Test of `posix_fadvise()` for Rust

commands_and_output.txt

$ # Generate 8 MB of random data
$ head -c 8M < /dev/urandom > data.txt
$ # Flush and clear the cache so we get the data from disk
$ sync && echo 3 | sudo tee /proc/sys/vm/drop_caches && cargo script posix_fadvise_test.rs
3
   Compiling posix_fadvise_test v0.1.0 (file:///home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286)
Advised result: HashResult { hash: [173, 168, 157, 204, 185, 126, 165, 177, 195, 111, 94, 179, 252, 59, 25, 169], time: Duration { secs: 0, nanos: 10050785 } }
$ # Comment out the call to advise_willneed, save, flush again and re-run
$ sync && echo 3 | sudo tee /proc/sys/vm/drop_caches && cargo script posix_fadvise_test.rs
3
   Compiling posix_fadvise_test v0.1.0 (file:///home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286)
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:40:1: 53:2 warning: function is never used: `advise_willneed`, #[warn(dead_code)] on by default
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:40 fn advise_willneed(file: &File) {
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:41     let len = file.metadata().unwrap().len();
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:42     
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:43     unsafe {
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:44         assert_eq!(
/home/austin/.cargo/script-cache/file-posix_fadvise_test-8d6a7e80a3131286/posix_fadvise_test.rs:45             libc::posix_fadvise(
                                                                                                   ...
Advised result: HashResult { hash: [173, 168, 157, 204, 185, 126, 165, 177, 195, 111, 94, 179, 252, 59, 25, 169], time: Duration { secs: 0, nanos: 33213398 } }
$ # Significantly slower!

posix_fadvise_test.rs

//! ```cargo
//!
//! [dependencies]
//! md5 = "*"
//!
//! [dependencies.libc]
//! path = "/home/austin/Rust/libc/"
//! ```
#![feature(time2)]

extern crate libc;
extern crate md5;

use md5::Digest;

use std::io::prelude::*;
use std::fs::File;
use std::os::unix::io::AsRawFd;
use std::time::{Duration, Instant};
use std::thread;

#[derive(Debug)]
struct HashResult {
    hash: Digest,
    time: Duration,
}

fn main() {
    let mut data_buf = File::open("data.txt").expect("Expected data.txt");

    advise_willneed(&data_buf);

    // Give the system time to read the file into the cache; in practice we'd be doing other stuff here in userspace.
    thread::sleep(Duration::from_millis(1000));

    let buf = read_and_hash(&mut data_buf);

    println!("Advised result: {:?}", buf);
}

fn advise_willneed(file: &File) {
    let len = file.metadata().unwrap().len();
    
    unsafe {
        assert_eq!(
            libc::posix_fadvise(
                file.as_raw_fd(),
                0, len as libc::off_t,
                libc::POSIX_FADV_WILLNEED
            ), 
            0
        );
    }
}

fn read_and_hash(file: &mut File) -> HashResult {
    let start = Instant::now();

    let mut data = Vec::new();

    file.read_to_end(&mut data).unwrap();

    let read_time = start.elapsed();

    let digest = md5::compute(&data);

    HashResult {
        hash: digest,
        time: read_time,
    }
}