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A lot of websockets in Haskell

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Reading time: ~20 minutes.

I really enjoyed reading how the phoenix-framework people managed to get to two million active websocket connections.

I’ve heard some very smart people say that Haskell has an amazing runtime with very cheap threads. I have no reason to disbelieve them but we thought it’d be fun to see how Haskell fares in a comparable setup.

Test servers

Unlike the Phoenix people we don’t have Rackspace sponsorship so we had to resort to the common man’s cheap machines: EC2 spot instances. We bid $0.10 on two m4.xlarge machines with 16G of RAM and 4 cores which usually cost around $0.05 per hour in eu-west.

As usual we’re using Nix to deploy tsung and a very simple Haskell chat program that just broadcasts incoming messages to everyone.

tsung is a tcp/web load generator written in Erlang configured through a XML domain-specific language (website).

The core handler of our chat program looks like this (full source here):

handleWS :: InChan ByteString -> PendingConnection -> IO ()
handleWS bcast pending = do
    localChan <- dupChan bcast
    connection <- acceptRequest pending
    -- Yes, we're leaking resources here because this forkIO
    -- doesn't terminate when the socket closes :)
    forkIO $ forever $ do
        message <- readChan localChan
        sendTextData connection message
    -- loop forever
    let loop = do
            Text message <- receiveDataMessage connection
            writeChan bcast message

To run the ec2 machines we’re using nixops which also does the spot-price bidding for us:

nixops create '<nix/test-setup.nix>'
nixops deploy

(See here for the full configuration including kernel tuning).

tsung setup

Unfortunately I could not get the distributed tsung going: The distributed testing uses an Erlang function called slave:start which connects through SSH and spawns Erlang on the remote host. This failed for reasons I didn’t have time to debug.

But without the distributed testing there’s a problem: A single server can only open ~65000 connections because ports are limited to 16 bits. We want more connections though!

Luckily tsung supports using multiple virtual IP addresses for a single network interface out of the box. We went to Amazon and clicked “Assign new IP” a few times to assign more private IPs to our tsung box.

Now we associate the new IPs with our network interface:

ip addr add dev eth0
ip addr add dev eth0
ip addr add dev eth0
ip addr add dev eth0
ip addr add dev eth0
ip addr add dev eth0
ip addr add dev eth0
ip addr add dev eth0

We have a slightly different tsung config from the Phoenix people which we copy to our tsung box. The differences are using the recently added tsung websockets connection type, and to terminate on messages instead of a delay.

$ nixops scp --to tsung-1 code/src/tsung-conf.xml tsung-conf.xml
code/src/tsung-conf.xml -> [email protected]:tsung-conf.xml
tsung-conf.xml                                                  100% 1494     1.5KB/s   00:00

Running tsung

Our Nix config tuned the TCP stack and increased kernel limits, but we still need to run ulimit to make sure we’re not hitting the default limit of 1024 file descriptors:

$ nixops ssh tsung-1
[[email protected]:~]# ulimit -n 2000000
[[email protected]:~]# tsung -f tsung-conf.xml start
Starting Tsung
Log directory is: /root/.tsung/log/20151104-1622

tsung exports some data via a web interface on port 8091. We use an extra SSH tunnel so we can access this data on

$ ssh [email protected] -L 8091:

Problem 1: The firewall

All our Nix boxes are configured with a firewall enabled. This is because I start from a template configuration instead of starting from scratch.

The firewall uses connection tracking to make decisions, and connection tracking requires memory. When that memory is full the dmesg logs look like this:

[ 2960.570157] nf_conntrack: table full, dropping packet
[ 2960.575060] nf_conntrack: table full, dropping packet
[ 2960.629764] nf_conntrack: table full, dropping packet
[ 2960.678016] nf_conntrack: table full, dropping packet
[ 2992.936177] TCP: request_sock_TCP: Possible SYN flooding on port 8080. Sending cookies.  Check SNMP counters.
[ 2998.005969] net_ratelimit: 364 callbacks suppressed

That log also shows that we triggered the kernel’s DOS protection against SYN flooding. We fixed that by increasing net.ipv4.tcp_max_syn_backlog and net.core.somaxconn.

Now when running tsung we got up to about 120k connections on the Haskell websocket box:

[root@websock-server:~]# netstat -ntp  | grep -v TIME_WAIT | wc -l

Problem 2: The Erlang process limit

But then tsung’s web UI would suddenly throw 500 errors and drop all connections. Initially we could not figure out what was going on because tsung is really slow at writing logs. Waiting for 5 minutes and then checking the logs reveals the message:

=ERROR REPORT==== 4-Nov-2015::18:03:45 ===
Too many processes

We noticed that tsung supports changing the maximum number of internal Erlang processes and we tried this:

tsung -p 1250000 -f tsung-conf.xml start

But no luck – the same problem occurs. Turns out that the -p switch doesn’t actually work (we filed a bug which was fixed within hours. Thanks!).

We patched tsung ourselves for now.

Some performance numbers

So far we spent most of our time fighting tsung and the slighly bizarre Erlang ecosystem. Here’s what 100k users look like for CPU and memory for the Haskell server:

 1944 root      20   0 7210960 2.656g  22524 S 177.7 16.9   2:58.50 haskell-websock

2.6G, not bad! With all problems fixed we ran another test with 256k users:

 2252 root      20   0 11.237g 4.714g  22532 S 128.3 30.1   6:58.25 haskell-websock

More addresses

In order to go higher we needed more IP addresses for tsung. This is where we learnt that EC2 limits the number of additional private IPs based on the instance type. You’ll see a message like this:

eni-5af8fa3d: Number of private addresses will exceed limit.

For m4.xlarge the limit is 15 addresses so we got another 6:

ip addr add  dev eth0
ip addr add  dev eth0
ip addr add  dev eth0
ip addr add  dev eth0
ip addr add  dev eth0
ip addr add  dev eth0

With 15 addresses in total we should get close to one million connections:

>>> 15 * 64000

But tsung needs more memory than our Haskell server and died at ~500k connections:

/run/current-system/sw/bin/tsung: line 60: 29721 Killed                  [...]

The Haskell server was still running quite comfortably below 10G at 500k connections:

 2320 root      20   0 16.879g 9.395g  22300 S   0.0 59.9  14:38.75 haskell-websock

That was certainly a fun afternoon! Time to clean up:

$ nixops destroy
warning: are you sure you want to destroy EC2 machine ‘tsung-1’? (y/N) y
warning: are you sure you want to destroy EC2 machine ‘websock-server’? (y/N) y

The whole experiment took ~2.5 hours and cost us a grand total of $0.25. To get to 4x the connections (two million) we’d need two m4.4xlarge or r2.2xlarge instances but that’s for another day.


Our graphs show very nicely that we add a bit more than 1000 connections a second, and that the connection count follows the user count closely. I.e. there is no delay from the Haskell server.

Some unscientific testing also showed that propagating a message to all 256k clients takes 10-50 milliseconds, so the 2 seconds quoted by the Phoenix team for two million users sounds about right.

256k connections:

~500k connections (tsung died):