Introduce HashCompressor for name compression
#396
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September 23, 2024 15:07
I noticed that 'TreeCompressor' will likely have a lot of overhead, as it has to copy each label into its internal tree and it maintains a separate hashmap for every known name (where each name likely has one or two parents at the most, even though hashmaps have a large capacity). 'HashCompressor' is an alternative name compressor built on top of the 'hashbrown' crate, which offers lower-level hash table access that lets entries indirectly reference the built message for hashing and equality checks. It maintains a single hashmap, it should be faster, and it does not require as much memory. The implementation here makes it clear that domain names should have been stored in reverse order, at least in the wire format. A decent chunk of the logic goes into correctly reversing the domain name. In fact, the implementation likely quadratic runtime because of this (at least when 'Name' or 'RelativeName' are used, as backward iteration on them has quadratic runtime). While 'std' essentially exposes the same data structures as 'hashbrown', it does not currently offer a way to perform low-level hash table access (although there is a nightly "raw entry" feature). This may change in the future. If not for the added 'hashbrown' dependency, I would suggest deprecating 'TreeCompressor' entirely.
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I noticed that 'TreeCompressor' will likely have a lot of overhead, as it has to copy each label into its internal tree and it maintains a separate hashmap for every known name (where each name likely has one or two parents at the most, even though hashmaps have a large capacity).
'HashCompressor' is an alternative name compressor built on top of the 'hashbrown' crate, which offers lower-level hash table access that lets entries indirectly reference the built message for hashing and equality checks. It maintains a single hashmap, it should be faster, and it does not require as much memory.
The implementation here makes it clear that domain names should have been stored in reverse order, at least in the wire format. A decent chunk of the logic goes into correctly reversing the domain name. In fact, the implementation likely quadratic runtime because of this (at least when 'Name' or 'RelativeName' are used, as backward iteration on them has quadratic runtime).
While 'std' essentially exposes the same data structures as 'hashbrown', it does not currently offer a way to perform low-level hash table access (although there is a nightly "raw entry" feature). This may change in the future.
If not for the added 'hashbrown' dependency, I would suggest deprecating 'TreeCompressor' entirely.