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Scheduling Module for XSched

Schedulers

Application Managed Scheduler

AppManagedScheduler is a empty scheduler. User need to manually add and execute xsched API (refering to xsched API) to control xqueues's behavior.

Local Scheduler

local.cpp implements processing logic of a scheduler. When running a LocalScheduler, it will start a thread running function Worker. Worker contains a loop to take events in event_queue_ and call Sched API of policy. local.cpp also defines following function:

  • Suspend and Resume are called by policy to really realize control to xqueues
  • SetPolicy is used to changes policy online

Note:

  • If a process's scheduler type is LocalScheduler, it means the process can only schedule xqueues created by itself.
  • The scheduler type of XServer must be GlobalScheduler.

Global Scheduler

Global Scheduler Framework

Actually, global scheduler only achieves two IPC channel. Its Worker thread just receive Operation by recv_chan_ and call Execute bound when created (refering to SetExecutor() of sched::Scheduler in file scheduler.h).

  • send_chan_ uses name XSCHED_SERVER_CHANNEL_NAME and its type is sender. When preempt::SchedAgent call RecvEvent to force scheduler to receive events, send_chan_ will send event to receiver and do nothing else.
  • recv_chan_ uses name bound to its PID and its type is receiver.

If a process wants to use GlobalScheduler, user must first run XServer. XServer will create a LocalScheduler and two IPC channels to communicate with process's scheduler, receiving events sended by process's send_chan_ and generating operations by policy in its LocalScheduler, then send these operations back to process's recv_chan_.

Protocol

Event

event are used in communication among preempt::XQueue, preempt::SchedAgent, sched::Scheduler, service::Server.

When XQueues' status change, e.g. become ready when new commands enqueue, become idle when all commands are completed, XQueue will send event to SchedAgent. SchedAgent will process these events by itself or sent them to the XServer according to the type of its scheduler.

Operation

operation is used in communication among service::Server, sched::Scheduler, preempt::SchedAgent.

When policy in sched::Scheduler (LocalScheduler) generate scheduling operations, it will call Execute() bound to service::Server. Then Execute() finds corresponding application process and send these operations to preempt::SchedAgent.

Hint

hint are customized data structures to change policy parameters. Developers can define their own hint types when implementing a new policy. The hint types we support now are as follows:

Name Used Policy Description
kHintTypePriority HPF, HHPF Change priority of the xqueue
kHintTypeUtilization UP, PUP Change utilization of the xqueue or process
kHintTypeTimeslice UP, PUP Change time slice
kHintTypeDeadline KEDF Change deadline of the xqueue
kHintTypeLaxity LAX Change laxity of the xqueue.
kHintTypeWindowActive AWF Change window activity

Policies

policy.cpp provide a uniform abstract sched::policy, all specific strategies are derived from it.

Now we have implemented several policies, including:

Value Full Name Description
HPF Highest Priority First Firstly run the xqueue with highest priority
HHPF Heterogeneous Highest Priority First Firstly run the xqueue with highest priority on heterogeneous devices
UP Utilization Partition Assign time slices with weights to each xqueue
PUP Process Utilization Partition Assign time slices with weights to each process
KEDF K-Earliest Deadline First Firstly run the k xqueues with earliest deadline
LAX Laxity-based Firstly run the xqueue with highest laxity
AWF Active Window First Firstly run the xqueue in the active window

How to Implement Your Own Policy

1. Create a new policy class

Create a new policy class in sched/include/xsched/sched/policy directory, and inherit from sched::policy. Implement the following two functions:

  • Sched is the main function of the policy, it will be called by sched::Scheduler to schedule xqueues.
  • RecvHint is used to receive hints and change policy parameters.

2. Register the new policy

  1. Add the new policy type to the enum PolicyType in include/xsched/types.h.
  2. Add the new policy type to the function CreatePolicy() in sched/src/policy/policy.cpp.
  3. Add the new policy type to the definition of XSCHED_POLICY in protocol/include/xsched/protocol/def.h.
  4. Add the new policy type to the map kPolicyNames in sched/src/protocol/names.cpp.
  5. Add the new hint type by need in hint.h and hint.cpp.