[FEATURE]: Add `perfect_hashing` probing scheme
维护者通常 2 天内回复
@njones93531 已经在做这个了。
开始于 2024年6月3日。
评估
这个 Issue 还没有评估数据。
描述
Is your feature request related to a problem? Please describe.
Perfect hash functions describe an injective mapping from the input key domain into the hash map's slot index domain. In other words, Each distinct key hashes to a distinct slot in the map.
This setup allows for a set of optimizations:
- Perfect hash functions don't require any probing logic, since there can't be any colliding key in a slot that is not equal to the probe key
- The injectivity constraint guarantees that any index value produced by the hash function is smaller than the map's capacity. Thus, we can get rid of the remainder computation.
- Key equality comparison can be simplified, i.e., we only have to check if the key in the slot is equal to a sentinel or not.
Describe the solution you'd like
Add a new class cuco::perfect_hashing<class Hash> to our probing scheme zoo which behaves as follows:
When the dereferencing operator of the probing iterator is called for the first time (at the initial probing position), return slots + hash(key). After incrementing the iterator, always return end(), meaning that there is at most one probing step.
A user must ensure that the Hash function in combination with the input key set actually forms a perfect hash function, and the maximum hash values is smaller than the map's capacity. Otherwise behavior is undefined.
Notes on the implementation:
- Currently each of our probing schemes uses the same
probing_iteratorclass. This new probing scheme doesn't fit into the logic of the existing iterator. Thus I propose to let each probing scheme define its ownprobing_iteratoras a member class. - Since perfect hashing only requires bitwise comparison against the sentinel, we ignore any user-specified
KeyEqualoperator.
Describe alternatives you've considered
There is one more optimization we could additionally apply, but I would vote against it due to technical reasons:
Perfect hashing guarantees that there are no collisions. Thus, we could insert keys using non-atomic STG instructions, which has proven to be significantly faster compared to atomic CAS operations.
This however leads to some undesireful side effects due to the relaxed memory ordering of the GPU, which ultimately leads to implausible return values from some of our APIs (insert_and_find and also bulk insert; see example in the bottom paragraph of https://github.com/NVIDIA/cuCollections/issues/475#issuecomment-2113437463).
If this optimization is desired, it can still be enabled by specifying cuda::thread_scope_thread when instantiating the map type. This is a bit hacky but I think it's better than breaking the existing logic, introducing spurious errors in the aforementioned return values.
Additional context
See discussion #475
### Tasks
- 主要语言
- Cuda
- 星标
- 671
- 派生
- 122
- 平均合并
- 4 天 19 小时
- 30 天内合并 PR
- 10
环境准备
在浏览器里用你自己的 GitHub 账号启动这个项目的开发容器。
- 没有 Dockerfile 或 Docker Compose 文件
- 有 Pull Request 模板
- 阅读贡献指南
从这里开始
- 先读完整个 Issue,再读项目的贡献指南。
- 在 Issue 下留言说明你要接手 —— 这能避免两个人做同样的事。
- Fork 仓库,在一个分支上完成修改。
- 提交 Pull Request,并在描述里引用这个 Issue 编号。
NVIDIA/cuCollections 的其他 Issue
-
Add cuco::detail::stream_sync(cuda::stream_ref) to centralize CCCL version-specific API naming可能重新可做 @0z5a 于 22 天前认领,目前没有进行中的 PR。 未关闭
难度 2/5 1-3 小时 新手友好度 74/100
NVIDIA/cuCollections#840 · 1 条评论 ·
维护者通常 2 天内回复
-
nvidia-runners
难度 1/5 1-3 小时 新手友好度 25/100
NVIDIA/cuCollections#853 ·
维护者通常 2 天内回复
-
topic: performance type: feature request
难度 5/5 一周以上 新手友好度 35/100
NVIDIA/cuCollections#817 · 7 条评论 · 1 个 reaction ·
维护者通常 2 天内回复
-
good first issue P2: Nice to have type: improvement
难度 4/5 3-5 天 新手友好度 38/100
NVIDIA/cuCollections#805 · 4 条评论 ·
维护者通常 2 天内回复
-
[FEA] Add MPSC/MPMC concurrent queue可能重新可做 @sleeepyjack 于 262 天前认领,目前没有进行中的 PR。 未关闭type: feature request
NVIDIA/cuCollections#791 · 已指派 1 人 ·
维护者通常 2 天内回复