Why are multiple workspaces being created for a single workflow job execution

[darrell cabral]

I have a workflow that should only need one workspace but consumes 2 workspaces (1 for those steps that run on the dockerHostNode agent and 1 for those steps that run on a 'dockerHost' docker container agent).  At one time the build only consumed one workspace but at some point (after triggering 2 jobs simultaneously, I think) the build started using multiple workspaces per a single job execution.  

Jenkins version 1.609.3

workflow version is 1.12

Here is a simplified example of how my workflow looks:

stage name: 'sync workspace', concurrency: 2

   node(dockerHostNode) {

      checkout ...

   }

stage name: 'Build Source', concurrency: 2

   node(dockerHostNode) {

      sh  make ...

   }

stage name: 'Run unittest', concurrency: 2

   parallel( 

       unittest-a : { 

           node('dockerNode'){

               sh ...  // run unittest-a

           }

       },

       unittest-b : { 

           node('dockerNode'){

               sh ...  // run unittest-b

           }

       }

   )

stage name: 'Post Build Task', concurrency: 2

   node(hostNode) {

      sh  make ...  // collect artifacts, scan logs, etc

   }

Does anyone have any idea why my workflow is creating multiple workspaces during a single workflow job execution?  For now I am using ws() to force all job steps to use the same workspace, but I can still see that a second workspace is unnecessarily created during some of the allocate node start steps.