Introduction:

Use case:

As clusters get larger and workloads vary it is becoming critical that the jobs get evaluated in as short as time possible to ensure that the correct workload is being run. Using multiple schedulers to address this issue can allow for different scheduling policies and quicker turnaround time for large number of jobs or nodes.

Gist of design proposal::

PBS scheduler in it's current form can run easily run in multiple instances on the same machine. There are only two major problems that we have to deal with:

Managing the scheduler - This includes starting the scheduler, configuring them, making PBS server connect to each one of them and then make them run on specific events.
Make sure that schedulers do not overrun on each other's territory. Make sure that they run on clearly partitioned complex in terms of jobs and nodes.

Design proposal mentioned below tends to address both these problem.

Forum discussion

Interface 1: Extend PBS to support a list of scheduler objects
- Visibility: Public
- Change Control: Stable
- Details:
  - PBS supports a list of scheduler objects to be created using qmgr. It is similar to how we create nodes in server.
  - qmgr command can be used to create a scheduler object . It must be invoked by a PBS admin/manager.
  - To create a scheduler object and make it run, the following are the mandatory attributes that needs to be set by the user
    - Name of the scheduler is mandatory to be given while creating a scheduler object.
      - qmgr -c "c sched multi_sched_1"
        This will create/set the following attributes for the sched object
        port - If not defined by the user, It will start from 15050 and try to run the scheduler on the next available port number.
        host (read-only for now, Has the same value as PBS server host)
        queues = None (default)
        sched_priv = $PBS_HOME/multi_sched_1_priv (default)
        sched_log = $PBS_HOME/multi_sched_1_log (default)
        scheduling = False (default)
        scheduler_iteration = 600 (default)
    - Set the priv directory for the scheduler.
      - The directory must be root owned and should have permissions as "750". By default a sched object has
        it's priv directory set as $PBS_HOME/<sched-name>_priv
      - qmgr -c "s sched multi_sched_1 sched_priv=/var/spool/pbs/sched_priv_1"
    - Set the log directory for the scheduler.
      - The directory must be root owned and should have permissions as "755". By default a sched object has
        it's logs directory set as $PBS_HOME/<sched_name>_logs
      - qmgr -c "s sched multi_sched_1 sched_log=/var/spool/pbs/sched_logs"
    - To set scheduling on one of the newly created scheduler object one must make use of scheduler name.
      - By default a multi-sched object has scheduling set as False.
        If no name is specified then PBS server will enable/disable scheduling on default scheduler.
      - qmgr -c " s sched <scheduler name> scheduling = 1"
  - By default PBS server will configure a default scheduler which will run out of the box.
    - The name of this default scheduler will be "pbs_sched"
    - The sched_priv directory of this default scheduler will be set to the $PBS_HOME/sched_priv
    - Default scheduler will log in $PBS_HOME/sched_logs directory.
Interface 2: Changes to PBS scheduler
- Visibility: Public
- Change Control: Stable
- Details:
  - Scheduler now has additional attributes which can be set in order to run it.
    - sched_priv - to point to the directory where scheduler keeps the fairshare usage, resource_group, holidays file and sched_config
    - sched_logs - to point to the directory where scheduler logs.
    - policy - collection of various attributes (as mentioned below) which can be used to configure scheduler.
    - queues - list of all the queues for which this scheduler is going to schedule jobs.
    - host - hostname on which scheduler is running. For default scheduler it is set to pbs server hostname.
    - port - port number on which scheduler is listening.
    - job_accumulation_time - amount of time server will wait after the submission of a job before starting a new cycle.
    - state - This attribute shows the status of the scheduler. It is a parameter that is set only by pbs server.
  - One can set a queue or a list of queues to scheduler object. Once set, given scheduler object will only schedule jobs from the queues specified.
    - qmgr -c "s sched multi_sched_1 queues=hp_queue1,hp_queue2"
  - If no queues are specified with a given scheduler object then that scheduler will not schedule any jobs.
  - By default, All new queues created will be attached to the default scheduler, unless specified otherwise.
  - A queue once attached to a scheduler can not be attached to another scheduler. If tried, then it will throw following error:
    - qmgr -c "s sched multi_sched_1 queues=workq"
      Queue workq is already associated with scheduler <sched_name>.
  - Scheduler can now accept a set of policy that it can work on:
    - Policy can be specified by using - qmgr -c "s sched <sched_name> policy=<policy object>" command.
  - Scheduler object "state" attribute will show one of these 3 values - DOWN, IDLE, SCHEDULING
    - If a scheduler object is created but scheduler is not running for some reason state will be shown as "DOWN"
    - If a scheduler is up and running but waiting for a cycle to be triggered the state will be shown as "IDLE"
    - If a scheduler is up and running and also running a scheduling cycle then the state will be shown as "SCHEDULING"

Interface 3: New policy object

Visibility: Public
Change Control: Stable

Details:

Admins will now be allowed to create policy objects and give a name to these policy object.
Admins can then assign these policy objects to specific schedulers, they can have one policy object assigned to more than one scheduler.
One can delete a policy object only when it is not assigned to any scheduler.
Example:
qmgr -c "c policy p1"
qmgr -c "s p p1 by_queue=False, strict_ordering=True"
qmgr -c "s sched scheduler1 policy=p1"

Below is the list of policies that reside in the policy attribute of scheduler.

Policy name	Type	Default value	example
round_robin	Boolean	round_robin=False	qmgr -c "s sched sched1 policy.round_robin=True"
by_queue	Boolean	by_queue=True	qmgr -c "s sched sched1 policy.by_queue=True"
strict_ordering	Boolean	strict_ordering=False	qmgr -c "s sched sched1 policy.strict_ordering=True"
help_starving_jobs	Boolean	help_starving_jobs=True	qmgr -c "s sched sched1 policy.help_starving_jobs=True"
max_starve	string	max_starve="24:00:00"	qmgr -c "s sched sched1 policy.max_starve=24:00:00"
node_sort_formula	string	node_sort_formula="sort_priority"	qmgr -c "s sched sched1 policy.node_sort_formula="resources_available.ncpus - resources_assigned.ncpus"
provision_policy	string	provision_policy="aggressive_provision"	qmgr -c "s sched sched1 policy.provision_policy="aggressive_provision"
exclude_resources	array_string	NOT SET BY DEFAULT	qmgr -c 's sched sched1 policy.exclude_resources="vmem, color"'
load_balancing	Boolean	load_balancing=False	qmgr -c "s sched sched1 policy.load_balancing=True"
fairshare	Boolean	fairshare=False	qmgr -c "s sched sched1 policy.fairshare=True"
fairshare_usage_res	string	fairshare_usage_res=cput	qmgr -c "s sched sched1 policy.fairshare_usage_res=cput"
fairshare_entity	string	fairshare_entity=euser	qmgr -c "s sched sched1 policy.fairshare_entity=euser"
fairshare_decay_time	string	fairshare_decay_time="24:00:00"	qmgr -c "s sched sched1 policy.fairshare_decay_time=24:00:00"
fairshare_enforce_no_shares	Boolean	fairshare_enforce_no_shares=True	qmgr -c "s sched sched1 policy.fairshare_enforce_no_shared=True"
preemption	Boolean	preemption=True	qmgr -c " s sched sched1 policy.preemption=True"
preempt_queue_prio	integer	preempt_queue_prio=150	qmgr -c "s sched sched1 policy.preempt_queue_prio=190"
preempt_prio	string	preempt_prio="express_queue, normal_jobs"	qmgr -c 's sched sched1 policy.preempt_prio="starving_jobs, normal_jobs, starving_jobs+fairshare"'
preempt_order	string	preempt_order="SCR"	qmgr -c 's sched sched1 policy.preempt_order="SCR 70 SC 30"'
preempt_sort	string	preempt_sort="min_time_since_start"	qmgr -c 's sched sched1 policy.preempt_sort="min_time_since_start"'
peer_queue	array_string	NOT SET BY DEFAULT	qmgr -c 's sched sched1 policy.peer_queue=" workq workq@svr1"
server_dyn_res	array_string	NOT SET BY DEFAULT	qmgr -c 's sched sched1 policy.server_dyn_res="mem !/bin/get_mem"'
dedicated_queues	array_string	NOT_SET_BY_DEFAULT	qmgr -c 's sched sched1 policy.dedicated_queues="queue1,queue2"'
log_event	integer	log_event=3328	qmgr -c "s sched sched1 policy.log_event=255"
job_sort_formula	string	NOT SET BY DEFAULT	qmgr -c 's sched sched1 policy.job_sort_formula="ncpus*walltime"'
backfill_depth	integer	Set to 1 by default	qmgr -c 's sched sched1 policy.backfill_depth=1'

Following are the configurations that are moved/removed:
- mom_resources - removed (mom periodic hooks can update custom resources)
- unknown_shares - moved to resource_group file.
- smp_cluster_dist - It was already deprecated, removed now
- sort_queues - It was already deprecated, removed now
- nonprimetime_prefix - New policy object does not differentiate between prime/non-prime time (should we make a queue setting that indicates that it is a prime/non prime queue)
- primetime_prefix - New policy object does not differentiate between prime/non-prime time (should we make a queue setting that indicates that it is a prime/non prime queue)
- job_sort_key - uses job_sort_formula instead.
- node_sort_key - replaced by node_sort_formula instead.
- prime_spill - New policy object does not differentiate between prime/non-prime time
- prime_exempt_anytime_queues - New policy object does not differentiate between prime/non-prime time
- backfill_prime - New policy object does not differentiate between prime/non-prime time
- resources - New policy object will now list the resources that needs to be excluded from scheduling. By default all resources will be used for scheduling.
- dedicated_prefix - New policy object will expose "dedicated_queues" which is a list of queues associated with dedicated time.
- preemptive_sched - This has been renamed to "preemption".
- log_filter - log_filter has been renamed to "log_event" to be in sync with the option server object exposes.
Following are the configuration that have been moved from being a "PRIME OPTION" to " NO PRIME OPTION"
- round_robin - Now works irrespective of prime/non-prime time
- by_queue - Now works irrespective of prime/non-prime time
- strict_ordering - Now works irrespective of prime/non-prime time
- help_starving_jobs - Now works irrespective of prime/non-prime time
- load_balancing - Now works irrespective of prime/non-prime time
- fair_share - Now works irrespective of prime/non-prime time
- preemptive_sched - Now works irrespective of prime/non-prime time
If one wants to use policies mentioned under old sched config file then they need to keep a copy of the config file in the directory mentioned under "sched_priv" attribute.
If both policy and sched_config files are present then sched_config file will be ignored.
One can unset all the policies in one shot using "qmgr -c "unset sched <sched_name> policy" and this will make scheduler read the sched_config file in the next iteration.

Interface 4: Changes to PBS server.
- Visibility: Public
- Change Control: Stable
- Details:
  - PBS does not allow attributes like scheduling, scheduler_iteration to be set on PBS server object.
  - scheduling and scheduler_iteration now belong to the sched object
  - backfill_depth will also be an attribute of scheduler's policy object.
    - If scheduler is configured to use sched_config instead of policy object, then it will take value of backfill_depth from scheduler object. If not set on scheduler object then it will take what is set on the server object (We should deprecate backfill_depth on the server object).
    - If scheduler is configured to use policy object instead of sched_config file, then it will take value of backfill_depth from scheduler's policy object.
    - If there is backfill_depth set on per queue level then that value will take precedence over the value set in sched object or server object.
  - These attributes now belong to a scheduler object and needs to be set on scheduler object using a scheduler name
    - qmgr -c "s policy p1 backfill_depth=3"
    - qmgr -c "s sched multi_sched_1 policy = p1"
  - Setting these attributes on server will result into following warning:
    - qmgr -c "s s backfill_depth=3"
    - qmgr: Warning: backfill_depth in server is deprecated. Set backfill_depth in a scheduler policy object.
  - If no scheduler name is specified then also it will throw the following error:
    - qmgr -c "s sched policy.backfill_depth=3"
      No scheduler specified, nothing done
  - Attribute job_sort_formula has been moved from server to scheduler policy attribute.
Interface 5: Changes to PBS Nodes objects.
- Visibility: Public
- Change Control: Stable
- Details:
  - Each of the node object in PBS will have an additional attribute called "sched" which can be used to associate a node to a particular scheduler.
  - This attribute will by default be set to the default scheduler started by the server (which is pbs_sched)
  - PBS admin/manager can set node's sched attribute to an existing scheduler name which will be scheduling jobs on this node.
  - When a scheduler object is deleted all the queues/nodes that were associated to the deleted scheduler moves back to default scheduler.
Interface 6: How PBS server runs scheduler.
- Visibility: Public
- Change Control: Stable
- Details:
  - Upon startup PBS server will start all schedulers which have their scheduling attribute set to "True"
    - If "PBS_START_SCHED" is set to 0 in pbs.conf then server will not start any scheduler.
  - PBS server will connect to these schedulers on their respective hostnames and port number.
  - If server is unable to connect to these schedulers it will check to see if the scheduler is running, try to connect 5 times, and finally restart the scheduler.
  - Scheduling cycles for all configured schedulers are started by PBS server when a job is queued, finished, when scheduling attribute is set to True or when scheduler_iteration is elapsed.
    - When a job gets queued or finished, server will check it's corresponding queue and try to connect to it's corresponding scheduler to run a scheduling cycle.
    - If a scheduler is already running a scheduling cycle while server will just wait for the previous cycle to finish before trying to start another one.
    - If job_accumulation_time is set then server will wait until that time has passed after the submission of a job before starting a new cycle.
  - Each scheduler while querying server specifies it's scheduler name and then gets only a chunk of the universe which is relevant to this scheduler.
    - It gets all the running, queued, exiting jobs from the queues it is associated with.
    - It gets all the list of nodes which are associated with this scheduler and queues managed by the scheduler.
    - It gets the list of all the global policies like run soft/hard limits set on the server object.
Interface 7: What does not work when multiple scheduler objects are present.
- Visibility: Public
- Change Control: Experimental
- Details:
  - When there are multiple scheduler objects configures following things might be broken.
    - Run limits set on server may seem to be broken because a scheduler object may not have a view of whole of the universe.
    - Fairshare is now only limited to what a specific scheduler views, it can not be done complex wide with multiple schedulers.

PP-337: Multiple schedulers servicing the PBS cluster

Introduction: