I was disappointed to see the existing echo server and client examples for asyncio repeating the same mistakes every time. Mistakes that will get you in trouble when trying to do non-trivial work, namely:

• presenting code that will not work concurrently by default (which is the point of asyncio after all)

• conflation of concerns

  • hard-coding data within the protocol instead of letting the user parametrize it by default

  • exposing the event loop to the protocol, or even stopping it from within the protocol; this is not composable

  • needless scheduling of asynchronous post-processing, which increases fragility on high load

• lack of visibility into the current state of the client/server

• code that will cause ungraceful exception handling under load

• no retry support

• no mention of cost of blocking domain name resolution

This repo contains a minimal example avoiding these design problems. I can pump 10K connections through it without data loss on a 1.3GHz Macbook Air from 2013 with OS X 10.11, achieving ~1K concurrent clients. Try how many your box can handle!

Make sure to ulimit -n $NUMBER_OF_CONCURRENT_CONNECTIONS+100 on the server shell.

On the client you can experiment with any number. Low numbers will decrease concurrency (try 100) and thus speed, high numbers increase load, to the point of saturating CPU (try 15000), at which point no further speedup can be achieved. Also try varying ulimit and the number of connections you launch echocli with, observe results. Example output of time python3 echocli.py 10000::

PID(3505781) attempting 10000 connections
10000 tasks, 0 exceptions, 0 retries
python3 echocli.py 10000  4.59s user 1.26s system 98% cpu 5.952 total

On OS X it's also helpful to run something like::

sudo sysctl -w kern.maxfiles=40960 sudo sysctl -w kern.maxfilesperproc=20480 sudo sysctl -w net.inet.icmp.icmplim=10000

This will ensure the kernel doesn't treat your C10k attempt as a denial-of-service attack.

Report all remaining issues with this example on the Issues page, please.