<p>I'm running Kubernetes with AWS EKS. I'm performing some load tests for a nodeport service and seeing a concurrent connection limit of ~16k-20k when hitting a node the pod is not running on. I'm wondering if there's some way to increase the number of concurrent connections.</p>

<p>So I'm running a nodeport service with only 1 pod that is scheduled on node A. The load test I'm running tries to connect as many concurrent websocket connections as possible. The websockets just sleep and send heartbeats every 30s to keep the connection alive.</p>

<p>When I point the load tester (tsung) at node A, I can get upwards of 65k concurrent websockets before the pod gets OOMKilled so memory is the limiting factor and that's fine. The real problem is when I point the load tester at node B, and kube-proxy's iptables forward the connection to node A, all of sudden, I can only get about 16k-20k concurrent websocket connections before the connections start stalling. According to netstat, they are getting stuck in the <code>SYN_SENT</code> state.</p>

<pre><code>netstat -ant | awk '{print $6}' | sort | uniq -c | sort -n
...
20087 ESTABLISHED
30969 SYN_SENT
</code></pre>

<p>The only thing I can think of to check is my conntrack limit and it looks to be fine. Here is what I get for node B.</p>

<pre><code>net.netfilter.nf_conntrack_buckets = 16384
net.netfilter.nf_conntrack_max = 131072
net.nf_conntrack_max = 131072 
</code></pre>

<p>Here is the port range. I'm not sure if it matters (I'm not sure if DNAT and SNAT use up ports), but the range seems to be well above 16k.</p>

<pre><code>net.ipv4.ip_local_port_range = 32768    60999
</code></pre>

<p>The file descriptor limit and kernel TCP settings are the same for node A and node B so I <em>think</em> that rules them out.</p>

<p>Is there anything else that could be limiting the number of concurrent connections forwarded through iptables/netfilter?</p>

<p>You are always going to get worse performance when hitting the NodePort where your pod is not running. Essentially, your packets are going through extra hops trying (through iptables) to get its final destination.</p>

<p>I'd recommend using <a href="https://kubernetes.io/docs/tutorials/services/source-ip/" rel="nofollow noreferrer">source IP</a> for your <a href="https://kubernetes.io/docs/tutorials/services/source-ip/#source-ip-for-services-with-type-nodeport" rel="nofollow noreferrer">NodePort</a> service. Basically, patch your service with this:</p>

<pre><code>$ kubectl patch svc &lt;your-service&gt; -p '{"spec":{"externalTrafficPolicy":"Local"}}'
</code></pre>

<p>Then let your load balancer forward traffic only to NodePorts that are serving traffic.</p>

<p>Alternatively, if you'd like to consider something better performing you could consider using proxy mode <a href="https://kubernetes.io/docs/concepts/services-networking/service/#proxy-mode-ipvs" rel="nofollow noreferrer">ipvs</a> or something like <a href="https://cilium.readthedocs.io/en/latest/intro/" rel="nofollow noreferrer">BPF/Cillium</a> for your overlay.</p>
