title: Airflow Scheduler Pause Issue when scheduling multiple DAGs
body: <p>Everyone, I am using Airflow(v.1.10.12) for one of the project to schedule the Jobs daily. Wanted to execute multiple DAG’s(14) in parallel. Have updated the concurrency parameters in cfg file. However, have observed the behavior that: Scheduler starts the execution of tasks, assigns the tasks to queue but after certain amount of time it pauses and restart after 5 minutes. Due to this behavior the time taken by all Dag is huge and also some of tasks are getting failed. Can someone help me with the understanding why the Scheduler gets halt and if we should modify some parameter or shift to some other Airflow version?</p>
  <p><strong>Below is the configuration file</strong></p>
  <pre><code>[core]
  dags_folder = /home/airflow/dags
  base_log_folder = /home/airflow/logs
  remote_logging = False
  remote_log_conn_id =
  remote_base_log_folder =
  encrypt_s3_logs = False
  logging_level = INFO
  fab_logging_level = WARN
  logging_config_class =
  colored_console_log = True
  colored_log_format = [%%(blue)s%%(asctime)s%%(reset)s] {%%(blue)s%%(filename)s:%%(reset)s%%(lineno)d} %%(log_color)s%%(levelname)s%%(reset)s - %%(log_color)s%%(message)s%%(reset)s
  colored_formatter_class = airflow.utils.log.colored_log.CustomTTYColoredFormatter
  log_format = [%%(asctime)s] {%%(filename)s:%%(lineno)d} %%(levelname)s - %%(message)s
  simple_log_format = %%(asctime)s %%(levelname)s - %%(message)s
  log_filename_template = {{ ti.dag_id }}/{{ ti.task_id }}/{{ ts }}/{{ try_number }}.log
  log_processor_filename_template = {{ filename }}.log
  dag_processor_manager_log_location = /home/airflow/logs/dag_processor_manager/dag_processor_manager.log
  hostname_callable = socket:getfqdn
  default_timezone = utc
  executor = CeleryExecutor
  sql_alchemy_conn = postgresql+psycopg2://devairflow:airflow@localhost:5432/pcfdb 
  sql_engine_encoding = utf-8
  sql_alchemy_pool_enabled = True
  sql_alchemy_pool_size = 0
  sql_alchemy_max_overflow = -1    
  # The SqlAlchemy pool recycle is the number of seconds a connection
  # can be idle in the pool before it is invalidated. This config does
  # not apply to sqlite. If the number of DB connections is ever exceeded,
  # a lower config value will allow the system to recover faster.
  sql_alchemy_pool_recycle = 1800
  sql_alchemy_pool_pre_ping = True
  sql_alchemy_schema =
  parallelism = 50
  dag_concurrency = 50
  dags_are_paused_at_creation = True
  max_active_runs_per_dag = 50
  load_examples = False
  load_default_connections = True
  plugins_folder = /home/airflow/plugins
  fernet_key = M4dpP6f2Hd5p3N--CxtIoUo9XaSDifA42MPLs1UR7-g=
  donot_pickle = False
  dagbag_import_timeout = 41460
  dag_file_processor_timeout = 60
  task_runner = StandardTaskRunner
  default_impersonation =
  security =
  secure_mode = False
  unit_test_mode = False
  enable_xcom_pickling = True
  killed_task_cleanup_time = 60
  dag_run_conf_overrides_params = False
  worker_precheck = False
  dag_discovery_safe_mode = True
  default_task_retries = 0
  store_serialized_dags = False
  min_serialized_dag_update_interval = 30
  
  max_num_rendered_ti_fields_per_task = 100
  
  # On each dagrun check against defined SLAs
  check_slas = True
  
  [secrets]
  # Full class name of secrets backend to enable (will precede env vars and metastore in search path)
  # Example: backend = airflow.contrib.secrets.aws_systems_manager.SystemsManagerParameterStoreBackend
  backend =
  
  # The backend_kwargs param is loaded into a dictionary and passed to __init__ of secrets backend class.
  # See documentation for the secrets backend you are using. JSON is expected.
  # Example for AWS Systems Manager ParameterStore:
  # ``{&quot;connections_prefix&quot;: &quot;/airflow/connections&quot;, &quot;profile_name&quot;: &quot;default&quot;}``
  backend_kwargs =
  
  [cli]
  # In what way should the cli access the API. The LocalClient will use the
  # database directly, while the json_client will use the api running on the
  # webserver
  api_client = airflow.api.client.local_client
  
  # If you set web_server_url_prefix, do NOT forget to append it here, ex:
  # ``endpoint_url = http://localhost:8080/myroot``
  # So api will look like: ``http://localhost:8080/myroot/api/experimental/...``
  endpoint_url = http://localhost:8080
  
  [debug]
  # Used only with DebugExecutor. If set to True DAG will fail with first
  # failed task. Helpful for debugging purposes.
  fail_fast = False
  
  [api]
  # How to authenticate users of the API. See
  # https://airflow.apache.org/docs/stable/security.html for possible values.
  # (&quot;airflow.api.auth.backend.default&quot; allows all requests for historic reasons)
  auth_backend = airflow.api.auth.backend.deny_all
  
  [lineage]
  # what lineage backend to use
  backend =
  
  [atlas]
  sasl_enabled = False
  host =
  port = 21000
  username =
  password =
  
  [operators]
  # The default owner assigned to each new operator, unless
  # provided explicitly or passed via ``default_args``
  default_owner = airflow
  default_cpus = 1
  default_ram = 1024
  default_disk = 1024
  default_gpus = 0
  
  [hive]
  # Default mapreduce queue for HiveOperator tasks
  default_hive_mapred_queue =
  
  [webserver]
  # The base url of your website as airflow cannot guess what domain or
  # cname you are using. This is used in automated emails that
  # airflow sends to point links to the right web server
  base_url = http://localhost:8080
  
  # Default timezone to display all dates in the RBAC UI, can be UTC, system, or
  # any IANA timezone string (e.g. Europe/Amsterdam). If left empty the
  # default value of core/default_timezone will be used
  # Example: default_ui_timezone = America/New_York
  default_ui_timezone = UTC
  
  # The ip specified when starting the web server
  web_server_host = 0.0.0.0
  
  # The port on which to run the web server
  web_server_port = 8080
  
  # Paths to the SSL certificate and key for the web server. When both are
  # provided SSL will be enabled. This does not change the web server port.
  web_server_ssl_cert =
  
  # Paths to the SSL certificate and key for the web server. When both are
  # provided SSL will be enabled. This does not change the web server port.
  web_server_ssl_key =
  
  # Number of seconds the webserver waits before killing gunicorn master that doesn't respond
  web_server_master_timeout = 41460
  
  # Number of seconds the gunicorn webserver waits before timing out on a worker
  web_server_worker_timeout = 41460
  
  # Number of workers to refresh at a time. When set to 0, worker refresh is
  # disabled. When nonzero, airflow periodically refreshes webserver workers by
  # bringing up new ones and killing old ones.
  worker_refresh_batch_size = 1
  
  # Number of seconds to wait before refreshing a batch of workers.
  worker_refresh_interval = 30
  
  # If set to True, Airflow will track files in plugins_folder directory. When it detects changes,
  # then reload the gunicorn.
  reload_on_plugin_change = False
  
  # Secret key used to run your flask app
  # It should be as random as possible
  secret_key = temporary_key
  
  # Number of workers to run the Gunicorn web server
  workers = 4
  
  # The worker class gunicorn should use. Choices include
  # sync (default), eventlet, gevent
  worker_class = sync
  
  # Log files for the gunicorn webserver. '-' means log to stderr.
  access_logfile = -
  
  # Log files for the gunicorn webserver. '-' means log to stderr.
  error_logfile = -
  
  # Expose the configuration file in the web server
  expose_config = True
  
  # Expose hostname in the web server
  expose_hostname = True
  
  # Expose stacktrace in the web server
  expose_stacktrace = True
  
  # Set to true to turn on authentication:
  # https://airflow.apache.org/security.html#web-authentication
  authenticate = False
  
  # Filter the list of dags by owner name (requires authentication to be enabled)
  filter_by_owner = False
  
  # Filtering mode. Choices include user (default) and ldapgroup.
  # Ldap group filtering requires using the ldap backend
  #
  # Note that the ldap server needs the &quot;memberOf&quot; overlay to be set up
  # in order to user the ldapgroup mode.
  owner_mode = user
  
  # Default DAG view. Valid values are:
  # tree, graph, duration, gantt, landing_times
  dag_default_view = tree
  
  # &quot;Default DAG orientation. Valid values are:&quot;
  # LR (Left-&gt;Right), TB (Top-&gt;Bottom), RL (Right-&gt;Left), BT (Bottom-&gt;Top)
  dag_orientation = LR
  
  # Puts the webserver in demonstration mode; blurs the names of Operators for
  # privacy.
  demo_mode = False
  
  # The amount of time (in secs) webserver will wait for initial handshake
  # while fetching logs from other worker machine
  log_fetch_timeout_sec = 5
  
  # Time interval (in secs) to wait before next log fetching.
  log_fetch_delay_sec = 2
  
  # Distance away from page bottom to enable auto tailing.
  log_auto_tailing_offset = 30
  
  # Animation speed for auto tailing log display.
  log_animation_speed = 1000
  
  # By default, the webserver shows paused DAGs. Flip this to hide paused
  # DAGs by default
  hide_paused_dags_by_default = False
  
  # Consistent page size across all listing views in the UI
  page_size = 100
  
  # Use FAB-based webserver with RBAC feature
  rbac = False
  
  # Define the color of navigation bar
  navbar_color = #007A87
  
  # Default dagrun to show in UI
  default_dag_run_display_number = 25
  
  # Enable werkzeug ``ProxyFix`` middleware for reverse proxy
  enable_proxy_fix = False
  
  # Number of values to trust for ``X-Forwarded-For``.
  # More info: https://werkzeug.palletsprojects.com/en/0.16.x/middleware/proxy_fix/
  proxy_fix_x_for = 1
  
  # Number of values to trust for ``X-Forwarded-Proto``
  proxy_fix_x_proto = 1
  
  # Number of values to trust for ``X-Forwarded-Host``
  proxy_fix_x_host = 1
  
  # Number of values to trust for ``X-Forwarded-Port``
  proxy_fix_x_port = 1
  
  # Number of values to trust for ``X-Forwarded-Prefix``
  proxy_fix_x_prefix = 1
  
  # Set secure flag on session cookie
  cookie_secure = False
  
  # Set samesite policy on session cookie
  cookie_samesite =
  
  # Default setting for wrap toggle on DAG code and TI log views.
  default_wrap = False
  
  # Allow the UI to be rendered in a frame
  x_frame_enabled = True
  
  # Send anonymous user activity to your analytics tool
  # choose from google_analytics, segment, or metarouter
  # analytics_tool =
  
  # Unique ID of your account in the analytics tool
  # analytics_id =
  
  # Update FAB permissions and sync security manager roles
  # on webserver startup
  update_fab_perms = True
  
  # Minutes of non-activity before logged out from UI
  # 0 means never get forcibly logged out
  force_log_out_after = 0
  
  # The UI cookie lifetime in days
  session_lifetime_days = 30
  
  [email]
  email_backend = airflow.utils.email.send_email_smtp
  
  [smtp]
  
  # If you want airflow to send emails on retries, failure, and you want to use
  # the airflow.utils.email.send_email_smtp function, you have to configure an
  # smtp server here
  #smtp_host = localhost
  
  
  # SMTP Address
  # 192.168.152.213
  
  # SMTP Port
  # 25
  
  # User Name
  # etf@csopasset.com
  
  
  
  
  smtp_host = *.*.*.*
  smtp_starttls = True
  
  smtp_ssl = False
  # smtp_user = etf@csopasset.com
  # smtp_password = etfGen2013
  smtp_port = 25
  smtp_mail_from = etf@***.com
  
  [sentry]
  
  # Sentry (https://docs.sentry.io) integration
  sentry_dsn =
  
  [celery]
  
  # This section only applies if you are using the CeleryExecutor in
  # ``[core]`` section above
  # The app name that will be used by celery
  celery_app_name = airflow.executors.celery_executor
  
  # The concurrency that will be used when starting workers with the
  # ``airflow celery worker`` command. This defines the number of task instances that
  # a worker will take, so size up your workers based on the resources on
  # your worker box and the nature of your tasks
  worker_concurrency = 50
  
  # The maximum and minimum concurrency that will be used when starting workers with the
  # ``airflow celery worker`` command (always keep minimum processes, but grow
  # to maximum if necessary). Note the value should be max_concurrency,min_concurrency
  # Pick these numbers based on resources on worker box and the nature of the task.
  # If autoscale option is available, worker_concurrency will be ignored.
  # http://docs.celeryproject.org/en/latest/reference/celery.bin.worker.html#cmdoption-celery-worker-autoscale
  # Example: worker_autoscale = 16,12
  # worker_autoscale =
  
  # When you start an airflow worker, airflow starts a tiny web server
  # subprocess to serve the workers local log files to the airflow main
  # web server, who then builds pages and sends them to users. This defines
  # the port on which the logs are served. It needs to be unused, and open
  # visible from the main web server to connect into the workers.
  worker_log_server_port = 8793
  
  # The Celery broker URL. Celery supports RabbitMQ, Redis and experimentally
  # a sqlalchemy database. Refer to the Celery documentation for more
  # information.
  # http://docs.celeryproject.org/en/latest/userguide/configuration.html#broker-settings
  #roker_url = sqla+mysql://airflow:airflow@localhost:3306/airflow
  
  broker_url = amqp://guest:guest@localhost:5672//
  # The Celery result_backend. When a job finishes, it needs to update the
  # metadata of the job. Therefore it will post a message on a message bus,
  # or insert it into a database (depending of the backend)
  # This status is used by the scheduler to update the state of the task
  # The use of a database is highly recommended
  # http://docs.celeryproject.org/en/latest/userguide/configuration.html#task-result-backend-settings
  #result_backend = db+mysql://airflow:airflow@localhost:3306/airflow
  result_backend = db+postgresql+psycopg2://devairflow:airflow@localhost:5432/pcfdb
  
  # Celery Flower is a sweet UI for Celery. Airflow has a shortcut to start
  # it ``airflow flower``. This defines the IP that Celery Flower runs on
  flower_host = 0.0.0.0
  
  # The root URL for Flower
  # Example: flower_url_prefix = /flower
  flower_url_prefix =
  
  # This defines the port that Celery Flower runs on
  flower_port = 5555
  
  # Securing Flower with Basic Authentication
  # Accepts user:password pairs separated by a comma
  # Example: flower_basic_auth = user1:password1,user2:password2
  flower_basic_auth =
  
  # Default queue that tasks get assigned to and that worker listen on.
  default_queue = default
  
  # How many processes CeleryExecutor uses to sync task state.
  # 0 means to use max(1, number of cores - 1) processes.
  sync_parallelism = 0
  
  # Import path for celery configuration options
  celery_config_options = airflow.config_templates.default_celery.DEFAULT_CELERY_CONFIG
  
  # In case of using SSL
  ssl_active = False
  ssl_key =
  ssl_cert =
  ssl_cacert =
  
  # Celery Pool implementation.
  # Choices include: prefork (default), eventlet, gevent or solo.
  # See:
  # https://docs.celeryproject.org/en/latest/userguide/workers.html#concurrency
  # https://docs.celeryproject.org/en/latest/userguide/concurrency/eventlet.html
  pool = prefork
  
  # The number of seconds to wait before timing out ``send_task_to_executor`` or
  # ``fetch_celery_task_state`` operations.
  operation_timeout = 50
  
  [celery_broker_transport_options]
  
  # This section is for specifying options which can be passed to the
  # underlying celery broker transport. See:
  # http://docs.celeryproject.org/en/latest/userguide/configuration.html#std:setting-broker_transport_options
  # The visibility timeout defines the number of seconds to wait for the worker
  # to acknowledge the task before the message is redelivered to another worker.
  # Make sure to increase the visibility timeout to match the time of the longest
  # ETA you're planning to use.
  # visibility_timeout is only supported for Redis and SQS celery brokers.
  # See:
  # http://docs.celeryproject.org/en/master/userguide/configuration.html#std:setting-broker_transport_options
  # Example: visibility_timeout = 21600
  # visibility_timeout =
  
  [dask]
  
  # This section only applies if you are using the DaskExecutor in
  # [core] section above
  # The IP address and port of the Dask cluster's scheduler.
  cluster_address = 127.0.0.1:8786
  
  # TLS/ SSL settings to access a secured Dask scheduler.
  tls_ca =
  tls_cert =
  tls_key =
  
  [scheduler]
  # Task instances listen for external kill signal (when you clear tasks
  # from the CLI or the UI), this defines the frequency at which they should
  # listen (in seconds).
  job_heartbeat_sec = 5
  
  # The scheduler constantly tries to trigger new tasks (look at the
  # scheduler section in the docs for more information). This defines
  # how often the scheduler should run (in seconds).
  scheduler_heartbeat_sec = 5
  
  # After how much time should the scheduler terminate in seconds
  # -1 indicates to run continuously (see also num_runs)
  run_duration = -1
  
  # The number of times to try to schedule each DAG file
  # -1 indicates unlimited number
  num_runs = -1
  
  # The number of seconds to wait between consecutive DAG file processing
  processor_poll_interval = 1
  
  # after how much time (seconds) a new DAGs should be picked up from the filesystem
  min_file_process_interval = 0
  
  # How often (in seconds) to scan the DAGs directory for new files. Default to 5 minutes.
  dag_dir_list_interval = 300
  
  # How often should stats be printed to the logs. Setting to 0 will disable printing stats
  print_stats_interval = 30
  
  # If the last scheduler heartbeat happened more than scheduler_health_check_threshold
  # ago (in seconds), scheduler is considered unhealthy.
  # This is used by the health check in the &quot;/health&quot; endpoint
  scheduler_health_check_threshold = 30
  child_process_log_directory = /home/airflow/logs/scheduler
  
  # Local task jobs periodically heartbeat to the DB. If the job has
  # not heartbeat in this many seconds, the scheduler will mark the
  # associated task instance as failed and will re-schedule the task.
  scheduler_zombie_task_threshold = 1800
  
  # Turn off scheduler catchup by setting this to False.
  # Default behavior is unchanged and
  # Command Line Backfills still work, but the scheduler
  # will not do scheduler catchup if this is False,
  # however it can be set on a per DAG basis in the
  # DAG definition (catchup)
  catchup_by_default = True
  
  # This changes the batch size of queries in the scheduling main loop.
  # If this is too high, SQL query performance may be impacted by one
  # or more of the following:
  # - reversion to full table scan
  # - complexity of query predicate
  # - excessive locking
  # Additionally, you may hit the maximum allowable query length for your db.
  # Set this to 0 for no limit (not advised)
  max_tis_per_query = 512
  
  # Statsd (https://github.com/etsy/statsd) integration settings
  statsd_on = False
  statsd_host = localhost
  statsd_port = 8125
  statsd_prefix = airflow
  
  # If you want to avoid send all the available metrics to StatsD,
  # you can configure an allow list of prefixes to send only the metrics that
  # start with the elements of the list (e.g: scheduler,executor,dagrun)
  statsd_allow_list =
  
  # The scheduler can run multiple threads in parallel to schedule dags.
  # This defines how many threads will run.
  max_threads = 2
  authenticate = False
  
  # Turn off scheduler use of cron intervals by setting this to False.
  # DAGs submitted manually in the web UI or with trigger_dag will still run.
  use_job_schedule = True
  
  # Allow externally triggered DagRuns for Execution Dates in the future
  # Only has effect if schedule_interval is set to None in DAG
  allow_trigger_in_future = False
  
  [ldap]
  # set this to ldaps://&lt;your.ldap.server&gt;:&lt;port&gt;
  uri =
  user_filter = objectClass=*
  user_name_attr = uid
  group_member_attr = memberOf
  superuser_filter =
  data_profiler_filter =
  bind_user = cn=Manager,dc=example,dc=com
  bind_password = insecure
  basedn = dc=example,dc=com
  cacert = /etc/ca/ldap_ca.crt
  search_scope = LEVEL
  
  # This setting allows the use of LDAP servers that either return a
  # broken schema, or do not return a schema.
  ignore_malformed_schema = False
  
  [mesos]
  # Mesos master address which MesosExecutor will connect to.
  master = localhost:5050
  
  # The framework name which Airflow scheduler will register itself as on mesos
  framework_name = Airflow
  
  # Number of cpu cores required for running one task instance using
  # 'airflow run &lt;dag_id&gt; &lt;task_id&gt; &lt;execution_date&gt; --local -p &lt;pickle_id&gt;'
  # command on a mesos slave
  task_cpu = 1
  
  # Memory in MB required for running one task instance using
  # 'airflow run &lt;dag_id&gt; &lt;task_id&gt; &lt;execution_date&gt; --local -p &lt;pickle_id&gt;'
  # command on a mesos slave
  task_memory = 256
  
  # Enable framework checkpointing for mesos
  # See http://mesos.apache.org/documentation/latest/slave-recovery/
  checkpoint = False
  
  # Failover timeout in milliseconds.
  # When checkpointing is enabled and this option is set, Mesos waits
  # until the configured timeout for
  # the MesosExecutor framework to re-register after a failover. Mesos
  # shuts down running tasks if the
  # MesosExecutor framework fails to re-register within this timeframe.
  # Example: failover_timeout = 604800
  # failover_timeout =
  
  # Enable framework authentication for mesos
  # See http://mesos.apache.org/documentation/latest/configuration/
  authenticate = False
  
  # Mesos credentials, if authentication is enabled
  # Example: default_principal = admin
  # default_principal =
  # Example: default_secret = admin
  # default_secret =
  
  # Optional Docker Image to run on slave before running the command
  # This image should be accessible from mesos slave i.e mesos slave
  # should be able to pull this docker image before executing the command.
  # Example: docker_image_slave = puckel/docker-airflow
  # docker_image_slave =
  
  [kerberos]
  ccache = /tmp/airflow_krb5_ccache
  
  # gets augmented with fqdn
  principal = airflow
  reinit_frequency = 3600
  kinit_path = kinit
  keytab = airflow.keytab
  
  [github_enterprise]
  api_rev = v3
  
  [admin]
  # UI to hide sensitive variable fields when set to True
  hide_sensitive_variable_fields = True
  
  [elasticsearch]
  # Elasticsearch host
  host =
  
  # Format of the log_id, which is used to query for a given tasks logs
  log_id_template = {dag_id}-{task_id}-{execution_date}-{try_number}
  
  # Used to mark the end of a log stream for a task
  end_of_log_mark = end_of_log
  
  # Qualified URL for an elasticsearch frontend (like Kibana) with a template argument for log_id
  # Code will construct log_id using the log_id template from the argument above.
  # NOTE: The code will prefix the https:// automatically, don't include that here.
  frontend =
  
  # Write the task logs to the stdout of the worker, rather than the default files
  write_stdout = False
  
  # Instead of the default log formatter, write the log lines as JSON
  json_format = False
  
  # Log fields to also attach to the json output, if enabled
  json_fields = asctime, filename, lineno, levelname, message
  
  [elasticsearch_configs]
  use_ssl = False
  verify_certs = True
  </code></pre>
  <p><strong>Below are the Worker logs:</strong>
  <a href="https://docs.google.com/document/d/1BM6JiOSnLT5WGxfiqYTMR12041dYVRY4Pdhh3HAjJrc/edit?usp=sharing" rel="nofollow noreferrer">Worker logs File</a></p>