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95 lines
4.2 KiB
95 lines
4.2 KiB
# `shuffle3-lean` - Improved 3 stage byte shuffler
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Deterministically and reversably shuffle a file's bytes around.
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## Shuffling
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Shuffle a file in place
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``` shell
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$ shuffle3 -s file
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```
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## Unshuffling
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Unshuffle a file in place
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``` shell
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$ shuffle3 -u file
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```
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## Other options
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Run with `-h` for more options.
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# Improvements from `v1`
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* **~70-80x** speedup from shuffle3 v1.0
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* Huge reduction in syscalls
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* Takes advantage of the kernel's fs cache
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* Can properly handle large files without core dumping
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* Doesn't dump huge amounts of trash onto each stack frame
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## Performance
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[hyperfine](https://github.com/sharkdp/hyperfine) reports a **700-800%** speedup over `v1`.
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It's easy to see why.
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### V1 flamegraph
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V1 uses a pesudo-array adaptor to perform filesystem reads, seeks, and writes. This causes a massive syscall overhead.
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### V2 flamegraph
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Whereas V2 uses a single `mmap()`.
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## Memory usage
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The [memusage](https://www.systutorials.com/docs/linux/man/1-memusage/) graph for `v1` shows extremely inefficient stack usage.
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( the green is supposed to be a line, not a bar! )
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This is due to how the unshuffler buffers RNG results.
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`v1` naively used VLAs to store this buffer, which can baloon to 8 times the size of the file being unshuffled.
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It dumps this massive buffer onto the stack frame of a function that is called multiple times, causing massive and inefficient stack usage.
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This can cause a segfault when attempting to unshuffle a large file, while shuffling a file of the same size might succeed.
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### V2 improvement
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The `memusage` graph for `v2` is a lot more sane.
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`v2` instead allocates this buffer on the heap. Note the stable stack and heap usage.
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# Building
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Run `make` to build the normal binary. It will output to `shuffle3-release`.
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## Release target
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The `release` (default) target uses the variables `RELEASE_CFLAGS`, `RELEASE_CXXFLAGS` and `RELEASE_LDFLAGS` to specify opitimisations, as well as the `OPT_FLAGS` variable. These can be set by you if you wish.
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### Note
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The default `OPT_FLAGS` contains the flag `-march=native`. This may be underisable for you, in which case set the variable or modify the makefile to remove it.
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## Debug target
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To build with debug information, run `make debug`. Extra debug flags can be provided with the `DEBUG_CFLAGS`, `DEBUG_CXXFLAGS` and `DEBUG_LDFLAGS` variables which have default values in the Makefile.
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The build and unstripped binary will be `shuffle3-debug`.
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## PGO target
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To build with Profile Guided Optimisation run `make pgo`, the stripped and optimised binary will be output to `shuffle3-pgo`.
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## Notes
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Before switching between `release` and `debug` targets, remember to run `make clean`.
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To disable stripping of release build binaries, run with `make STRIP=: release`
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### Compile-time flags
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There are some build-time flags you can switch while building by appending to the `FEATURE_FLAGS` variable.
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| Flag | Description |
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| `DEBUG` | Pretend we're building a debug release even though we're not. |
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| `_FS_SPILL_BUFFER` | Spill buffers into a file if they grow over a threshold. Can cause massive slowdowns but prevent OOMs while unshuffling on systems with low available memory. See [shuffle3.h](./include/shuffle3.h) for more details |
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## Gentoo ebuild
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There is a gentoo ebuild for this project in the overlay [test-overlay](https://git.flanchan.moe/birb/test-overlay).
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[direct link](https://git.flanchan.moe/birb/test-overlay/src/branch/master/app-misc/shuffle3/shuffle3-2.0.0.ebuild)
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# License
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GPL'd with <3
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