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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

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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
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


Driver versions int dpc thr res
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


Interrupt rate
Interrupt Rate

3 different phases, interrupts very regular


Elapsed times in isr int
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


Cpu utilization
CPU Utilization

16% sustained CPU load


Cpu reservation abstraction and implementation
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


File transfer times
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


Modem reservation ranges
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


Conclusions
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


For more information
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