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123 lines
4.7 KiB
Markdown
123 lines
4.7 KiB
Markdown
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__collisionsTest__ is a brute force hash analyzer
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which will measure a 64-bit hash algorithm's collision rate
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by generating billions of hashes,
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and comparing the result to an "ideal" target.
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The test requires a very large amount of memory.
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By default, it will generate 24 billion of 64-bit hashes,
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requiring __192 GB of RAM__ for their storage.
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The number of hashes can be modified using command `--nbh=`.
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Be aware that testing the collision ratio of 64-bit hashes
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requires a very large amount of hashes (several billion) for meaningful measurements.
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To reduce RAM usage, an optional filter can be requested, with `--filter`.
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It reduces the nb of candidates to analyze, hence associated RAM budget.
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Note that the filter itself requires a lot of RAM
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(32 GB by default, can be modified using `--filterlog=`,
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a too small filter will not be efficient, aim at ~2 bytes per hash),
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and reading and writing into filter cost a significant CPU budget,
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so this method is slower.
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It also doesn't allow advanced analysis of partial bitfields,
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since most hashes will be discarded and not stored.
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When using the filter, the RAM budget consists of the filter and a list of candidates,
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which will be a fraction of the original hash list.
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Using default settings (24 billion hashes, 32 GB filter),
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the number of potential candidates should be reduced to less than 2 billion,
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requiring ~14 GB for their storage.
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Such a result also depends on hash algorithm's efficiency.
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The number of effective candidates is likely to be lower, at ~ 1 billion,
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but storage must allocate an upper bound.
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For the default test, the expected "optimal" collision rate for a 64-bit hash function is ~18 collisions.
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#### How to build
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```
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make
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```
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Note: the code is a mix of C99 and C++14,
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it's not compatible with a C90-only compiler.
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#### Build modifier
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- `SLAB5`: use alternative pattern generator, friendlier for weak hash algorithms
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- `POOL_MT`: if `=0`, disable multi-threading code (enabled by default)
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#### How to integrate any hash in the tester
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The build script will compile files found in `./allcodecs`.
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Put the source code here.
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This also works if the hash is a single `*.h` file.
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The glue happens in `hashes.h`.
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In this file, there are 2 sections:
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- Adds the required `#include "header.h"`, and creates a wrapper
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to respect the format expected by the function pointer.
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- Adds the wrapper, along with the name and an indication of the output width,
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to the table, at the end of `hashes.h`
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Build with `make`. Locate your new hash with `./collisionsTest -h`,
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it should be listed.
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#### Usage
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```
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usage: ./collisionsTest [hashName] [opt]
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list of hashNames: (...)
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Optional parameters:
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--nbh=NB Select nb of hashes to generate (25769803776 by default)
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--filter Enable the filter. Slower, but reduces memory usage for same nb of hashes.
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--threadlog=NB Use 2^NB threads
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--len=NB Select length of input (255 bytes by default)
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```
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#### Some advises on how to setup a collisions test
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Most tests are primarily driven by the amount of RAM available.
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Here's a method to decide the size of the test.
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Presuming that RAM budget is not plentiful, for this example 32 GB,
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the `--filter` mode is actually compulsory to measure anything meaningful.
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Let's plan 50% of memory for the filter, that's 16 GB.
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This will be good enough to filter about 10% less hashes than this size.
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Let's round down to 14 G.
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By requesting 14G, the expectation is that the program will automatically
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size the filter to 16 GB, and expect to store ~1G candidates,
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leaving enough room to breeze for the system.
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The command line becomes:
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```
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./collisionsTest --nbh=14G --filter NameOfHash
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```
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#### Examples:
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Here are a few results produced with this tester:
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| Algorithm | Input Len | Nb Hashes | Expected | Nb Collisions | Notes |
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| --- | --- | --- | --- | --- | --- |
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| __XXH3__ | 255 | 100 Gi | 312.5 | 326 | |
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| __XXH64__ | 255 | 100 Gi | 312.5 | 294 | |
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| __XXH128__ low64 | 512 | 100 Gi | 312.5 | 321 | |
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| __XXH128__ high64| 512 | 100 Gi | 312.5 | 325 | |
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| __XXH128__ | 255 | 100 Gi | 0.0 | 0 | a 128-bit hash is expected to generate 0 collisions |
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Test on small inputs:
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| Algorithm | Input Len | Nb Hashes | Expected | Nb Collisions | Notes |
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| --- | --- | --- | --- | --- | --- |
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| __XXH64__ | 8 | 100 Gi | 312.5 | __0__ | `XXH64` is bijective for `len==8` |
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| __XXH3__ | 8 | 100 Gi | 312.5 | __0__ | `XXH3` is also bijective for `len==8` |
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| __XXH3__ | 16 | 100 Gi | 312.5 | 332 | |
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| __XXH3__ | 32 | 14 Gi | 6.1 | 3 | |
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| __XXH128__ | 16 | 25 Gi | 0.0 | 0 | test range 9-16 |
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| __XXH128__ | 32 | 25 Gi | 0.0 | 0 | test range 17-128 |
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| __XXH128__ | 100 | 13 Gi | 0.0 | 0 | test range 17-128 |
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| __XXH128__ | 200 | 13 Gi | 0.0 | 0 | test range 129-240 |
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