Two Case Studies in Predictable Application Scheduling Using RialtoNT

1
Two Case Studies in Predictable Application
Scheduling Using Rialto/NT

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

2
Application Case Studies

• Two applications needing predictable execution on
  Windows 2000
• Soft Modem Driver
• Digital Audio Player
• The case studies
• analyze behavior on normal Windows 2000
• study improvements possible using Rialto/NT CPU
  Reservation mechanism

3
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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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
• Case studies use only 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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First Case Study

• Predictable Scheduling for a Soft Modem

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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

8
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 Version - 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
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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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Samples Pending to be Processed (RES 2ms/8ms
25)
Fits well within 512-sample buffer size
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Coexisting Thread Latencies (RES 2ms/8ms 25)
Maximum 1971µs between wakeups
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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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Soft Modem 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 try 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 only 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 Soft Modem Studies

• Software-based Digital Subscriber Line (SoftDSL)
  studies
• Multiple Soft Modems within the same machine
• Similar studies on multiprocessors

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Second Case Study

• Predictable Scheduling for Digital Audio

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Methodology

• Empirically reverse-engineer thread requirements
  in a complex, legacy soft real-time application
• without use of source code
• Assign CPU reservations to threads
• without modifying the application
• Measure application behavior during contention

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Windows Media Player

• Default player for mp3, wav, avi, mpeg
• Experimental method
• Modelled contention using spinning thread at
  various priorities
• Gave CPU Reservations to media player threads
• Played an mp3 song
• Listened for glitches
• Used instrumented kernel to detect buffer
  under-runs

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Media Player Thread Structure (Simplified)
() Received CPU Reservations in some experiments.
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MP3 Playback w/o Contention

• Kmixer thread (top) runs every 10ms
• MP3 decoder (4th line) runs every 100ms
• Works fine

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Starvation Caused by Competing Thread _at_ Priority
10

• Media Player runs only when NT priority inversion
  avoidance logic kicks in

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Media Player Reservation

• 1ms every 16ms reserved for decoder thread
• Competing with priority 10 thread
• Works fine

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Priority Inversion Caused by Competing Thread
x
x

• Competitor thread (priority 9) preempts MP3
  decoder while holding Kmixer buffer lock
• Kmixer misses next two time slots (x)
• Starves, causes audio glitch
• Fix raise decoder priority before grabbing lock

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Media Player Deadlock

• Circular wait among Media Player threads
• Deadlock broken by a timeout
• Fix file a bug report

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Media Player Results

• Expected
• In the presence of contention, the Windows
  priority scheduler allows real-time apps to
  starve
• This can be fixed by giving real-time threads CPU
  Reservation
• Unexpected
• Competitor thread changes sequencing, exposes
  races in Media Player
• Hard to write correct programs with many threads
  mutexes
• Fixed using priority ceiling emulation

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Implications of Results

• Periods of threads in complex legacy apps can be
  reverse engineered
• Amounts are platform-dependent and are harder
• Next step to store application requirements and
  use middleware to automatically assign
  reservations
• No application support needed
• Potentially a way around the chicken/egg problem
  of using reservations in a world of legacy OSs
  and applications

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Possible ContinuedMedia Experiments

• Study software DVD player
• CPU intensive and time sensitive

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Overall Conclusions

• Status quo insufficient
• Applications either inflate their priorities
• as did the soft modem driver
• or are at the mercy of applications that may be
  run at higher priorities
• as is the case with the digital audio player
• CPU Reservations solve this problem
• by allowing applications to reliably obtain the
  time they need
• while allowing other applications to do the same

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For More Information

• See Mike Jones (mbj_at_microsoft.com)
• http//research.microsoft.com/mbj/
• or John Regehr (regehr_at_cs.utah.edu)
• http//www.cs.utah.edu/regehr/
• or Stefan Saroiu (tzoompy_at_cs.washington.edu)
• http//www.cs.washington.edu/homes/tzoompy/
• Related papers at Mikes web site