How to Integrate Rayon for Parallel File Extraction in a Rust Project with tar and flate2 Crates?

I'm working on a Rust project where I need to extract files from a tar.gz archive. I'm using the tar and flate2 crates for handling the archives. My goal is to speed up the extraction process, and I believe parallel processing could be a key to achieving this. I've considered using the Rayon crate for this purpose, but I'm facing challenges in implementing it correctly due to trait constraints.

Here's a simplified version of my current code:

use clap::{Arg, Command};
use flate2::read::GzDecoder;
use std::{env, fs::File, io::BufReader, path::PathBuf};
use tar::Archive;

fn main() {
    // Code for command-line interface handling omitted for brevity

    let file_path = env::current_dir().unwrap().join(filename);
    let file = File::open(&file_path).unwrap();
    let buf_reader = BufReader::with_capacity(32 * 1024, file);
    let gz_decoder = GzDecoder::new(buf_reader);
    let archive = Archive::new(gz_decoder);

    if let Err(e) = extract_files(archive) {
        eprintln!("Error extracting files: {}", e);
    }
}

fn extract_files(mut archive: Archive<GzDecoder<BufReader<File>>>) -> Result<(), std::io::Error> {
    for file in archive.entries()? {
        let mut file = file?;
        let path = file.path()?.into_owned();

        if let Err(e) = file.unpack(&path) {
            eprintln!("Error unpacking file {}: {}", path.display(), e);
        }
    }

    println!("All files successfully extracted.");
    Ok(())
}

The challenge I'm facing is how to parallelize the extraction of files from the tar.gz archive. The Archive and Entries iterators from the tar crate, when combined with flate2::GzDecoder, do not seem to implement the Send trait, which is necessary for safe parallel processing with Rayon.

I'm looking for advice or examples on how to effectively integrate Rayon into this setup. Specifically, I need guidance on:

  1. How to structure the code to leverage parallel processing for extracting files from the archive.
  2. Handling any potential issues with Send trait requirements or other parallel processing constraints.

Any suggestions or code examples would be greatly appreciated!

Unfortunately, the tarball format is inherently single-threaded.

The problem is that the entire tar.gz file is a single gzip stream, and every byte in gzip depends on its previous bytes. So you're forced to decode gzip single-threaded.

After that, decoding of the tar format is too simple to benefit from parallelization, and also the format isn't well suited for that.

zip files are parallelizable. There are variants of gzip like pigz that can be decoded in parallel, but that requires files to be created using a special parallel-gzip encoder.

If you're really determined, it is sometimes possible to use heuristics to seek in gzip streams, like it was done in ACropalypse. However, you may get garbage data this way. Then you can use heuristics to seek in the partial tar file, which again isn't guaranteed to be reliable, and get some parallelization this way, at a risk of sometimes decoding nonsense. But that requires writing custom tooling from scratch, and is not supported by these Rust crates.

2 Likes

Since tar::Entries (an Iterator) and tar::Entry (what the iterator produces) aren't Send or Sync, they can't be passed to other threads using Rayon or any other method.

GzDecoder is Send and Sync, but Read::read has this interface:

fn read(&mut self, buf: &mut [u8]) -> Result<usize>
        ^^^^

The &mut prevents more than 1 thread from decompressing the same stream at the same time. Since GzDecoder isn't Clone, there's no easy workaround other than creating multiple GzDecoder instances. Unfortunately the gzip format itself is stream-oriented instead of block-oriented; it can't jump around to random locations.

Thank you for this information! Maybe I’ll rethink my project, is there anyway to speed up the extraction of tar files? It seems rather slow, nearly 3x slower than base tar.

You can configure the flate2 crate to use faster gzip backends.

and of course use the --release flag.

1 Like

Take a look at the minitar crate, it's TarNode struct is Send and Sync. I used it as a basis for doing essentially what you mentioned doing, but wound up having to fork the crate to add features I needed and fix a couple other little paper cuts. If memory serves the crate just punts on getting the file's user and group names, which is understandable because all the OS gives you is uid/gid numbers. I think I wound up writing my own little parser for /etc/passwd to fill that part in. Won't matter if all you're doing is extracting archives though.

Kornel is correct that you are better off doing all of your read operations in a single thread, but then you can pass each node off to a child thread to be extracted while you continue reading in the main thread. The speed gains were worth it in my non-scientific testing. It also can potentially keep the overall memory usage down as you are writing the data and then freeing that memory right after reading it rather than reading an entire archive into memory before doing any writing.

You can scan down the headers of a tar file easily enough, sure, but that's trivial compared to the gzip stream that contains it, and that's effectively unparallizable / unseekable as mentioned.

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