Why is my Hello World go server getting crushed by ApacheBench?

package main

import (
	"io"
	"net/http"
)

func hello(w http.ResponseWriter, r *http.Request) {
	io.WriteString(w, "Hello world!\n")
}

func main() {
	http.HandleFunc("/", hello)
	http.ListenAndServe(":8000", nil)
}

I've got a couple of incredibly basic HTTP servers, and all of them are exhibiting this problem.

$ ab -c 1000 -n 10000 http://127.0.0.1:8000/
This is ApacheBench, Version 2.3 <$Revision: 1604373 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking 127.0.0.1 (be patient)
Completed 1000 requests
Completed 2000 requests
Completed 3000 requests
Completed 4000 requests
Completed 5000 requests
apr_socket_recv: Connection refused (61)
Total of 5112 requests completed

With a smaller concurrency value, things still fall over. For me, the issue seems to show up around the 5k-6k mark usually:

$ ab -c 10 -n 10000 http://127.0.0.1:8000/
This is ApacheBench, Version 2.3 <$Revision: 1604373 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking 127.0.0.1 (be patient)
Completed 1000 requests
Completed 2000 requests
Completed 3000 requests
Completed 4000 requests
Completed 5000 requests
Completed 6000 requests
apr_socket_recv: Operation timed out (60)
Total of 6277 requests completed

And in fact, you can drop concurrency entirely and the problem still (sometimes) happens:

$ ab -c 1 -n 10000 http://127.0.0.1:8000/
This is ApacheBench, Version 2.3 <$Revision: 1604373 $>
Copyright 1996 Adam Twiss, Zeus Technology Ltd, http://www.zeustech.net/
Licensed to The Apache Software Foundation, http://www.apache.org/

Benchmarking 127.0.0.1 (be patient)
Completed 1000 requests
Completed 2000 requests
Completed 3000 requests
Completed 4000 requests
Completed 5000 requests
Completed 6000 requests
apr_socket_recv: Operation timed out (60)
Total of 6278 requests completed

I can't help but wonder if I'm hitting some kind of operating system limit somewhere? How would I tell? And how would I mitigate?

In short, you're running out of ports.

The default ephemeral port range on osx is 49152-65535, which is only 16,383 ports. Since each ab request is http/1.0 (without keepalive in your first examples), each new request takes another port.

As each port is used, it get's put into a queue where it waits for the tcp "Maximum Segment Lifetime", which is configured to be 15 seconds on osx. So if you use more than 16,383 ports in 15 seconds, you're effectively going to get throttled by the OS on further connections. Depending on which process runs out of ports first, you will get connection errors from the server, or hangs from ab.

You can mitigate this by using an http/1.1 capable load generator like wrk, or using the keepalive (-k) option for ab, so that connections are reused based on the tool's concurrency settings.

Now, the server code you're benchmarking does so little, that the load generator is being taxed just as much as the sever itself, with the local os and network stack likely making a good contribution. If you want to benchmark an http server, it's better to do some meaningful work from multiple clients not running on the same machine.