<p>On AWS EKS
I'm adding deployment with 17 replicas (requesting and limiting 64Mi memory) to a small cluster with 2 nodes type t3.small.</p>

<p>Counting with kube-system pods, total running pods per node is 11 and 1 is left pending, i.e.:</p>

<p>Node #1: <br>
aws-node-1<br>
coredns-5-1as3<br>
coredns-5-2das<br>
kube-proxy-1<br>
+7 app pod replicas       <br>
<br>
Node #2:<br>
aws-node-1<br>
kube-proxy-1<br>
+9 app pod replicas<br>
<br>
I understand that t3.small is a very small instance. I'm only trying to understand what is limiting me here. Memory request is not it, I'm way below the available resources.</p>

<p>I found that there is IP addresses limit per node depending on instance type.
<a href="https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html?shortFooter=true#AvailableIpPerENI" rel="noreferrer">https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html?shortFooter=true#AvailableIpPerENI</a> .<br>
I didn't find any other documentation saying explicitly that this is limiting pod creation, but I'm assuming it does.
Based on the table, t3.small can have 12 IPv4 addresses. If this is the case and this is limiting factor, since I have 11 pods, where did 1 missing IPv4 address go?</p>

<p>The real maximum number of pods per EKS instance are actually listed in <a href="https://github.com/awslabs/amazon-eks-ami/blob/master/files/eni-max-pods.txt" rel="noreferrer">this document</a>.</p>

<p>For t3.small instances, it is 11 pods per instance. That is, you can have a maximum number of 22 pods in your cluster. 6 of these pods are system pods, so there remains a maximum of 16 workload pods.</p>

<p>You're trying to run 17 workload pods, so it's one too much. I guess 16 of these pods have been scheduled and 1 is left pending.</p>

<hr>

<p>The <a href="https://github.com/awslabs/amazon-eks-ami/blob/master/files/eni-max-pods.txt" rel="noreferrer">formula</a> for defining the maximum number of pods per instance is as follows:</p>

<pre><code>N * (M-1) + 2
</code></pre>

<p>Where:</p>

<ul>
<li>N is the number of Elastic Network Interfaces (ENI) of the instance type</li>
<li>M is the number of IP addresses of a single ENI</li>
</ul>

<p>So, for t3.small, this calculation is <code>3 * (4-1) + 2 = 11</code>.</p>

<p>Values for <code>N</code> and <code>M</code> for each instance type in <a href="https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-eni.html#AvailableIpPerENI" rel="noreferrer">this document</a>.</p>
