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Drafter: belliott


The scheduler is subject to a race condition which can cause it to incorrectly identify available resources on a particular compute host. The problem occurs if multiple scheduler instances/threads concurrently issue an instance build request (i.e. run_instance) to the same compute host. This situation may oversubscribe the given compute host and cause one or more run_instance requests to fail.


Compute host C has 3 GB of ram free.

  1. Scheduler A sends a run_instance request R1 to C trying to build a 2GB instance.
  2. Scheduler B sends a run_instance request R2 to C trying to build a 2GB instance.
  3. Assume processing of R1 and R2 begins concurrently on C.

Obviously C cannot handle both requests, so at least 1 will fail.


Instance build requests may fail, even if other compute hosts are available with free resources.


  • Compute hosts should have the final say over whether a run_instance request can be properly serviced. To this end, the compute host must be capable of identify whether it has free resources when a new run_instance request arrives.
  • Compute hosts should serially verify resources available for run_instance requests to avoid concurrent competition by multiple callers.
  • Schedulers should read the response to run_instance and possibly retry the request at a different compute host.


Launchpad bug