Difference between revisions of "SchedulerRaceReduction"
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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. | 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. | ||
+ | |||
+ | == Example == | ||
+ | |||
+ | Compute host ''C'' has 3 GB of ram free. | ||
+ | |||
+ | Scheduler ''A'' sends a ''run_instance'' request ''R1'' to ''C'' trying to build a 2GB instance. | ||
+ | Scheduler ''B'' sends a ''run_instance'' request ''R2'' to ''C'' trying to build a 2GB instance. | ||
+ | |||
+ | Assuming processing of ''R1'' and ''R2'' begins concurrently on ''C''. Obviously ''C'' cannot handle both requests, so at least 1 will fail. | ||
== Impact == | == Impact == |
Revision as of 21:49, 11 June 2012
SchedulerRaceReduction
Time: <<DateTime(2012-06-11T20:42:43Z)>>
Drafter: belliott
Overview
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.
Example
Compute host C has 3 GB of ram free.
Scheduler A sends a run_instance request R1 to C trying to build a 2GB instance. Scheduler B sends a run_instance request R2 to C trying to build a 2GB instance.
Assuming processing of R1 and R2 begins concurrently on C. Obviously C cannot handle both requests, so at least 1 will fail.
Impact
Instance build requests may fail, even if other compute hosts are available with free resources.
Solution
- 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.