The problem reported in https://issues.jenkins-ci.org/browse/JENKINS-23902 (Matrix parent takes executor slot and blocks children) is still occuring.

I have 4 build executors and 4 (or more) jobs should be started by build triggers with the option "Block until the triggered projects finish their builds" set.

This results in 4 parent matrix jobs filling up the build executors so no actual build jobs can be started leading to a dead lock. Only killing some of these parent jobs results in the start of the actuall build jobs.