BasPH commented on a change in pull request #18356:
URL: https://github.com/apache/airflow/pull/18356#discussion_r711984761
##########
File path: docs/apache-airflow/concepts/scheduler.rst
##########
@@ -138,12 +141,101 @@ The following databases are fully supported and provide
an "optimal" experience:
Microsoft SQLServer has not been tested with HA.
+
+Fine-tuning your Scheduler
+--------------------------
+
+When you deploy Airflow in production you often would like to optimize its
performance and
+fine-tune Scheduler behaviour. Firs of all you need to remember that Scheduler
performs two
+operations:
Review comment:
```suggestion
The Scheduler is responsible for two operations:
```
##########
File path: docs/apache-airflow/concepts/scheduler.rst
##########
@@ -138,12 +141,101 @@ The following databases are fully supported and provide
an "optimal" experience:
Microsoft SQLServer has not been tested with HA.
+
+Fine-tuning your Scheduler
+--------------------------
+
+When you deploy Airflow in production you often would like to optimize its
performance and
+fine-tune Scheduler behaviour. Firs of all you need to remember that Scheduler
performs two
+operations:
+
+* continuously parses DAG files and updates their starts in ``Serialized DAG``
form in the database
+* continuously finds and schedules for execution the next tasks to run and
sends those tasks for
+ execution to the executor you have configured
+
+Those two tasks are executed in parallel by scheduler, they are fairly
independent from each other and
+they are run using different processes. You can fine tune the behaviour of
both components, however
+in order to fine-tune your scheduler, you need to included a number of factors:
+
+* The kind of deployment you have
+ * what kind of filesystem you have to share the DAGS
+ * how fast the filesystem is (in many cases of distributed cloud
filesystem you can pay extra to get
+ more throughput/faster filesystem
+ * how much memory you have for your processing
+ * how much CPU you have available
+ * how much networking throughput you have available
+
+* The logic and definition of your DAG structure:
+ * how many DAG files you have
+ * how many DAGs you have in your files
+ * how large the DAG files are (remember scheduler needs to read and parse
the file every n seconds)
+ * how complex they are
+ * whether parsing your DAGs involves heavy processing (Hint! It should
not. See:doc:`/best-practices`)
+
+* The scheduler configuration
+ * How many schedulers you have
+ * How many parsing processes you have in your scheduler
+ * How much time scheduler waits between re-parsing of the same DAG (it
happens continuously)
+ * How many task instances scheduler processes in one loop
+ * How many new dag runs should be created/scheduled per loop
+ * Whether to execute "mini-scheduler" after completed task to speed up
scheduling dependent tasks
+ * How often the scheduler should perform cleanup and check for orphaned
tasks/adopting them
+ * Whether scheduler uses row-level locking
+
+
+Airflow gives you a lot of "knobs" to turn to fine tune the performance but
it's a separate task,
+depending on your particular deployment, your DAG structure, hardware
availability and expectations,
+to decide which knobs to turn to get best effect for you. Part of the job when
managing the
+deployment is to decide what you are going to optimize for. Some users are ok
with
+30 seconds delays of new DAG parsing, at the expense of lower CPU usage, where
some other users
+expect the DAGs to be parsed almost instantly when they appear in the DAGs
folder at the
+expense of higher CPU usage for example.
+
+Airflow gives you the flexibility to decide, but you should find out what
aspect of performance is
+most important for you and decide which knobs you want to turn in which
direction.
+
+Generally for fine-tuning, your approach should be the same as for any
performance improvement and
+optimizations (we will not recommend any specific tools - just use the tools
that you usually use
+to observe and monitor your systems):
+
+* decide which aspect of performance is most important for you (what you want
to improve)
+* observe your system to see where your bottlenecks are: CPU, memory, I/O are
the usual limiting factors
+* based on your expectations and observations - decide what is your next
improvement and go back to
+ the observation of your performance, bottlenecks. Performance improvement is
an iterative process
+
+The improvements that you can consider are:
+
+* improve utilization of your resources. This is when you have a free capacity
in your system that
+ seems underutilized (again CPU, memory I/O, networking are the prime
candidates) - you can take
+ actions like increasing number of schedulers, parsing processes or
decreasing intervals for more
+ frequent actions might bring improvements in performance at the expense of
higher utilization of those.
+* increase hardware capacity (for example if you see that CPU is limiting you
or tha I/O you use for
+ DAG filesystem is at its limits). Often the problem with scheduler
performance is
+ simply because your system is not "capable" enough and this might be the
only way. For example if
+ you see that you are using all CPU you have on machine, you might want to
add another scheduler on
+ a new machine - in most cases, when you add 2nd or 3rd scheduler, the
capacity of scheduling grows
+ linearly (unless the shared database or filesystem is a bottleneck).
+* experiment with different values for the "scheduler tunables". Often you
might get better effects by
+ simply exchanging one performance aspect for another. For example if you
want to decrease the
+ cpu usage, you might increase file processing interval (but the result will
be that new DAGs will
+ appear with bigger delay). Usually performance tuning is the art of
balancing different aspects.
+* sometimes you change scheduler behaviour slightly (for example change
parsing sort order)
+ in order to get better fine-tuned results for your particular deployment.
+
+In order to perform fine-tuning, it's good to understand how Scheduler works
under-the-hood.
+You can take a look at the ``Airflow Summit 2021``
+`Deep Dive into the Airflow Scheduler talk <https://youtu.be/DYC4-xElccE>`_ to
perform the fine-tuning.
+
+Here are the most important tunables you can use to impact various performance
aspects of the Scheduler:
+
.. _scheduler:ha:tunables:
-Scheduler Tuneables
-"""""""""""""""""""
+Scheduler Tunables
Review comment:
Would keep this aligned with Airflow terminology
```suggestion
Scheduler configuration options
```
##########
File path: docs/apache-airflow/concepts/scheduler.rst
##########
@@ -138,12 +141,101 @@ The following databases are fully supported and provide
an "optimal" experience:
Microsoft SQLServer has not been tested with HA.
+
+Fine-tuning your Scheduler
+--------------------------
+
+When you deploy Airflow in production you often would like to optimize its
performance and
+fine-tune Scheduler behaviour. Firs of all you need to remember that Scheduler
performs two
+operations:
+
+* continuously parses DAG files and updates their starts in ``Serialized DAG``
form in the database
+* continuously finds and schedules for execution the next tasks to run and
sends those tasks for
+ execution to the executor you have configured
Review comment:
Nothing wrong here, but would try to avoid internal details not
necessary to the story
```suggestion
1. continuously parsing DAG files and synchronizing with the DAG in the
database
2. continuously scheduling tasks for execution
```
##########
File path: docs/apache-airflow/concepts/scheduler.rst
##########
@@ -187,3 +279,36 @@ The following config settings can be used to control
aspects of the Scheduler HA
:ref:`config:scheduler__scheduler_health_check_threshold`) any running or
queued tasks that were launched by the dead process will be "adopted" and
monitored by this scheduler instead.
+
+- :ref:`config:scheduler__dag_dir_list_interval`
+ How often (in seconds) to scan the DAGs directory for new files.
+
+- :ref:`config:scheduler__file_parsing_sort_mode`
+ The scheduler will list and sort the dag files to decide the parsing order.
+
+- :ref:`config:scheduler__max_tis_per_query`
+ 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)
+
+- :ref:`config:scheduler__min_file_process_interval`
+ Number of seconds after which a DAG file is parsed. The DAG file is parsed
every
+ min_file_process_interval number of seconds. Updates to DAGs are reflected
after
+ this interval. Keeping this number low will increase CPU usage.
+
+- :ref:`config:scheduler__parsing_processes`
+ The scheduler can run multiple processes in parallel to parse dags. This
defines
+ how many processes will run.
+
+- :ref:`config:scheduler__processor_poll_interval`
+ The number of seconds to wait between consecutive DAG file processing
+
+- :ref:`config:scheduler__schedule_after_task_execution`
+ Should the Task supervisor process perform a “mini scheduler” to attempt to
schedule more tasks of
+ the same DAG. Leaving this on will mean tasks in the same DAG execute
quicker,
+ but might starve out other dags in some circumstances
Review comment:
What circumstances?
##########
File path: docs/apache-airflow/concepts/scheduler.rst
##########
@@ -138,12 +141,101 @@ The following databases are fully supported and provide
an "optimal" experience:
Microsoft SQLServer has not been tested with HA.
+
+Fine-tuning your Scheduler
+--------------------------
+
+When you deploy Airflow in production you often would like to optimize its
performance and
+fine-tune Scheduler behaviour. Firs of all you need to remember that Scheduler
performs two
+operations:
+
+* continuously parses DAG files and updates their starts in ``Serialized DAG``
form in the database
+* continuously finds and schedules for execution the next tasks to run and
sends those tasks for
+ execution to the executor you have configured
+
+Those two tasks are executed in parallel by scheduler, they are fairly
independent from each other and
+they are run using different processes. You can fine tune the behaviour of
both components, however
+in order to fine-tune your scheduler, you need to included a number of factors:
Review comment:
```suggestion
Those two tasks are executed in parallel by the scheduler and run
independently of each other in different processes. In order to fine-tune your
scheduler, you need to include a number of factors:
```
##########
File path: docs/apache-airflow/concepts/scheduler.rst
##########
@@ -138,12 +141,101 @@ The following databases are fully supported and provide
an "optimal" experience:
Microsoft SQLServer has not been tested with HA.
+
+Fine-tuning your Scheduler
+--------------------------
+
+When you deploy Airflow in production you often would like to optimize its
performance and
+fine-tune Scheduler behaviour. Firs of all you need to remember that Scheduler
performs two
+operations:
+
+* continuously parses DAG files and updates their starts in ``Serialized DAG``
form in the database
+* continuously finds and schedules for execution the next tasks to run and
sends those tasks for
+ execution to the executor you have configured
+
+Those two tasks are executed in parallel by scheduler, they are fairly
independent from each other and
+they are run using different processes. You can fine tune the behaviour of
both components, however
+in order to fine-tune your scheduler, you need to included a number of factors:
+
+* The kind of deployment you have
+ * what kind of filesystem you have to share the DAGS
+ * how fast the filesystem is (in many cases of distributed cloud
filesystem you can pay extra to get
+ more throughput/faster filesystem
+ * how much memory you have for your processing
+ * how much CPU you have available
+ * how much networking throughput you have available
+
+* The logic and definition of your DAG structure:
+ * how many DAG files you have
+ * how many DAGs you have in your files
+ * how large the DAG files are (remember scheduler needs to read and parse
the file every n seconds)
+ * how complex they are
+ * whether parsing your DAGs involves heavy processing (Hint! It should
not. See:doc:`/best-practices`)
+
+* The scheduler configuration
+ * How many schedulers you have
+ * How many parsing processes you have in your scheduler
+ * How much time scheduler waits between re-parsing of the same DAG (it
happens continuously)
+ * How many task instances scheduler processes in one loop
+ * How many new dag runs should be created/scheduled per loop
+ * Whether to execute "mini-scheduler" after completed task to speed up
scheduling dependent tasks
+ * How often the scheduler should perform cleanup and check for orphaned
tasks/adopting them
+ * Whether scheduler uses row-level locking
+
+
+Airflow gives you a lot of "knobs" to turn to fine tune the performance but
it's a separate task,
+depending on your particular deployment, your DAG structure, hardware
availability and expectations,
+to decide which knobs to turn to get best effect for you. Part of the job when
managing the
+deployment is to decide what you are going to optimize for. Some users are ok
with
+30 seconds delays of new DAG parsing, at the expense of lower CPU usage, where
some other users
+expect the DAGs to be parsed almost instantly when they appear in the DAGs
folder at the
+expense of higher CPU usage for example.
+
+Airflow gives you the flexibility to decide, but you should find out what
aspect of performance is
+most important for you and decide which knobs you want to turn in which
direction.
+
+Generally for fine-tuning, your approach should be the same as for any
performance improvement and
+optimizations (we will not recommend any specific tools - just use the tools
that you usually use
+to observe and monitor your systems):
+
+* decide which aspect of performance is most important for you (what you want
to improve)
+* observe your system to see where your bottlenecks are: CPU, memory, I/O are
the usual limiting factors
+* based on your expectations and observations - decide what is your next
improvement and go back to
+ the observation of your performance, bottlenecks. Performance improvement is
an iterative process
+
+The improvements that you can consider are:
+
+* improve utilization of your resources. This is when you have a free capacity
in your system that
+ seems underutilized (again CPU, memory I/O, networking are the prime
candidates) - you can take
+ actions like increasing number of schedulers, parsing processes or
decreasing intervals for more
+ frequent actions might bring improvements in performance at the expense of
higher utilization of those.
+* increase hardware capacity (for example if you see that CPU is limiting you
or tha I/O you use for
+ DAG filesystem is at its limits). Often the problem with scheduler
performance is
+ simply because your system is not "capable" enough and this might be the
only way. For example if
+ you see that you are using all CPU you have on machine, you might want to
add another scheduler on
+ a new machine - in most cases, when you add 2nd or 3rd scheduler, the
capacity of scheduling grows
+ linearly (unless the shared database or filesystem is a bottleneck).
+* experiment with different values for the "scheduler tunables". Often you
might get better effects by
+ simply exchanging one performance aspect for another. For example if you
want to decrease the
+ cpu usage, you might increase file processing interval (but the result will
be that new DAGs will
+ appear with bigger delay). Usually performance tuning is the art of
balancing different aspects.
+* sometimes you change scheduler behaviour slightly (for example change
parsing sort order)
+ in order to get better fine-tuned results for your particular deployment.
+
+In order to perform fine-tuning, it's good to understand how Scheduler works
under-the-hood.
+You can take a look at the ``Airflow Summit 2021``
+`Deep Dive into the Airflow Scheduler talk <https://youtu.be/DYC4-xElccE>`_ to
perform the fine-tuning.
+
+Here are the most important tunables you can use to impact various performance
aspects of the Scheduler:
+
.. _scheduler:ha:tunables:
-Scheduler Tuneables
-"""""""""""""""""""
+Scheduler Tunables
+""""""""""""""""""
-The following config settings can be used to control aspects of the Scheduler
HA loop.
+The following config settings can be used to control aspects of the Scheduler.
+However you can also look at other scheduler configuration parameters
available at
+:doc:`../configurations-ref` in ``[scheduler]`` section.
Review comment:
Would leave this out, or mention something like "non-performance related
configuration options"
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