std::io::copy performance may be 20x slower than it could be?

[spease]

I was using std::io::copy from an SSD to memory using ProgressBarReader, which reported an eye-raising 30 MB/s. Wrapping it in a BufReader (either as the outer or middle layer) made no real difference.

So I googled around a bit and found the recommended size was 64KB according to some random Microsoft article, so I decided to copy-paste the std::io::copy implementation and increase the buffer size. This got up to 600 MB/s at about 256KB which sounded a lot more reasonable...then it continued going upwards from there past the point of believability (unless the files were being pulled from RAM as a consequence of my repeated accesses). With a 10MB Vec, I got 228.36 GB/s 🙃.

Unrealistic values aside, has anybody checked the buffer value to make sure there isn't some huge low-hanging fruit for std::io::copy speeds?

[the8472]

You mention windows. This may be platform-specific behavior. On unix systems I would fadvise POSIX_FADV_SEQUENTIAL on the input file for copying. Googling tells me that the windows equivalent is FILE_FLAG_SEQUENTIAL_SCAN. Have you tried that?

[Voultapher]

After a coarse overview, my intuition is that there is some funny business going on with the length determined by seek and the subsequent inc calls on the ProgressBar, that could explain the unrealistic results.

[spease]

It’s possible. The actual times did increase dramatically from the std::io::copy default though - my suspicion is that there is probably some caching to RAM going on or something as cold runs were generally slower than hot runs.

This was for some work code that I’m no longer actively developing. I’ll leave this email marked as unread for awhile in case I have some spare time to write a benchmark.

I don’t think the other flag that was mentioned is accessible with the default open file api, but in any case it at least appears there’s some low hanging fruit wrt to std::io::copy perf

[ljedrz]

I tried benchmarking io::copy on Windoze before and after changing DEFAULT_BUF_SIZE to 64KB (instead of the original 8KB) and the results were slightly worse (up to ~3%) with the modified setup, both on HDD and SSD. However, I'm not sure if my benchmarks were good enough - I was having a hard time tweaking them so that I could see any difference between results for different file sizes.

FWIW, I'm providing the benchmark code below; it can easily be expanded to different file sizes:

#[bench]
fn bench_copy_4MB(b: &mut Bencher) {
    let mut file = test::black_box(File::open("4MB").unwrap());
    let mut writer: Vec<u8> = Vec::with_capacity(4 * 1024 * 1024);

    b.iter(|| {
        let mut x = Ok(0);
        for _ in 0..100 { x = io::copy(&mut file, &mut writer); writer.clear(); }
        x
    })
}

[spease]

4MB isn’t a whole lot. That’s well within the size of hard drive caches or possibly an OS memory-based cache. I was noticing the difference with files several gigabytes in size.

As a sanity check, maybe look to see what the actual time is, not just the % difference between timings with different buffer sizes. The actual time may be wildly different than the actual steady state performance of the disk if it’s reading the data from a cache.

It may also be possible at a low level to pass a flag to encourage the OS to disable or flush the cache, but I don’t know if there’s any way to instruct the hard drive to do the same.

[ljedrz]

I only pasted the benchmark for 4MB as a reference - I tested files from 4KB up to 16MB. I'm not an expert with low-level hard drive stuff, just hoping my attempt can help someone else.

[retep998]

Ideally you would test with a file approximately twice as big as you have RAM, so if you have 8GB of RAM you would want to test with a 16GB file.

[rbtcollins]

@spease extended open flags are accessible via https://doc.rust-lang.org/std/os/windows/fs/trait.OpenOptionsExt.html
FWIW lots of bennchmarks exist showing synchronous IO calls improve when matching IO operations to the underlying block size (e.g. 4MB for modern SSD's) across multiple OS's, so its very likely that there are improvements to gain here. However, 4MB is stack-breaking size by default, and heap allocations will destroy potential improvements (and further, if we're optimising, we probably want overlapped IO and a ring of buffers rather than just one buffer which will always leave one device idle).... so its complex;)

[main--]

heap allocations will destroy potential improvements

How come? io::copy would allocate once at the beginning. Heap memory isn't inherently slower and one memory allocation is basically free compared to the heavy IO that's happening afterwards.

[retep998]

But we also don't want to be allocating 4MB every time we do io::copy as someone might be doing a bunch of small copies.

[the8472]

possible approaches:

• thread local buffers or a buffer pool
• stat the input before deciding on the buffer size (this is what gnu cp does I think)
• heap-allocate a bigger buffer after a few iterations of the copy loop with an on-stack buffer
• specialize copy operations from file to memory to read directly into the already allocated target, thus skipping the need for a buffer

[main--]

specialize copy operations from file to memory to read directly into the already allocated target, thus skipping the need for a buffer

I think some kind of specialization for BufRead sources would be ideal. Because in this case, io::copy should not buffer at all and simply use the reader's buffer. Effectively this also gives you a customizable buffer size by wrapping your File in a BufReader with whatever size you want.

As of today, if your source is BufRead then io::copy is always the wrong answer. I got a substantial performance improvement using a simple loop instead:

loop {
    let buf = reader.fill_buf()?;
    if buf.is_empty() { break; }
    let consumed = file_out.write(buf)?;
    drop(buf);
    reader.consume(consumed);
}