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Optimizing CPU Utilization in Saints Row: Scheduler and Job Management Solutions

This document discusses the challenges and solutions in CPU-bound processing for the Saints Row project, focusing on the implementation of an efficient scheduler and job management system. The integration of hardware and software threads is explored, highlighting the need for a more flexible system that can dynamically allocate resources. Key concepts include worker threads, job queues, independent job scheduling, and critical section management to enhance performance. Strategies for maximizing CPU utilization and minimizing processing delays are presented, alongside practical examples and references to relevant literature.

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Optimizing CPU Utilization in Saints Row: Scheduler and Job Management Solutions

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  1. Saints Row Scheduler • Randall Turner, Volition, Inc. • Randall.turner@volition-inc.com • Discussion Areas: • Saints Row project • Saint’s Row scheduler

  2. Saint’s Row

  3. Single Thread, CPU Bound

  4. Alpha Kits • Two Hardware Threads • Dual Core • Hardware thread • Software thread • Many software threads on one hardware thread • Software threads != concurrency. Hardware threads do.

  5. First Step: Alpha kit solution • Sim/Render Split (Still CPU bound, not as bad.) • Two HW Threads

  6. Beta Kits • Betas = Six hardware threads, Woo Hoo! • But each hardware thread was slower than Alphas. • Homogeneous processing, unified memory

  7. First Beta Soluton

  8. Problem • Still only using mostly two HW threads (Sim, Render) • Badly CPU bound • No more major “natural” separation points

  9. Interim Solution • Manually move processing to other HW threads. • Example – Shadow Calculations • Create a SW thread to process half of the object list • Process half the list when triggered.

  10. Fixed Concurrent Processing • Uses an entire SW thread for one type of processing. • Wastes system resources • Hard coded to execute that processing type only. • Inflexible, Scales up badly.

  11. Solution… Scheduler & Jobs

  12. Goal • Make all movable so will use available space

  13. Scheduler (Job Manager) • Jobs • Worker threads • Job Queues • Triggers

  14. Jobs • Independently schedulable entity • No dependencies on other jobs. • Atomic with respect to other jobs • Essentially a function/data pair

  15. Creating Jobs • Extract parallelism from loops. (Splatting) • Segregate memory usage • Thread safe interactions with systems • critical sections • double-buffering

  16. Examples • Compute shadow silhouettes • Character animation • Object visibility culling • Building command buffers • Bad candidate - AI processing • Not easily turned into jobs due to interaction with large, difficult to thread safe data set.

  17. Basic Scheduler Flow

  18. Worker Threads • One per HW thread • Draws jobs from job queues • Associated with job “type” • Sim jobs, Render jobs, or “any” job. • “Soft” or “Hard” linkage • Dynamically configurable • Wakes up when jobs go into job queue • Can be deactivated

  19. Job Queues • One per job type • Sim or Render • Implicit prioritization • High frequency, contention sensitive • Used critical sections on version 1.0

  20. Triggers • Fire on completion of jobs • Scheduling Triggers – schedule more jobs • Event Triggers - activate suspended threads • Can tie together to create state machines.

  21. First Pass

  22. Final Pass • 90% CPU Utilization

  23. Flow-Around • Callback to scheduler on interrupt • Allow “small” jobs to complete, then resume • Jobs contain duration info

  24. Havok • Timestep called from Scheduler job • More flexible • Problem: Has own internal scheduler • Timestep Goes “Serial” • Jobs don’t flow • Solution: break out and splat serial processing ourselves.

  25. Havok - serial extraction

  26. Performance • Critical Sections • Scheduler v1.0 all critical sections • Blockage risk – CS owner preemption • Can stall ALL threads • Lock-Free • Faster – x2 no contention, x10-20 high-contention situations • NO blockage risk (Woo Hoo!) • GP Gems 6, Fober

  27. Job Size • Scheduler overhead determines job size. • Small as possible. • Assume 5% overhead • Saints Row “ideal” size about 250 microseconds • 7 microseconds per job, 5% = 150 microsec, • 250 drops to ~ 3% overhed

  28. Questions? For latest slides: • http://www.volition-inc.com/gdc07/

  29. Extras • Pix with PVS on, night, high load • Release, w/o PIX samples, ~ 35ms

  30. Lock Free URL’s • Fober implementation: • http://www.grame.fr/pub/LockFree.pdfhttp://www.grame.fr/pub/TR-050523.pdf • Toby Jones: • GP Gems 6 • Good coding style

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