Plug.Conn.read_body vs. Plug.Conn.chunk performance

Hello there,

I’m currently working on implementing a WebDAV server using Plug/Cowboy, inspired by the great Python wsgidav library. So far read only access works fine and when downloading files using Plug.Conn.chunk I’m seeing very good performance (1,88GB/s - GigaBYTES from local SSD to /dev/null) with less than 50% scheduler utilization:

But while implementing initial write support I’ve noticed that the other direction, using Plug.Conn.read_body maxes out at about 200MB/s - even when just discarding the output:

You can try this yourselves by cloning the repository (write_support branch) and running the demo project from the demo folder. Then starting the server is as easy as using iex -S mix after the usual mix deps.get step.
For uploading, you can use curl -X PUT -F 'data=@/dev/random' http://demo:demo@
For downloading, first create/download a sufficiently large file and then use wget -O /dev/null "" --user demo --password demo (replace /tmp/10G with the file path).

I tried tweaking the Plug.Conn.read_body opts, but did not find anything that improved this.

Maybe someone with deeper insight into plug/cowboy has an idea why the Plug.Conn.read_body direction performs so bad. I’d expect both directions to have roughly the same performance and scheduler usage.

Thanks for any hints!


I am not an expert in this area, but in my opinion isn’t the hint for the reply in the name of the functions?

The read body function may need to wait for the whole file to came in to process it, while the chunck function will process it in chunks?

That’s not the case. As documented, read_body reads the body up to a given length. So both are processing the data in chunks. Otherwise the memory usage would explode when reading indefinitely from /dev/random.