S3 slow download compared to aws cli

[CAReyes]

I'm working on a custom file downloader using the aws rust sdk. I'm testing the performance of it versus the aws cli and noticed that I get capped at about 10-15 MiB/s download using the sdk. The aws cli is getting about up to 40 MiB/s download. I've scratched my head for a while trying figure out where performance was lost.

Here is the code snippet of what I am doing in the SDK:

let byte_stream = client
    .get_object()
    .bucket(bucket)
    .key(path)
    .send()
    .await?
    .body
    .into_async_read();

let mut reader = BufReader::new(byte_stream);
let mut writer = BufWriter::new(OpenOptions::new().create(true).write(true).open(&file_path).await?);

let mut buf = vec![0; 1024]; // 1KB buffer 
while let Ok(size) = reader.read(&mut buf).await {
    if size == 0 {
        break;
    }
    writer.write_all(&buf).await?;
}

versus running the aws s3 cp command to a local directory.

I've played around with buffer sizes and sync vs async but no luck.

[rcoh]

First, of course, is to double check that you're building in --release. At only 10-15MiB/s this is a possible culprit.

If you are using a new version of the CLI, the CLI is using the CRT, a C library to automatically split the download into multiple ranged GETs—the SDK does not do that automatically. You would need to write code to perform ranged GETs and use write_at to write the chunks to disk.

We have a code snippet in our S3 benchmark that shows how to use ranged GETs.

Note that build a library for customers to do this directly is high on our priority list for 2024.

We've benchmarked the Rust SDK achieving multiple Gib/S on large instance types so let me know if you're unable to match the performance.

[CAReyes]

Finally got around to implementing this and it worked out great. Thanks for the pointer.

[pavlospt]

@rcoh Hello, happened to stumble upon this today too. Why isn't this part of the SDK? 🤔

[Velfi]

The boring answer is that we have limited time and we have to make tough choices regarding what to focus on. We are tracking many tasks for things that people have asked for but we are a small team so there's always going to be more work to do than can be done.

[pavlospt]

That is understandable, it just feels a bit weird to be executing in Enhanced Network instances and S3 transfers from same VPC being the bottleneck :/ Is there something that we could help with?