1 / 13

Predictable Scheduling for a Soft Modem

Predictable Scheduling for a Soft Modem. Michael B. Jones – Microsoft Research Stefan Saroiu – University of Washington. Why Study Soft Modems ?. Signal Processing done on host CPU: requires predictable scheduling requires low latency responses While coexisting with other system activities

moe
Download Presentation

Predictable Scheduling for a Soft Modem

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Predictable Scheduling for a Soft Modem Michael B. Jones – Microsoft Research Stefan Saroiu – University of Washington

  2. Why Study Soft Modems ? • Signal Processing done on host CPU: • requires predictable scheduling • requires low latency responses • While coexisting with other system activities • Soft Modem is a background real-time task • Successful in home computer market: • Low cost • Easy to update – software upgrade

  3. Driver versions (INT/DPC/THR/RES) • Vendor version (INT) : 1. DMA transfers between A/D and D/A and physical memory 2. when enough data samples, the modem raises an interrupt 3. inside ISR, process incoming data and provide outgoing samples, before buffers exhausted • Signal processing routines executed: • in a DPC context (DPC) • in a thread context (THR) scheduled by NT scheduler • in a thread context (RES) scheduled by a real-time scheduler based on Rialto/NT

  4. Interrupt Rate 3 different phases, interrupts very regular

  5. Elapsed Times in ISR (INT) 1.8 ms on a Pentium II 450 with a repeatable worst case of 3.3 ms PC 99 recommends maximum time during which a driver-based modem disables interrupts should not exceed 100 µs

  6. CPU Utilization 16% sustained CPU load

  7. Elapsed Times in Queued DPC Interrupt durations now typically < 6µs But now long DPCs (as long as old interrupts) PC 99 recommends at any instant in time, the total execution time required for all DPCs that have been queued but not executed should not exceed 500 µs

  8. Samples Pending to be Processed(INT & THR 24) Small relative to 512 sample buffer size

  9. CPU Reservation Abstraction and Implementation • CPU Reservation abstraction: • ongoing reservation for X time units out of every Y units for a thread • Implementation limitation: • CPU Reservations must be multiples of milliseconds

  10. File Transfer Times Results for 10 copies of 200,000 bytes each For 1/8, 2/15, 3/17, 4/17, 7/20 no test passed

  11. Modem Reservation Ranges Nonlinear behavior If period < 12.5ms, must get 16% to work If period > 12.5ms, (period – amount) >= 12.5ms must also hold

  12. Conclusions • Signal Processing in interrupt context is: • Unnecessary • Detrimental to the predictability and latencies of the coexisting activities • The DPC version has similar problems • Threads help alleviate these problems • Modem runs well with real-time priorities and non-real-time competition • Real-time scheduler allows control over modem’s degree of interference with other time-sensitive activities

  13. For More Information • See Mike Jones (mbj@microsoft.com): • research.microsoft.com/~mbj/ • or me, Stefan Saroiu (tzoompy@cs.washington.edu): • www.cs.washington.edu/homes/tzoompy • Tech Report available shortly

More Related