This document is a short synthesis of the semester project I am doing at DSLab, EPFL. A more detailed Companion to the proof is provided here

To build the benchmarks, run make all. To compile individual binaries, use the GENERATOR macro. If it is defined, main.c will be preprocessed to use map_generator.h header instead of map.h.

Double-hashing is a collision resolution method for open-addressing hash tables.

Opposed to separated chaining, in open-addressing, the buckets of the table directly contain the data to record. In separate chaining, each bucket contains a reference to a list containing all data which have the same hash.

A naive way to implement a collision resolution mechanism for open-addressing tables is to insert data at the first empty bucket after the hash of the data. However, when multiple collision occur for the same hash, each insertion collide with all previously inserted data.

Double-hashing sole this problem by using an independent second hash function to compute the offset. Hence, if three insertions collide (i.e. h1(i1) == h1(i2) == h1(i3)), i1 will be placed in the cell h1(i1), i2 in h1(i2) + h2(i2) and i3 in h1(i3) + h2(i3). Since there is very low probability that both h1(i2) == h1(i3) and h2(i2) == h2(i3), one collision is avoided.

The different implementations are tested with randomly generated access sequences. Different parameters are adjustable:

• The size of the map
• The length of the sequence (i.e. the number of accesses)
• The range of the key inserted
• The target load of the map
• The proportion of read access and write accesses
• Whether the test tries to access data not in the table (test for existence)

The generated test contains two parts:

1. A warm-up step, in which data is inserted until the target load is reached
2. A timed step, in which n accesses are done, where n is the given length

Given a probability p of doing a read access, write accesses are done with probability 1-p. If a write access is done, it is either an insertion or a deletion. The probability of each is computed in function of the current load.

The evaluation works in three steps.

1. Generate mapctrl files. Those are files which contains a sequence of controls for map.
2. Convert the mapctrl into includable files.
3. Compile the different implementations.
4. Synthesize the results into graph plots.

The script test_load.sh run all these steps. As generating files is quite long, editing and commenting a line in this script allows to reuse previously generated files.

In the current state, the script reuses the files in the test_files folder.

The relevant performance graph are in the sub-directories load_test_contains and load_test_wo_contains. These folders contain both graphs and numerical data. One can see that the time used by the double-hash table is almost always comparable to the C++ standard implementation.

In particular, when only existing data is accessed (do not test for existence), the performance are always better than C++.

The for loop increase by offset instead of +1, where offset is provided as an argument. Notice that function signatures change due to that.

[ ] gcd_mul (i.e. coprime(a, c) and coprime(b, c) => coprime(ab, c))

[ ] Benchmark on non-bugged GCC.

[ ] gcd_mul.

[ ] Merge into Arseniy's repo.