer should be freed with CPLFree() when it is no longer
> used.
>
> I think it would be useful to point out, in addition to the need to
> CPLFree the buffer that if you are using this example in a loop to read the
> image a line at a time, you should FlushCache every line (or periodically)
> to keep from accumulating the entire image in the cache (which is invisible
> to the user). It seems (to me) that it would be better to put this in the
> API Tutorial rather than as a code snippet in the Wiki, since the tutorial
> is where most people would look when starting out.
>
>
>
>
>
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>



-- 
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I set the clouds in motion - turn up   | Frank Warmerdam,
warmerdam at pobox.com
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and watch the world go round - Rush    | Geospatial Software Developer
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