Unix Scheduling

1
Unix Scheduling

• Multilevel feedback queues
• 128 priority queues (value 0-127)
• Round Robin per priority queue
• Every scheduling event the scheduler picks the
  lowest priority non-empty queue and runs jobs in
  round-robin
• Scheduling events (when to schedule)
• Clock interrupt
• Process does a system call
• Process gives up CPU,e.g. to do I/O

2
UNIX Scheduling

• All processes assigned a baseline priority based
  on the type and current execution status
• swapper 0
• waiting for disk 20
• waiting for lock 35
• user-mode execution 50
• At scheduling events, all processs priorities
  are adjusted

3
Multiprocessor Scheduling

• Targeted applications Single-process jobs.
  Multi-process jobs. Multi-threaded jobs.
• Multiprogrammed vs. dedicated running several
  applications concurrently?
• Granularity of parallelism.
• Scheduling policy?

4
Granularity of Parallelism

• Independent parallelism independent processes.
  No need to synchronize.
• Coarse-grained parallelism relatively
  independent processes with occasional
  synchronization.
• Medium-grained parallelism. E.g. multi-threads
  which sync frequently.
• Fine-grained parallelism synchronization every
  few instructions.

5
Scheduling Challenges in Multiprogrammed
Environments

• Why multiprogramming?
• Improve system utilization
• Applications with independent tasks or coarse
  grained parallelism.
• Regular OS scheduling policy
• Applications with medium grained parallelism
• Special scheduling policy is needed. E.g.
  Consider a set of cooperative threads
  collectively.

6
Assigning Processes to Processors

• Master/slave approach Single scheduler. One
  processor is dedicated for processor assignment
• Good Centralized decision making
• But single point of failure. Not scalable.
• Peer assignment OS can run any processor. Each
  processor does self-scheduling.

7
Scheduler Organization in Peer Assignment

• Single global ready queue with locks. CPUs
  schedule jobs from this queue.
• Use FCFS or other scheduling policies.
• Limited scalability, but reasonable.
• One ready queue per CPU. Each CPU schedules its
  processes.
• Static process assignment cache friendly
  (Affinity), but difficult to balance load
• Dynamic assignment with job stealing.
• may need process migration.

8
Self Scheduling vs. Gang Scheduling

• Self-scheduling (load sharing) Pool of threads,
  pool of processors.
• Better utilization
• Cooperative threads may not run in parallel.
• Gang-scheduling (co-scheduling) Schedule
  related threads/processes at once on different
  CPUs.
• Maximum concurrency for parallel applications.
• Minimize intra-task communication latency.
• More resource waste.

9
Other Issues

• Time sharing and space sharing
• Space sharing Should processors be statically
  or dynamically divided among applications?
• Time sharing should a processor be
  multiprogrammed?
• Resource allocation
• How many processors should be assigned to a
  process (a user job)? How can the OS know about
  the resource requirement of a job (process).
• In Solaris, a user needs to declare threads to be
  scheduled in a system scope.
• Should the resource allocation be dynamically
  adjusted? A job may not utilize all processors
  assigned.

10
Lottery Scheduling for Resource Share Allocation

• Carl A. Waldspurger and William E. Weihl. Lottery
  Scheduling Flexible Proportional-Share Resource
  Management. In OSDI94
• Probabilistic approach for proportional
  resource sharing.
• Assign a number of lottery tickets to each
  process.
• The OS selects winning ticket randomly.
• Tickets are transferable to among processes for
  share adjustment.
• The of tickets a process owns represents the
  percentage of its share.