It isn't tile *allocation* that matters here, it is tile *locking*,
and locking only happens in one place, tile_lock() in
app/base/tile.c. It should be pretty straightforward for
you to throw some debugging code in there.
As I wrote before, though, tile locking happens *a lot*.
Many internal operations lock tiles transiently, and also
any time tile data is transferred to a plugin, the tile is
locked during the process. You might consider experimenting
with an image small enough to fit entirely within a single
tile (i.e., less than 64x64), if you are going to play with this
stuff without getting flooded.
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