feat(s2n-quic-dc): Use a new fixed-size map for path secret storage #2337
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Mark-Simulacrum
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| // Balance of speed of access (put or get) and likelihood of false positive eviction. | ||
| const SLOT_CAPACITY: usize = 32; |
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Is this something we'll need to revisit in the future with benchmarks?
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Yes, we'll want to revisit this. Hopefully the entropy will be enough to make this a non-issue in practice.
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| // Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved. | |||
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Not blocking but as a follow up we should probably fuzz the map. Additionally, it would be nice to add a stress test where we have a bunch of threads performing random operations and trying to break it.
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Agreed on both, though I'm not particularly worried - logic seems relatively simple. Let's follow up with this.
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This tightly bounds the maximum memory usage of the path secret storage.
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This tightly bounds the maximum memory usage of the path secret storage.
Description of changes:
This introduces a new data structure (fixed-space HashMap) and refactors our existing flurry maps for by-IP and by-ID path secret storage to utilize it. Under the new design, a given entry is hashed and placed in at most one of 8 slots -- currently replacement just rotates through if there are no empty slots left (this can be improved with LFU/LRU in the future). The new data structure is all safe code and has some tests inline (plus the existing testing from the path secret map).
Call-outs:
Due to the fixed-size slots, we are very likely much more prone to evicting entries from the map. In some sense, with as few as 32 peers (particularly for the IP map) we might start evicting entries (and permanently be unable to make use of much of the map). It seems likely that we should consider tweaking the fixed constant upwards -- or re-design around an open-addressing map that is able to make use of ~50% or more of the map (albeit chasing pointers) area if needed on particularly long hash collision chains.
On the other hand, the current map avoids slowing down as occupancy increases -- we expect at most 32 checks, and since they're memory-local it should be super fast to do so.
Testing:
See added tests.
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.