Generate layer unique id for raster cache key#8637
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The raw pointer isn't a reliable id as the allocator can reuse an address that's just been released for another layer. This will fix a Fuchsia retained rendering bug. This problem was not affecting non-Fuchsia Flutter probably because non-Fuchsia Flutter purges the raster cache key much more frequently so we won't see a key collision. In Fuchsia, as the key has to wait for the Vulkan surface to render asynchronously, this suddenly becomes an issue.
liyuqian
requested review from
apwilson,
chinmaygarde,
mikejurka,
nathanrogersgoogle and
zanderso
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| Layer::~Layer() = default; | ||
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| uint32_t Layer::NextUniqueID() { |
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What is the rate that these ids are generated at, and how long are these objects expected to live?
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Usually a few (mostly fewer than 10, I never see more than 100) per frame. So that's about 100-200 at full 60fps animations, and I've never seen more than 1000 per second.
A layer may live for a very long time if the scene doesn't change at all. But I'd never expect to have many layers alive at one time. I think it's safe to say that most of the time, no more than 100 are alive at the same time. In extreme cases, maybe never more than 1000 at a time (otherwise compositing all those layers will be very janky).
If we take 1000 layers/second as the generation speed, it'll take 49 days to cycle the unique id.
If we take that no more than 1000 layers are alive at a time, the chance of collision (given that we've cycled the unique id after 49 days, and those 1000 layers live forever) is less than 2^-20 per layer and less than 2^-10 (0.1%) per second.
If that doesn't sound safe enough, I'm happy to use uint64_t for the id.
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If it's not hard to switch to uint64_t, then let's switch.
3 participants
The raw pointer isn't a reliable id as the allocator can reuse an
address that's just been released for another layer.
This will fix Fuchsia bug FL-216.
This problem was not affecting non-Fuchsia Flutter probably because
non-Fuchsia Flutter purges the raster cache key much more frequently so
we won't see a key collision. In Fuchsia, as the key has to wait for the
Vulkan surface to render asynchronously, this suddenly becomes an issue.