PaCHash

An object store for variable-sized objects, which has small internal-memory space usage and still guarantees a limited number of external IO operations. For a given parameter a, the internal memory space usage is 2 + log(a) bits per block. Queries for objects of size |x| take constant time and fetch 1 + |x|/B + 1/a blocks from external memory.

Library usage

Add the following to your CMakeLists.txt.

add_subdirectory(path/to/PaCHash)
target_link_libraries(YourTarget PRIVATE PaCHash)

Building the examples

git clone --recursive git@github.com:ByteHamster/PaCHash.git
mkdir PaCHash/build
cd PaCHash/build
cmake -DCMAKE_BUILD_TYPE=Release ..
make -j

Benchmarks

Alternative methods are implemented for benchmarking. You can find the full benchmark scripts in an independent repository: https://github.com/ByteHamster/PaCHash-Experiments

Method	External memory utilization	Internal memory usage	IOs per query
PaCHash	100%	~6 bits/page	1 (variable size)
Separator Hashing	Up to 98%¹	~6 bits/page	1
Cuckoo Hashing	Up to 98%¹	Constant	2 parallel

¹ Depending on the input distribution. The methods are originally designed for fixed size objects. Adversarial input can therefore bring the utilization down to 51% or even make construction impossible.

License

This code is licensed under the GPLv3. If you use the project in an academic context or publication, please cite our paper:

@inproceedings{kurpicz2023pachash,
  author       = {Florian Kurpicz and
                  Hans{-}Peter Lehmann and
                  Peter Sanders},
  title        = {PaCHash: Packed and Compressed Hash Tables},
  booktitle    = {{ALENEX}},
  pages        = {162--175},
  publisher    = {{SIAM}},
  year         = {2023},
  doi          = {10.1137/1.9781611977561.CH14}
}