I've observed the same issue in 1.19 of the plugin. Looking at the code (specifically, EC2Cloud.addProvisionedSlave() and EC2Cloud.countCurrentEC2Slaves()), it appears the plugin counts all instances in the EC2 account, not just the ones launched by Jenkins. In other words, if you have other instances in your EC2 account, they count, as well.

To work around the bug, I did the following:

• Cleared out the global "Instance Cap"
• Set a cap per "Cloud"

With those changes, only instances of the same AMI count against the cap. This is still a bug, though, since it is quite possible for the same AMI to be used for other things (in our case, we have an old Hudson cluster that happens to use the same AMI).

In order to make this more dependable, I'd recommend using tags to remember which instances were launched by this particular Jenkins master. Maybe one tag (JenkinsCluster) that's the URL of the master, and a second tag (JenkinsCloud) that's the name of the cloud configuration. The tags would then be used to find the instances again.

UPDATE – it's the same problem. I found a second Instance Cap field, tucked into (yet another) Advanced button. Changing that from 1 to 2 let jenkins start a machine.

I've got similar symptoms, but not exactly the same. Everything was fine up until about a month ago.

I didn't originally set a cap, tried setting the cap to 5, (with only the Jenkins server and one other machine are running in this account). No machine is starting when I start a new build (but the new job is queued). I get the log I've pasted below. I upgraded everything. No luck.

Any thoughts?

Thanks,

Dave

May 13, 2014 3:20:51 AM INFO hudson.plugins.ec2.EC2Cloud provision

Excess workload after pending Spot instances: 1

May 13, 2014 3:20:51 AM INFO hudson.plugins.ec2.EC2Cloud addProvisionedSlave

AMI Instance cap of 1 reached for ami ami-e318378a, not provisioning.

May 13, 2014 3:20:51 AM INFO hudson.plugins.ec2.EC2Cloud provision

Excess workload after pending Spot instances: 1

May 13, 2014 3:20:51 AM INFO hudson.plugins.ec2.EC2Cloud addProvisionedSlave

AMI Instance cap of 1 reached for ami ami-e318378a, not provisioning.

This issue still exists in the 1.21 code. It looks like it also trickles down to the individual server-type definitions, where the "instance cap count" is generated by looking for the # of instances running a particular AMI image.

Instead, the EC2 plugin should simply tag the machines with identifiers, and be able to search for them that way. It should also be able to recover lists of live running EC2 slaves by searching for these nodes with these tags.

I have two issues.

1) Changing the instance cap does not get reflected. Jenkins log continues to show the following

"INFO: Total instance cap of 5 reached, not provisioning.
Sep 16, 2014 4:40:10 AM hudson.plugins.ec2.EC2Cloud provision
INFO: Excess workload after pending Spot instances: 3"

I learned that from Matt Wise that the count might be related to the AMI. Since I have 20 instances running using the current AMI. I have created a new AMI and change the configuration to use the new AMI instead. However, it still does not launch new EC2 instance when jobs in the queue.

Code changed in jenkins
User: Roland Groen
Path:
src/main/java/hudson/plugins/ec2/EC2Cloud.java
http://jenkins-ci.org/commit/ec2-plugin/dd2d1e63d423e0cfb4ee0a9ca38bfdc2c0c223eb
Log:
JENKINS-19845, EC2 plugin incorrectly reports current instance count

• Improvement, not a fix. This code checks if the tags match the configured tags

Code changed in jenkins
User: Roland Groen
Path:
src/main/java/hudson/plugins/ec2/EC2Cloud.java
src/main/java/hudson/plugins/ec2/SlaveTemplate.java
http://jenkins-ci.org/commit/ec2-plugin/e81094cc897ddeb456d20bc8c3cc03fafbfa970c
Log:
JENKINS-19845, EC2 plugin incorrectly reports current instance count

• Now using the specific 'ec2slave' tag

Code changed in jenkins
User: Roland Groen
Path:
src/main/java/hudson/plugins/ec2/EC2Cloud.java
http://jenkins-ci.org/commit/ec2-plugin/ace707c4a3498384d8d6c382a84200152f17cb1e
Log:
JENKINS-19845, EC2 plugin incorrectly reports current instance count

• Now using the specific 'ec2slave' tag

Code changed in jenkins
User: Roland Groen
Path:
src/main/java/hudson/plugins/ec2/EC2Cloud.java
http://jenkins-ci.org/commit/ec2-plugin/f2a97d5204df32f177e18d5ac9e84324c876215d
Log:
JENKINS-19845, EC2 plugin incorrectly reports current instance count

• Now using the specific 'ec2slave' tag

Code changed in jenkins
User: Roland Groen
Path:
src/main/java/hudson/plugins/ec2/EC2Cloud.java
http://jenkins-ci.org/commit/ec2-plugin/58fbb7f3a492f4f04c7a32881e6c9946241524fb
Log:
JENKINS-19845, EC2 plugin incorrectly reports current instance count

• Now using the specific 'ec2slave' tag