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Experiences with Client-based Speculative Remote Display

Experiences with Client-based Speculative Remote Display. John R. Lange & Peter A. Dinda Department of Electrical Engineering and Computer Science Northwestern University Sam Rossoff Department of Computer Science University of Victoria Prescience Lab http://presciencelab.org

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Experiences with Client-based Speculative Remote Display

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  1. Experiences with Client-based Speculative Remote Display John R. Lange & Peter A. Dinda Department of Electrical Engineering and Computer Science Northwestern University Sam Rossoff Department of Computer Science University of Victoria Prescience Lab http://presciencelab.org http://empathicsystems.org

  2. Speculative Remote Display • Remote display interactivity fundamentally constrained by network latency • Critical for WANs • Speculation can improve interactivity • Can be done without server support • Significant predictability exists in remote display protocol streams • Speculation can be introduced into remote display clients • User-controlled tradeoff between latency and display errors • User studies currently give mixed results for tradeoff

  3. Outline • Speculative Remote Display • Predictability study of Remote Display protocol streams • RDP and VNC • Design and implementation of a client-based speculative remote display system • VNC-SRD • User study of VNC-SRD • Examination of user-controlled speculation • Conclusions

  4. Remote Display Systems • Historically popular in many forms • VNC, RDP, THINC, Citrix, X • Thin clients, remote administration, mobile users, etc • Implemented at all levels of the stack • Virtual hardware, device drivers, window system extensions • Large variance in semantic properties of the protocols • Lots of work has gone into optimizing Remote Displays • Predominantly to decrease bandwidth requirements

  5. Latency vs. BandwidthOptimization • Large amount of work in optimizing bandwidth usage of Remote Displays • Compression/encoding, caching, protocol design • Bandwidth is important… • But so is interactivity • User interaction implies waiting for server updates • Network latency is a critical component

  6. Prospects for Speculative Remote Display User Events Screen Events (Windows RDP) • Can we predict screen events well? • If so, can we speculatively execute screen events before server responds?

  7. Optimizing Latency Through Speculation • Speculation can improve interactivity • Display screen events before they arrive • RTT is no longer limiting factor • UI Interaction intuitively appears to be predictable • Menus, dialog boxes, etc • Often tied to specific user action

  8. Questions • Is there predictability? • How do we design and implement such a tool • Client • Client/Server • Predictor • … • Can users tolerate display errors? • Can users control tradeoff between display errors and interactivity?

  9. Analysis of Predictability • Effectiveness of speculation requires predictable event sequences • Examined predictability with state-limited kth order Markov models • Simple • General prediction symbols • Unique event signatures as symbols • Can provide multi step ahead predictions

  10. Trace Collection • Instrumented versions of rdesktop and VNC • User studies • Northwestern students & faculty participants • 5 users for each protocol • Single study model • Full screen display of remote Windows machine • Private local network • Standardized task sequence • Event traces from typical Windows workloads • Word, PowerPoint, Web browsing • 15 minute periods • VNC: 12K-24K user events, 8.5K-17K screen events • RDP: 47K-77K user events, 712K-1M screen events

  11. Analysis of Predictability • Generated symbols from event sequences • Ran symbol streams through Markov model • Model is continuously being updated, just as in system we later built • Screen->Screen, Screen+User->Screen • Different model orders considered • Different constraints on state space size considered • Analyzed one-step-ahead predictability • Percentage of predictions that were correct • Predictor Accuracy • Percentage of total events correctly predicted • Protocol Predictability

  12. RDP Prediction AccuracyScreen+User -> Screen

  13. RDP PredictabilityScreen+User -> Screen

  14. VNC Prediction AccuracyScreen+User->Screen

  15. VNC PredictabilityScreen+User->Screen

  16. Implications • RDP displays a high degree of predictability • Screen events are highly correlated • RDP results are much better than VNC • Most likely due to number of messages in an update • Differences in protocol semantics • RDP screen event rate much higher than VNC • Errors are possible • Users will experience inconsistent screen states • Implementation must correct mis-predictions • Can users tolerate this?

  17. User Control • Errors will occur, but… • Sensitivity to errors varies… • Between applications • Between users • Let user decide tradeoff between aggressive use of predictions to reduce latency, and increasing display errors • Can users make that decision? • What is the best way to present the control to users?

  18. Why not RDP/SRD? • Implementation simplicity • VNC has much less client state • VNC has fewer event types • VNC execution is straightforward • If it works with VNC then it surely works with RDP

  19. VNC-SRD • Implementation of SRD inside VNC client • ~2300 LOC (C++) added to VNC client • ~850 LOC (Perl) Markov predictor implementation • User-driven prediction application policy • Allows user to adjust amount of speculation • Generic predictor interface • Event signature-based predictions • Error correction and rollback

  20. Prediction Application Policy • System predicts event signatures • Constructs events from signatures • Events directed to one of two queues • Applied Queue • Applied events currently displayed to the user • Held until validated by arrival of server events • Pending Queue • Predictions waiting to be displayed • Events applied to screen at a given rate • Rate: predicted server event rate • Rate * (desired latency reduction) = # of steps ahead

  21. Users/workloads have varying error tolerance Allow direct user control VNC-SRD adds a user adjustable scale to the interface User specifies desired network latency reduction User Control

  22. Error Handling • Speculation can result in wrong display • System must determine and correct errors • Correction • VNC-SRD fetches entire modified region • All applied predictions are rolled back • Local rollback is straightforward • Validation • VNC-SRD validates prediction against next event • Error on mismatch • Conservative approach • Out of order predictions may be displayed correctly • All errors have equal weight

  23. User Study • Students from Northwestern and University of Victoria • 16 total users • 4 network scenarios • Varied latency and drop rate • 300ms, 100ms, 50ms, 10ms + 10% drop rate • 3 randomly presented to each user • 3 Application workloads • Word, PowerPoint, and Web Browsing • Performed in each of the network scenarios • 5 minute runs • Predictor state cleared for each run

  24. Purpose of Study • Determine whether users can tolerate screen artifacts due to speculation • Determine whether users can trade off using our interface

  25. Study Questionnaire • Users rated the system after each scenario • Ability to find a comfortable predictor setting • Responsiveness at the most comfortable setting • Acceptability of display errors at the most comfortable setting • Scale: 1 (very bad) to 10 (very good)

  26. User Study Caveats • Few predictions were made • Study runs not long enough to sufficiently seed predictor • VNC has low predictability • Ratings should thus be taken with a large grain of salt • Future work planned to improve this

  27. Able to find a comfortable setting?

  28. Are Display Errors Acceptable?

  29. Is the System Responsive?

  30. Results • We still do not know for sure whether users… • Can tolerate the screen artifacts • Can trade off between screen artifacts and latency • More work needed

  31. Alternative Design Approaches • Speculative support on the server • Synchronized predictors • Predictions used for other purposes • Cache hints • Update regions • Update ordering • Other prediction algorithms/symbols • Time series predictions of event coordinates

  32. Future Work • Exploration of predictability in Remote Display protocols • How far ahead can predictions be made? • What prediction algorithm should be used? • To what degree can SRD decrease latency? • Comparison of more protocols • Exploration of different SRD designs

  33. Conclusions • Remote display interactivity fundamentally constrained by network latency • Critical for WANs • Speculation can improve interactivity • Can be done without server support • Significant predictability exists in remote display protocol streams • Speculation can be introduced into remote display clients • User-controlled tradeoff between latency and display errors • User studies currently give mixed results for tradeoff

  34. Prescience Lab • http://presciencelab.org • Empathic Systems Project • http://empathicsystems.org • John Lange • http://www.artifex.org/~jarusl

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