Sagemaker Batch Transform - "upstream prematurely closed connection" - Unable to serve requests that take longer than 30 minutes

This is a duplicate of a question I asked on stack overflow

I am serving a sagemaker model through a custom docker container using the guide that AWS provides. This is a docker container that runs a simple nginx->gunicorn/wsgi->flask server

I am facing an issue where my transform requests time out around 30 minutes in all instances, despite should being able to continue to 60 minutes. I need requests to be able to go to sagemaker maximum of 60 minutes due to data intense nature of request.

Through experience working with this setup for some months, I know that there are 3 factors that should affect the time my server has to respond to requests:

1. Sagemaker itself will cap invocations requests according to the InvocationsTimeoutInSeconds paremeter set when creating the batch transform job.
2. The nginx.conf file must be configured such that keepalive_timeout, proxy_read_timeout, proxy_send_timeout, and proxy_connect_timeout are all equal or greater than maximum timeout
3. gunicorn server must its timeout configured to be equal or greater than maximum timeout

I have verified that when I create my batch transform job InvocationsTimeoutInSeconds is set to 3600 (1 hour)

My nginx.conf looks like this:

worker_processes 1;
daemon off; # Prevent forking


pid /tmp/nginx.pid;
error_log /var/log/nginx/error.log;

events {
  # defaults
}

http {
  include /etc/nginx/mime.types;
  default_type application/octet-stream;
  access_log /var/log/nginx/access.log combined;

  sendfile        on;
  client_max_body_size 30M;
  keepalive_timeout  3920s;
  
  upstream gunicorn {
    server unix:/tmp/gunicorn.sock;
  }

  server {
    listen 8080 deferred;
    client_max_body_size 80m;

    keepalive_timeout 3920s;
    proxy_read_timeout 3920s;
    proxy_send_timeout 3920s;
    proxy_connect_timeout 3920s;
    send_timeout 3920s;

    location ~ ^/(ping|invocations) {
      proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
      proxy_set_header Host $http_host;
      proxy_redirect off;
      proxy_pass http://gunicorn;
    }

    location / {
      return 404 "{}";
    }
  }
}`

I start the gunicorn server like this:

def start_server():
    print('Starting the inference server with {} workers.'.format(model_server_workers))
    print('Model server timeout {}.'.format(model_server_timeout))

    # link the log streams to stdout/err so they will be logged to the container logs
    subprocess.check_call(['ln', '-sf', '/dev/stdout', '/var/log/nginx/access.log'])
    subprocess.check_call(['ln', '-sf', '/dev/stderr', '/var/log/nginx/error.log'])

    nginx = subprocess.Popen(['nginx', '-c', '/opt/program/nginx.conf'])
    gunicorn = subprocess.Popen(['gunicorn',
                                 '--timeout', str(3600),
                                 '-k', 'sync',
                                 '-b', 'unix:/tmp/gunicorn.sock',
                                 '--log-level', 'debug',
                                 '-w', str(1),
                                 'wsgi:app'])

    signal.signal(signal.SIGTERM, lambda a, b: sigterm_handler(nginx.pid, gunicorn.pid))

    # If either subprocess exits, so do we.
    pids = set([nginx.pid, gunicorn.pid])
    while True:
        pid, _ = os.wait()
        if pid in pids:
            break

    sigterm_handler(nginx.pid, gunicorn.pid)
    print('Inference server exiting')

Despite all this, whenever a transform job takes longer than approx 30 minutes I will see this message in my logs and the transform job status becomes failed:

2023/01/07 08:23:14 [error] 11#11: *4 upstream prematurely closed connection while reading response header from upstream, client: 169.254.255.130, server: , request: "POST /invocations HTTP/1.1", upstream: "http://unix:/tmp/gunicorn.sock:/invocations", host: "169.254.255.131:8080"

I am close to thinking there is a bug in AWS batch transform, but perhaps I am missing some other variable (perhaps in the nginx.conf) that could lead to premature upstream termination of my request.