1

Predictable Scheduling for Predictable Scheduling for a Soft Modema Soft Modem

Michael B. Jones – Microsoft ResearchStefan Saroiu – University of Washington

2

Consumer Real-TimeConsumer 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

3

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

4

MethodologyMethodology• 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

5

Vendor Driver - Signal Vendor Driver - Signal Processing in Interrupt (INT)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)

6

Three Additional VersionsThree 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

7

Interrupt RateInterrupt Rate3 different phases, interrupts very

regularRate of Interrupts (INT)

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Time (seconds)

Mil

lise

con

ds

On-hook ConnectedTrainingDialing

Falls within PC 99 recommended interrupt rates of 3-16ms

8

Elapsed Times in ISR (INT)Elapsed Times in ISR (INT)

PC 99 recommends maximum time during which a driver-based modem disables interrupts should not exceed

100 µs

1.8 ms with repeatable worst case of 3.3 ms

Elapsed Times in Interrupt Handler (INT)

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30

Time (seconds)

Mil

lis

ec

on

ds

On-hook ConnectedTrainingDialing

9

CPU UtilizationCPU Utilization14.7% sustained load on 450MHz

Pentium IICPU Load

0%

5%

10%

15%

20%

25%

30%

35%

0 5 10 15 20 25 30

Time (seconds)

CP

U L

oad

On-hook ConnectedTrainingDialing

10

Elapsed Times in ISR Elapsed Times in ISR (DPC)(DPC)

ISR times now small, typically < 6µs

Elapsed Times In Interrupt Handler (DPC)

0

2

4

6

8

10

12

14

16

0 5 10 15 20 25 30

Time (seconds)

Mic

ros

ec

on

ds

On-hook ConnectedTrainingDialing

11

Elapsed Times in Queued DPCElapsed Times in Queued DPC

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

µs

But now long DPC times: 1.8ms avg., 3.3 max (same as elapsed times in ISR for INT)

Elapsed Times In Queued DPC (DPC)

0

0.5

1

1.5

2

2.5

3

3.5

0 5 10 15 20 25 30

Time (seconds)

Mil

lis

ec

on

ds

On-hook ConnectedTrainingDialing

12

Samples Pending to be Samples Pending to be ProcessedProcessed

(INT & THR 24)(INT & THR 24)Small relative to 512 sample buffer sizeSamples Pending to be Processed (INT)

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Time (seconds)

Un

pro

ce

ss

ed

Sa

mp

les

On-hook ConnectedTrainingDialing

Samples Pending to be Processed (THR 24)

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30

Time (seconds)

Un

pro

ce

ss

ed

Sa

mp

les

On-hook ConnectedTrainingDialing

13

Samples Pending to be Samples Pending to be Processed (THR 8)Processed (THR 8)

Unsurprisingly, contention kills modem

Samples Pending to be Processed (THR 8)

0

100

200

300

400

500

600

0 5 10 15 20 25 30 35

Time (seconds)

Un

pro

cess

ed S

amp

les

On-hook "Please hang up and try your call again"Dialing

14

Latency ResultsLatency 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

15

Coexisting Thread Coexisting Thread Latencies (Control Case - Latencies (Control Case -

No Modem)No Modem)Maximum 1978µs between

wakeupsControl Case - No Modem

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

Latency (microseconds)

Per

cen

tag

e o

f C

allb

acks

96.8%

16

Coexisting Thread Coexisting Thread Latencies (INT)Latencies (INT)

Maximum 5313µs between wakeupsINT Version

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

Latency (microseconds)

Per

cen

tag

e o

f C

allb

acks

83.1%

17

Coexisting Thread Coexisting Thread Latencies (DPC)Latencies (DPC)

Maximum 4396µs between wakeupsDPC Version

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

Latency (microseconds)

Per

cen

tag

e o

f C

allb

acks

82.6%

18

Coexisting Thread Coexisting Thread Latencies (THR 24)Latencies (THR 24)Maximum 2239µs between

wakeupsTHR Version (24)

0.0%

0.5%

1.0%

1.5%

2.0%

2.5%

3.0%

Latency (microseconds)

Per

cen

tag

e o

f C

allb

acks

93.8%

19

What Have We Learned So Far?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

20

Two Possible SolutionsTwo 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

21

Rialto/NT AbstractionsRialto/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

22

Rialto/NT ImplementationRialto/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

23

Samples Pending to be Samples Pending to be Processed (RES 2ms/8ms – Processed (RES 2ms/8ms –

25%)25%)Fits well within 512-sample buffer size

Samples Pending to be Processed (RES 2ms/8ms)

0

20

40

60

80

100

120

140

160

0 5 10 15 20 25 30 35

Time (seconds)

Un

pro

cess

ed S

amp

les

On-hook ConnectedTrainingDialing

24

Coexisting Thread Coexisting Thread Latencies (RES 2ms/8ms – Latencies (RES 2ms/8ms –

25%)25%)Maximum 1971µs between

wakeupsRES Version (2ms/8ms)

0.0%

1.0%

2.0%

3.0%

4.0%

5.0%

6.0%

7.0%

Latency (microseconds)

Per

cen

tag

e o

f C

allb

acks

85.5%

25

File Transfer TimesFile Transfer Times

Min Max Mean Std Dev Passed

INT 36.334 36.398 36.367 0.029 10DPC 36.272 36.447 36.396 0.048 10THR Pri 24 36.319 36.475 36.384 0.056 10RES 1ms/7ms 36.333 36.724 36.426 0.112 10RES 2ms/13ms 36.288 36.975 36.547 0.232 10RES 2ms/14ms 38.631 91.713 65.172 37.535 2RES 3ms/15ms 36.275 36.586 36.387 0.108 10RES 3ms/16ms 97.289 180.415 110.523 26.408 9RES 4ms/16ms 36.255 37.116 36.415 0.256 10RES 8ms/20ms 36.347 36.476 36.394 0.039 10

Results for 10 copies of 200,000 bytes each

For 1/8, 2/15, 3/17, 4/17, 7/20 no test passed

26

Modem Reservation RangesModem Reservation RangesSensitivity to both percentage and gaps

If period < 12.5ms, must get 14.7% to workIf period > 12.5ms, (period – amount) >=

12.5ms must also hold

Modem Reservation Operating Ranges

0

1

2

3

4

5

6

7

8

9

10

0 2 4 6 8 10 12 14 16 18 20 22Reservation Period (ms)

Re

se

rva

tio

n A

mo

un

t (m

s)

Sufficient MarginalInsufficient Actual14.7% of CPU 12.5ms Gaps

SufficientCPU Percentageand Frequency

GapsToo

Long

Insufficient Percentage

27

ConclusionsConclusions• 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 modem’s degree of interference

with other time-sensitive activities– Performance isolation for threads using

reservations

28

Industry PerspectiveIndustry 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

29

Soft DSL is ComingSoft 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

30

Further Research Further Research PossibilitiesPossibilities

• Soft DSL studies• Multiple soft devices within the

same machine• Similar studies on

multiprocessors

31

For More InformationFor More Information

• See the authors:– Mike Jones

+mbj@microsoft.com+http://research.microsoft.com/~mbj/

– Stefan Saroiu+tzoompy@cs.washington.edu+http://www.cs.washington.edu/homes/tzoompy/

• See related papers at Mike’s web site