Predictable Scheduling for a Soft Modem

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Predictable Scheduling for a Soft Modem

• Michael B. Jones Microsoft Research
• Stefan Saroiu University of Washington

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Consumer Real-Time

• General-purpose Operating Systems,such as
  Windows 2000
• maximize aggregate throughput
• approximate fair sharing of the resources
• Increasing use of time-dependent tasks
• signal processing, audio, video
• Need support for
• predictable scheduling for independently
  developed applications
• low latency responses
• explicit resource allocation mechanisms

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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

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Methodology

• Instrumented Windows 2000 performance kernel
• Logs predefined and custom events
• Writes them to a memory buffer
• Dumps buffers to disk at end of trace
• Driver Software
• No source for signal processing code
• Measurement Environment
• All experiments run with normal-priority spinning
  competitor thread
• System
• Windows 2000 Professional
• Pentium II 450 MHz (uniprocessor)
• 384 MB ECC SDRAM - 100 MB allocated to logging

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Vendor Driver - Signal Processing in Interrupt
(INT)

• Operation of the modem
• 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
• Uses input and output data buffers holding 512
  16-bit samples (1024 bytes/buffer)

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Three Additional Versions

• DPC Version (DPC)
• The ISR queues a DPC
• DPC performs signal processing
• Thread Version (THR)
• The ISR queues a DPC that signals a thread via a
  semaphore
• Thread performs signal processing
• Experimented with several different priorities
• Rialto/NT Version (RES)
• Same as THR, but thread scheduled using Rialto/NT
  real-time periodic CPU Reservation

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Interrupt Rate
3 different phases, interrupts very regular
Falls within PC 99 recommended interrupt rates of
3-16ms
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Elapsed Times in ISR (INT)
1.8 ms with 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

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CPU Utilization
14.7 sustained load on 450MHz Pentium II
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Elapsed Times in ISR (DPC)
ISR times now small, typically lt 6µs
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Elapsed Times in Queued DPC
But now long DPC times 1.8ms avg., 3.3 max
(same as elapsed times in ISR for INT)

• PC 99 recommends that the total execution time
  required for all queued DPCs should not exceed
  500 µs

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Samples Pending to be Processed(INT THR 24)
Small relative to 512 sample buffer size
13
Samples Pending to be Processed (THR 8)
Unsurprisingly, contention kills modem
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Latency Results

• Set the multimedia timers to fire once every
  millisecond
• Register a routine to be called every millisecond
• Routine does very little work
• Stores cycle counter value and sleeps again
• Histograms show differences between recorded
  times and ideal times

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Coexisting Thread Latencies (Control Case - No
Modem)
Maximum 1978µs between wakeups
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Coexisting Thread Latencies (INT)
Maximum 5313µs between wakeups
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Coexisting Thread Latencies (DPC)
Maximum 4396µs between wakeups
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Coexisting Thread Latencies (THR 24)
Maximum 2239µs between wakeups
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What Have We Learned So Far?

• Signal processing in the context of the interrupt
  handler is
• unnecessary
• detrimental to the latencies and predictability
  of coexisting activities
• Vendor choice understandable
• For any priority there is a potentially unbounded
  delay between the interrupt and the thread
  running
• In practice
• Delays are reasonable for well-configured systems
  Intel OSDI 99
• Using interrupts extreme form of priority
  inflation

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Two Possible Solutions

• Rate Monotonic Analysis determine the right
  priority assignments among all threads. Two
  problems
• Assumes cooperative priority assignment among all
  threads - unrealistic
• Working priority assignment dependent upon timing
  requirements of all threads
• Changes in application mix may require changes in
  priority assignments
• Use a time-based real-time scheduler
• Such as Rialto/NT

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Rialto/NT Abstractions

• Two real-time software abstractions
• CPU Reservations ongoing reservation for at
  least X time units out of every Y units for a
  thread
• Time Constraints one-shot time reservation for
  specified amount of work between start time and
  deadline
• The Soft Modem work only uses CPU Reservations

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Rialto/NT Implementation

• Rialto/NT developed on top of Windows 2000
  priority scheduler
• Limitations
• CPU Reservations must be integer multiples of
  milliseconds
• Frequency of reservations must be power-of-two
  multiple of 1ms

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Samples Pending to be Processed (RES 2ms/8ms
25)
Fits well within 512-sample buffer size
24
Coexisting Thread Latencies (RES 2ms/8ms 25)
Maximum 1971µs between wakeups
25
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
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Modem Reservation Ranges
Sensitivity to both percentage and gaps
If period lt 12.5ms, must get 14.7 to work If
period gt 12.5ms, (period amount) gt 12.5ms must
also hold
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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
• However modem threads may interfere with other
  threads
• Real-time scheduler allows
• Control over modems degree of interference with
  other time-sensitive activities
• Performance isolation for threads using
  reservations

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Industry Perspective

• Vendor did build their own THR version
• Worked fine during normal load
• However, modem was starved when
• copying data between two IDE devices
• using USB scanner (Intel 440BX chipset) that
  turned off interrupts for 30-50 ms
• Therefore they shipped the INT version
• Vendor is willing to be a good citizen
• if ensured that others would be as well
• Systematic latency timing verification of
  components is needed to enforce good behavior

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Soft DSL is Coming

• More demanding than soft modems
• 4ms processing period
• G.lite
• 1.531Mbps downstream and 512Kbps upstream
• 25 of a 600 MHz Pentium III
• Full rate DSL
• 3.062Mbps downstream and 512Kbps upstream
• Nearly 50 of a 600 MHz Pentium III
• Soft Bluetooth period 312.5µs

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Further Research Possibilities

• Soft DSL studies
• Multiple soft devices within the same machine
• Similar studies on multiprocessors

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For More Information

• See the authors
• Mike Jones
• mbj_at_microsoft.com
• http//research.microsoft.com/mbj/
• Stefan Saroiu
• tzoompy_at_cs.washington.edu
• http//www.cs.washington.edu/homes/tzoompy/
• See related papers at Mikes web site