IC/TS: Plug a hole that led to redundant tile reads #2557
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I was investigating thread behavior while fixing a totally unrelated bug, and I spotted a way that redundant I/O happens when the same tile is needed by multiple threads at once. (And it really did happen, I could see the extra reads in the stats.) A small additional method to `unordered_map_current` allows us to simplify the logic in `add_tile_to_cache` fully avoid the redundant read. Testing against our production benchmarks, this makes no measurable difference to performance, even under heavy threading. I think that's because we make sure that all our texture files are prepared with 'maketx', and thus are tiled, so even the presence of a few redundant tile reads is essentially undetectable in a full render. However, for somebody who is using untiled textures (such as jpegs), which will be read as whole images, this might be a bigger performance gain because those redundant reads are redundant reads of the entire file.
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I was investigating thread behavior while fixing a totally unrelated
bug, and I spotted a way that redundant I/O happens when the same tile
is needed by multiple threads at once. (And it really did happen, I
could see the extra reads in the stats.)
A small additional method to
unordered_map_currentallows us tosimplify the logic in
add_tile_to_cachefully avoid the redundantread.
Testing against our production benchmarks, this makes no measurable
difference to performance, even under heavy threading. I think that's
because we make sure that all our texture files are prepared with
'maketx', and thus are tiled, so even the presence of a few redundant
tile reads is essentially undetectable in a full render. However, for
somebody who is using untiled textures (such as jpegs), which will be
read as whole images, this might be a bigger performance gain because
those redundant reads are redundant reads of the entire file.