1. PredictRemainingTime
Timecard: Controlling User-Perceived Delays in Server-based Mobile Applications
Lenin Ravindranath, Jitendra Padhye, Ratul Mahajan, Hari Balakrishnan
Servers should adapt to external delays to control user-perceived delay
Timecard provides two APIs for server developers
Mobile apps becoming
dominant mode of data access
Developer
App
Service
Timecard.dll
PredictRemainingTime
(responseSize);
GetElapsedTime();
config
Cloud
Services
App Instrumenter
GetElapsedTime();
PredictRemainingTime
(responseSize);
Desired
end-to-end delay
Instrumented
App
Predicted downlink delay and app processing delay for a given data size
Time elapsed since the user initiated the request
App Store
Adapt
processing time
Mobile
Apps
Deployment
Modified two services and two apps to use Timecard
MobileAds Service
Within 50ms of the target delay 90% of the time
Adapt
response
Significant variability
in external delays
Server
Tradeoff between response time and quality of result
Server processing
Request
Response
Server
TCP state
Phone model
Uplink
Downlink
Phone model
Reading sensors
Link quality
(3G, HSPA+, LTE, Wifi)
Datasize
App
App
DNS and TCP connect
User
click
App
App
Parsing and
Rendering
Radio State
(Radio wakeup)
Overhead
Compute
Network
Memory
Battery
low
<1%
0.1%
Highly variable
User perceived delay
User perceived delay
Track Transaction
Time Synchronization
Predict Downlink Delay
Predict Processing Delay
Track transaction across asynchronous threads
and between mobile device and server
90% percentile data size: 37 KB
RTT matters than throughput
Predict RTT
TCP window state matters
Multiple RTTs
Estimate TCP Window & number of RTTs
Complicated by middleboxes
Periodic time sync probes from app to server
Find drift and offset between clocks
Use server clock as reference clock
Client maps local time to server time
Parsing and rendering delay depends on data size
Processing delay dependent on the phone hardware
Server
Send response
Data size
Request handler
Spawn workers
Server Threads
Server TC TC
Middlebox
Efficient technique for probing
Aware of the radio state and traffic
Minimal extra delays
Energy efficient
App
App
Model downlink delay
Recent RTT
Response size
Bytes already transferred
Network provider & client OS TC TC
Facebook
Background Thread
GPS callback
Web callback
Web request
UI dispatcher
Model processing delay
Response size
Phone model
Event
handler
GPS start
UI Thread
Thread start
Triggered by transaction tracking TC
Pass around transaction context TC