103 lines
5.4 KiB
Markdown
103 lines
5.4 KiB
Markdown
This is a compression algorithm focused on finding long distance matches.
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It is based upon lz4 and uses nearly the same block format (github.com/lz4/lz4/blob/dev/doc/lz4_Block_format.md). The number of bytes to encode the offset is four instead of two in lz4 to reflect the longer distance matching. The block format is described in `ldm.h`.
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### Build
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Run `make`.
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### Compressing a file
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`ldm <filename>`
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Decompression and verification can be enabled by defining `DECOMPRESS_AND_VERIFY` in `main.c`.
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The output file names are as follows:
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- `<filename>.ldm` : compressed file
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- `<filename>.ldm.dec` : decompressed file
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### Parameters
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There are various parameters that can be tuned. These parameters can be tuned in `ldm.h` or, alternatively if `ldm_params.h` is included, in `ldm_params.h` (for easier configuration).
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The parameters are as follows and must all be defined:
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- `LDM_MEMORY_USAGE` : the memory usage of the underlying hash table in bytes.
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- `HASH_BUCKET_SIZE_LOG` : the log size of each bucket in the hash table (used in collision resolution).
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- `LDM_LAG` : the lag (in bytes) in inserting entries into the hash table.
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- `LDM_WINDOW_SIZE_LOG` : the log maximum window size when searching for matches.
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- `LDM_MIN_MATCH_LENGTH` : the minimum match length.
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- `INSERT_BY_TAG` : insert entries into the hash table as a function of the hash. This increases speed by reducing the number of hash table lookups and match comparisons. Certain hashes will never be inserted.
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- `USE_CHECKSUM` : store a checksum with the hash table entries for faster comparison. This halves the number of entries the hash table can contain.
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The optional parameter `HASH_ONLY_EVERY_LOG` is the log inverse frequency of insertion into the hash table. That is, an entry is inserted approximately every `1 << HASH_ONLY_EVERY_LOG` times. If this parameter is not defined, the value is computed as a function of the window size and memory usage to approximate an even coverage of the window.
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### Benchmark
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Below is a comparison of various compression methods on a tar of four versions of llvm (versions `3.9.0`, `3.9.1`, `4.0.0`, `4.0.1`) with a total size of `727900160` B.
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| Method | Size | Ratio |
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|:---|---:|---:|
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|lrzip -p 32 -n -w 1 | `369968714` | `1.97`|
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|ldm | `209391361` | `3.48`|
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|lz4 | `189954338` | `3.83`|
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|lrzip -p 32 -l -w 1 | `163940343` | `4.44`|
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|zstd -1 | `126080293` | `5.77`|
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|lrzip -p 32 -n | `124821009` | `5.83`|
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|lrzip -p 32 -n -w 1 & zstd -1 | `120317909` | `6.05`|
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|zstd -3 -o | `115290952` | `6.31`|
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|lrzip -p 32 -g -L 9 -w 1 | `107168979` | `6.79`|
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|zstd -6 -o | `102772098` | `7.08`|
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|zstd -T16 -9 | `98040470` | `7.42`|
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|lrzip -p 32 -n -w 1 & zstd -T32 -19 | `88050289` | `8.27`|
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|zstd -T32 -19 | `83626098` | `8.70`|
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|lrzip -p 32 -n & zstd -1 | `36335117` | `20.03`|
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|ldm & zstd -6 | `32856232` | `22.15`|
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|lrzip -p 32 -g -L 9 | `32243594` | `22.58`|
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|lrzip -p 32 -n & zstd -6 | `30954572` | `23.52`|
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|lrzip -p 32 -n & zstd -T32 -19 | `26472064` | `27.50`|
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The method marked `ldm` was run with the following parameters:
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| Parameter | Value |
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|:---|---:|
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| `LDM_MEMORY_USAGE` | `23`|
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|`HASH_BUCKET_SIZE_LOG` | `3`|
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|`LDM_LAG` | `0`|
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|`LDM_WINDOW_SIZE_LOG` | `28`|
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|`LDM_MIN_MATCH_LENGTH`| `64`|
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|`INSERT_BY_TAG` | `1`|
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|`USE_CHECKSUM` | `1`|
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The compression speed was `220.5 MB/s`.
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### Parameter selection
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Below is a brief discussion of the effects of the parameters on the speed and compression ratio.
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#### Speed
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A large bottleneck in terms of speed is finding the matches and comparing to see if they are greater than the minimum match length. Generally:
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- The fewer matches found (or the lower the percentage of the literals matched), the slower the algorithm will behave.
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- Increasing `HASH_ONLY_EVERY_LOG` results in fewer inserts and, if `INSERT_BY_TAG` is set, fewer lookups in the table. This has a large effect on speed, as well as compression ratio.
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- If `HASH_ONLY_EVERY_LOG` is not set, its value is calculated based on `LDM_WINDOW_SIZE_LOG` and `LDM_MEMORY_USAGE`. Increasing `LDM_WINDOW_SIZE_LOG` has the effect of increasing `HASH_ONLY_EVERY_LOG` and increasing `LDM_MEMORY_USAGE` decreases `HASH_ONLY_EVERY_LOG`.
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- `USE_CHECKSUM` generally improves speed with hash table lookups.
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#### Compression ratio
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The compression ratio is highly correlated with the coverage of matches. As a long distance matcher, the algorithm was designed to "optimize" for long distance matches outside the zstd compression window. The compression ratio after recompressing the output of the long-distance matcher with zstd was a more important signal in development than the raw compression ratio itself.
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Generally, increasing `LDM_MEMORY_USAGE` will improve the compression ratio. However when using the default computed value of `HASH_ONLY_EVERY_LOG`, this increases the frequency of insertion and lookup in the table and thus may result in a decrease in speed.
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Below is a table showing the speed and compression ratio when compressing the llvm tar (as described above) using different settings for `LDM_MEMORY_USAGE`. The other parameters were the same as used in the benchmark above.
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| `LDM_MEMORY_USAGE` | Ratio | Speed (MB/s) | Ratio after zstd -6 |
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|---:| ---: | ---: | ---: |
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| `18` | `1.85` | `232.4` | `10.92` |
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| `21` | `2.79` | `233.9` | `15.92` |
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| `23` | `3.48` | `220.5` | `18.29` |
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| `25` | `4.56` | `140.8` | `19.21` |
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### Compression statistics
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Compression statistics (and the configuration) can be enabled/disabled via `COMPUTE_STATS` and `OUTPUT_CONFIGURATION` in `ldm.h`.
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