Austin Donnelly wrote:

> [Lots of people writing barking mad things about tile swapping]
> Look, you're all missing the point.
> Gimp does it's own tile swapping not because it wants to control the
> layout on disk.  As some of you have pointed out, this is futile.
> The only reason to swap a tile at a time is to do with controlling how
> far apart in memory neighbouring pixels are.
> Consider a very wide image.  If it is stored as a large linear array
> in memory (possibly paged by the OS to an OS-managed swap file), then
> the common operation of consulting a pixel above or below the current
> one results in needing to skip a large number of bytes through the
> linear array.  This results in poor CPU cache performance.  So, we use
> a tiled memory layout.  Once the data is in a tiled representation in
> memory, there seems little point in converting it into a linear buffer
> before writing it to disk.  This would certainly take more time than
> it would to just hand the data to the OS.
> Now, the size of a tile cache (ie number of tiles we'd like to be able
> to access as speedily as possible) should be a little over the number
> of tiles it takes to cover the width of the image.  This is so that
> filters which iterate over every single pixel from left to right, top
> to bottom, perform better on the horizontal boundary between adjacent
> strips of tiles.  Consider a 3x3 convolution (let's say a blur
> matrix).  When the center of the matrix is at the top of the second
> row of tiles, the top of the matrix needs to reference the first row
> of tiles.  It is helpful for performance to have this top row
> available.  Which means caching ceil(img_width / tile_width) + 1
> tiles.
> And gentlemen, this is not rocket science.  It's what undergraduates
> are normally taught in their basic "OS Functions" lectures.  The gimp
> is a good example of why application-specific paging control can be a
> performance boost.
> Now can we drop this silly subject, please?
> Austin

now what is the best size to set it to?

/me runs

Garrett LeSage - Art Director

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