github.com/Ilhicas/nomad@v1.0.4-0.20210304152020-e86851182bc3/website/content/docs/internals/scheduling/scheduling.mdx (about)

     1  ---
     2  layout: docs
     3  page_title: Scheduling
     4  sidebar_title: Internals
     5  description: Learn about how scheduling works in Nomad.
     6  ---
     7  
     8  # Scheduling in Nomad
     9  
    10  [![Nomad Data Model][img-data-model]][img-data-model]
    11  
    12  There are four primary "nouns" in Nomad; jobs, nodes, allocations, and
    13  evaluations. Jobs are submitted by users and represent a _desired state_. A job
    14  is a declarative description of tasks to run which are bounded by constraints
    15  and require resources. Tasks can be scheduled on nodes in the cluster running
    16  the Nomad client. The mapping of tasks in a job to clients is done using
    17  allocations. An allocation is used to declare that a set of tasks in a job
    18  should be run on a particular node. Scheduling is the process of determining
    19  the appropriate allocations and is done as part of an evaluation.
    20  
    21  An evaluation is created any time the external state, either desired or
    22  emergent, changes. The desired state is based on jobs, meaning the desired
    23  state changes if a new job is submitted, an existing job is updated, or a job
    24  is deregistered. The emergent state is based on the client nodes, and so we
    25  must handle the failure of any clients in the system. These events trigger the
    26  creation of a new evaluation, as Nomad must _evaluate_ the state of the world
    27  and reconcile it with the desired state.
    28  
    29  This diagram shows the flow of an evaluation through Nomad:
    30  
    31  [![Nomad Evaluation Flow][img-eval-flow]][img-eval-flow]
    32  
    33  The lifecycle of an evaluation begins with an event causing the evaluation to
    34  be created. Evaluations are created in the `pending` state and are enqueued
    35  into the evaluation broker. There is a single evaluation broker which runs on
    36  the leader server. The evaluation broker is used to manage the queue of pending
    37  evaluations, provide priority ordering, and ensure at least once delivery.
    38  
    39  Nomad servers run scheduling workers, defaulting to one per CPU core, which are
    40  used to process evaluations. The workers dequeue evaluations from the broker,
    41  and then invoke the appropriate scheduler as specified by the job. Nomad ships
    42  with a `service` scheduler that optimizes for long-lived services, a `batch`
    43  scheduler that is used for fast placement of batch jobs, a `system` scheduler
    44  that is used to run jobs on every node, and a `core` scheduler which is used
    45  for internal maintenance.
    46  
    47  Schedulers are responsible for processing an evaluation and generating an
    48  allocation _plan_. The plan is the set of allocations to evict, update, or
    49  create. The specific logic used to generate a plan may vary by scheduler, but
    50  generally the scheduler needs to first reconcile the desired state with the
    51  real state to determine what must be done. New allocations need to be placed
    52  and existing allocations may need to be updated, migrated, or stopped.
    53  
    54  Placing allocations is split into two distinct phases, feasibility checking and
    55  ranking. In the first phase the scheduler finds nodes that are feasible by
    56  filtering unhealthy nodes, those missing necessary drivers, and those failing
    57  the specified constraints.
    58  
    59  The second phase is ranking, where the scheduler scores feasible nodes to find
    60  the best fit. Scoring is primarily based on bin packing, which is used to
    61  optimize the resource utilization and density of applications, but is also
    62  augmented by affinity and anti-affinity rules. Nomad automatically applies a job
    63  anti-affinity rule which discourages colocating multiple instances of a task
    64  group. The combination of this anti-affinity and bin packing optimizes for
    65  density while reducing the probability of correlated failures.
    66  
    67  Once the scheduler has ranked enough nodes, the highest ranking node is
    68  selected and added to the allocation plan.
    69  
    70  When planning is complete, the scheduler submits the plan to the leader which
    71  adds the plan to the plan queue. The plan queue manages pending plans, provides
    72  priority ordering, and allows Nomad to handle concurrency races. Multiple
    73  schedulers are running in parallel without locking or reservations, making
    74  Nomad optimistically concurrent. As a result, schedulers might overlap work on
    75  the same node and cause resource over-subscription. The plan queue allows the
    76  leader node to protect against this and do partial or complete rejections of a
    77  plan.
    78  
    79  As the leader processes plans, it creates allocations when there is no conflict
    80  and otherwise informs the scheduler of a failure in the plan result. The plan
    81  result provides feedback to the scheduler, allowing it to terminate or explore
    82  alternate plans if the previous plan was partially or completely rejected.
    83  
    84  Once the scheduler has finished processing an evaluation, it updates the status
    85  of the evaluation and acknowledges delivery with the evaluation broker. This
    86  completes the lifecycle of an evaluation. Allocations that were created,
    87  modified or deleted as a result will be picked up by client nodes and will
    88  begin execution.
    89  
    90  [omega]: https://research.google.com/pubs/pub41684.html
    91  [borg]: https://research.google.com/pubs/pub43438.html
    92  [img-data-model]: /img/nomad-data-model.png
    93  [img-eval-flow]: /img/nomad-evaluation-flow.png