Real-Time Java* Programming
Christopher D. [email protected]
Center for Distributed Object ComputingDepartment of Computer ScienceWashington University, St. Louis
http://www.cs.wustl.edu/~cdgill/RTSJ/COOTS01_M4.pptCOOTS 2001 Tutorial M4
Monday, January 29, 2001
*JavaTM is a registered trademark of Sun Microsystems
Christopher D. Gill
Tutorial Objectives• Provide an overview of real-time programming issues• Describe a motivating real-time programming example
– An on-line stock market analysis tool• Exhibits canonical requirements and issues
common to other classes of real-time systems• Show through incremental evolution of the example
– How real-time programming issues can arise in a JavaTM (Java) programming environment
– How features of the Real-Time Specification for JavaTM (RTSJ) can be applied to resolve these issues
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool• Performs automated decision aiding for stock trading• Inputs arrive from real-time data streams• May run queries against on-line databases• Sends alerts to human operator and/or other
automated systems with specific recommendations (e.g., sell, buy, limit order, short, call, put)
• Timeliness of outputs is crucial– A functionally correct output sent too late can be
worse than no output at all• Several application layers compete in real-time for
system resources (i.e., CPU, memory)
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool• Inputs arrive in real-
time from data streams– Real-time (seconds)
arrival of data events– One feed per market
• May run queries on-line tables and databases: differences in latency and latency jitter– Analyst reports– Market histories– Sector P/E tables
DataFeed
NYSEFeed NasdaqFeed
DataStore
NasdaqStore
ResearchStore
NYSEStore
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool• Sends recommendations
as alerts to: – Human operators– Automated systems
• Documented quality of information is key– Decision path, triggers– Additional info, links
• Timeliness constraints must also be met– Incremental addition,
refinement is useful
MarketOrder Option
Call
AnnotationAlert
PutBuy Sell
*
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool
• Input events pass through an analysis pipeline– Each analysis filter handles the data and news
events in which it is interested, may search databases
– May attach additional information to event and pass it on or consume it, and/or produce alerts
– Composites combine other analysis filters
AnalysisPipelineAnalysisFilter
SectorPE PortfolioBalance
1+
Composite
1+
Real-Time Java Programming
Christopher D. Gill
Example: Roadmap
NasdaqAnnotation
AnalysisPipeline
AlertList
SectorPEFilter PortfolioBalanceFilterCompositeFilter
MarketOrderAlert OptionAlert
CallAlert
Alert
PutAlert
BuyAlert SellAlert
AnalysisFilter
AnalysisTool
Annotation
ResearchAnnotation
DataStore
NasdaqStore ResearchStore
AnnotationList
Portfolio
DataFeed DataFeedEvent
NasdaqDataFeed
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool// Input Event Streams Codepublic class DataFeedEvent{ private float bid; private float ask; private float change; private long volume; // ... public DataFeedEvent (float b, float a, float c, long v) {bid = b; ask = a; change = c; volume = v;} public float getBid () {return bid;} public float getAsk () {return ask;} public float getChange () {return change;} public long getVolume () {return volume;} // ...}
Real-Time Java Programming
data event
90 seconds
data feedMarket
market order
Christopher D. Gill
Example: Stock Market Analysis Tool// Input Event Streams Code, Continuedpublic abstract class DataFeed{ public abstract DataFeedEvent pullDataFeedEvent ();}
public class NasdaqDataFeed extends DataFeed{ // low-ish latency public DataFeedEvent pullDataFeedEvent () { return pullNasdaqDataFeedEvent (); } protected DataFeedEvent pullNasdaqDataFeedEvent () { float bid = 0.0F; float ask = 0.0F; float chg = 0.0F; long vol = 0; // read data from socket, etc... return new DataFeedEvent (bid, ask, chg, vol); }} /* ... Other DataFeed Classes ... */
Real-Time Java Programming
• Separate data feed for each market
• Low latency to pull an event from a market data feed
Christopher D. Gill
Example: RoadmapAnalysisPipeline
AlertList
SectorPEFilter PortfolioBalanceFilterCompositeFilter
AnalysisFilter
AnalysisTool
DataStore
NasdaqStore ResearchStore
Portfolio
DataFeed DataFeedEvent
NasdaqDataFeed
NasdaqAnnotation
MarketOrderAlert OptionAlert
CallAlert
Alert
PutAlert
BuyAlert SellAlert
Annotation
ResearchAnnotation
AnnotationList
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool// Alerts Codepublic abstract class Annotation { /* ... */ }public class AnnotationList{ private java.util.Vector alist; // list of annotations public void addSorted (Annotation a) { /* ... */ }}public abstract class Alert{ private AnnotationList anotes; private DataFeedEvent trigger; Alert (DataFeedEvent dfe) {anotes = new AnnotationList (); trigger = dfe;} public DataFeedEvent getTrigger () {return trigger;} public void addAnnotation (Annotation a) { anotes.addSorted (a); } public Annotation nextAnnotation (boolean restart) { /* move to next annotation in list, return it ... */ }}
Real-Time Java Programming
trigger
Alert
annotations
Christopher D. Gill
Example: Stock Market Analysis Tool// Alerts Code, Continuedpublic abstract class MarketOrderAlert extends Alert{ private float orderPrice; private String symbol; public MarketOrderAlert (DataFeedEvent dfe, float op, String s) {super (dfe); orderPrice = op; symbol = s;} protected String getSymbol () {return symbol;} protected float getOrderPrice () {return orderPrice;}} /* ... Similarly, for OptionAlert and its derived classes ... */
public class BuyAlert extends MarketOrderAlert{ public BuyAlert (DataFeedEvent dfe, float op, String s) {super (dfe, op, s);} float getBuyPrice () { return super.getOrderPrice (); }} /* ... Similarly for SellAlert, Other Alert Classes ... */
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool// Data Store Query Codepublic class NasdaqAnnotation extends Annotation{ private float sectorAvgEarnings; private float sectorPERatio; public NasdaqAnnotation (float e, float r) {sectorAvgEarnings = e; sectorPERatio = r;} public float getSectorAvgEarnings () {return sectorAvgEarnings;} public float getSectorPERatio () {return sectorPERatio;}}/* ... Other Annotation Classes */public class ResearchAnnotation extends Annotation{ // URLs for research reports private java.util.Vector research_reports; public void addReport (java.net.URL u) {reports.add (u);} public java.net.URL nextReport (boolean restart) { /* ... */ }}/* ... Other Annotation Classes */
Real-Time Java Programming
annotations
research reports
URL
sector analysis
table
P/E
Christopher D. Gill
Example: RoadmapAnalysisPipeline
AlertList
SectorPEFilter PortfolioBalanceFilterCompositeFilter
AnalysisFilter
AnalysisTool
Portfolio
DataFeed DataFeedEvent
NasdaqDataFeed
NasdaqAnnotation
MarketOrderAlert OptionAlert
CallAlert
Alert
PutAlert
BuyAlert SellAlert
Annotation
ResearchAnnotation
AnnotationList
DataStore
NasdaqStore ResearchStore
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool// Data Store Query Code, Continuedpublic abstract class DataStore { public abstract void annotateAlert (Alert a);}public class NasdaqStore extends DataStore{ public float getPE (String symbol, boolean sector) {/* medium duration */} public float getEarnings (String symbol) {/*...*/} public void annotateAlert (Alert a) { addNasdaqAnnotation (a); /* ... */ } protected void addNasdaqAnnotation (Alert a) { float e = 0.0F; float r = 0.0F; // compute PE and Earnings averages for the sector a.addAnnotation (new NasdaqAnnotation (e, r)); }}
Real-Time Java Programming
annotations
sector analysis
table
P/E
Nasdaqmarkethistory
database
analysisquery
Christopher D. Gill
Example: Stock Market Analysis Tool// Data Store Query Code, Continuedpublic class ResearchStore extends DataStore{ public void annotateAlert (Alert a) { addResearchAnnotation (a);} protected void addResearchAnnotation (Alert a) { // long duration: guided // search for research // reports, adding URLS // for relevant analyst // research reports to // the annotation // (ordered by relevance // & confidence factors)
// add annotation to alert a.addAnnotation (new ResearchAnnotation ()); }} /* ... Other DataStore Classes ... */
Real-Time Java Programming
annotations
hyper-linked
research reports
URL
report index search
agent
Christopher D. Gill
Example: Roadmap
DataFeed DataFeedEvent
NasdaqDataFeed
NasdaqAnnotation
MarketOrderAlert OptionAlert
CallAlert
Alert
PutAlert
BuyAlert SellAlert
Annotation
ResearchAnnotation
AnnotationList
DataStore
NasdaqStore ResearchStore
AnalysisPipeline
AlertList
SectorPEFilter PortfolioBalanceFilterCompositeFilter
AnalysisFilter
AnalysisTool
Portfolio
Real-Time Java Programming
Christopher D. Gill
Example: Stock Market Analysis Tool// Analysis Filter Codepublic class AlertList{// Alerts raised so far private java.util.Vector alerts; public void addAlert (Alert a) {alerts.add (a);} public Alert nextReport (boolean restart) { /* ... */ } public void reset () { alerts.clear ();}}
public abstract class AnalysisFilter {public abstract boolean handleDataEvent (DataFeedEvent d, AlertList a); // ...}
Real-Time Java Programming
data event
data feedAnalysis filter
alert list
Christopher D. Gill
Example: Stock Market Analysis Tool// Analysis Filter Code, Continuedpublic class CompositeFilter extends AnalysisFilter { // the composed filters private java.util.Vector filters; public void addFilter (AnalysisFilter af) { filters.add (af); } public boolean handleDataEvent (DataFeedEvent dfe, AlertList al) { boolean consumed = false; for (int i = 0; !consumed && i < filters.size (); ++i) { consumed = ((AnalysisFilter) filters.get(i)).handleDataEvent (dfe, al); } return consumed; }}
Real-Time Java Programming
data event
Composite Filter
Christopher D. Gill
Example: Stock Market Analysis Tool// Analysis Filter Code, Continuedpublic class SectorPEFilter extends AnalysisFilter { private NasdaqStore nh; private ResearchStore rr; public boolean handleDataEvent (DataFeedEvent dfe, AlertList al) { boolean consumed = false; // See if event is of interest, // compare its PE to the avg for // its sector, look at existing // alerts, possibly generate // new ones annotated with // relevant research reports rr.annotateAlert (alert) return consumed; }}
Real-Time Java Programming
research reports
sector analysis
table
Sector P/E Filter
data event
Christopher D. Gill
Example: Stock Market Analysis Tool// Analysis Filter Code, Continuedpublic class Portfolio { public float projectRiskDelta (DataFeedEvent d) {/*...*/} public float projectGainDelta (DataFeedEvent d) {/*...*/}}public class PortfolioBalanceFilter extends AnalysisFilter { protected Portfolio p; public boolean handleDataEvent (DataFeedEvent dfe, AlertList al) { boolean consumed = false; // issue/remove alerts based on // data feed event and projected // risk/gain to portfolio goals return consumed; }}
Real-Time Java Programming
PortfolioBalance Filter
alert list
goals
risk profile
data event
Christopher D. Gill
Example: Stock Market Analysis Tool// Analysis Pipeline Codepublic class AnalysisPipeline{ private CompositeFilter cf; private DataFeed df; private AlertList al; public void addFilter (AnalysisFilter af) {cf.addFilter (af);} public void sendAlerts () {/* Send all alerts, reset list */} public void run () { for (;;) { DataFeedEvent dfe = df.pullDataFeedEvent (); cf.handleDataEvent (dfe, al); // possibly long latency sendAlerts (); /* latency depends on alert count */} }}
Real-Time Java Programming
data event
Filter Pipeline
alert list
send alerts
Operator
Christopher D. Gill
Example: Stock Market Analysis Tool// Analysis Tool Codepublic class AnalysisTool { public static void main (String [] args) { AnalysisPipeline ap = new AnalysisPipeline (); ap.addFilter (new PortfolioBalanceFilter ()); ap.addFilter (new SectorPEFilter ());
ap.run (); // run the pipeline }}
Real-Time Java Programming
data event
data feed
Market
market order
alert list
send alerts
Christopher D. Gill
Review: RoadmapAnalysisPipeline
AlertList
SectorPEFilter PortfolioBalanceFilterCompositeFilter
MarketOrderAlert OptionAlert
CallAlert
DataFeed Alert
PutAlert
BuyAlert SellAlert
AnalysisFilter
AnalysisTool
AnnotationDataFeedEvent
NasdaqDataFeed
NasdaqAnnotation
ResearchAnnotation
DataStore
NasdaqStore ResearchStore
AnnotationList
Portfolio
Real-Time Java Programming
Christopher D. Gill
Example: Time Scales
AlertList
MarketOrderAlert OptionAlert
CallAlert
Alert
PutAlert
BuyAlert SellAlert
AnalysisTool
DataFeedEvent
ResearchAnnotation
ResearchStore
AnnotationList
Portfolio
Low MediumLatency: High
AnalysisPipeline
SectorPEFilter PortfolioBalanceFilterCompositeFilter
DataFeed
AnalysisFilter
NasdaqDataFeed
NasdaqAnnotationDataStore
NasdaqStore
Annotation
AnalysisTool
ResearchAnnotation
ResearchStore
Real-Time Java Programming
Christopher D. Gill
Java Real-Time Issues• Existing JavaTM facilities take us several important
steps in the direction of real-time application behavior• Threads
– Liveness (what and how much happens)– Threads are used to decouple activity time scales
• Synchronization– Safety (nothing “unsafe” happens)– Careful application of monitors can preserve
liveness• We’ll start in a bottom-up liveness-first design mode,
using thread adapters (Lea, “Concurrent Programming in JavaTM”)
Real-Time Java Programming
Christopher D. Gill
• Separate threads of execution are useful to improve liveness by doing the following concurrently:– Getting and handling
market data events• Medium latency
– Searching stores to add annotations• High latency
– Issuing alerts• Low latency
// Analysis Tool Code, Revisited
// Separate high latency activitypublic class StoreThreadAdapter
implements Runnable{ private DataStore store; private Alert alert; public StoreThreadAdapter
(DataStore ds, Alert a) { store = ds; alert = a;} public void run () { store.annotateAlert (alert); }}
Java: Threading IssuesReal-Time Java Programming
Christopher D. Gill
Java: Threading Issues// Analysis Filter Code, Revisitedpublic class SectorPEFilter extends AnalysisFilter { private NasdaqStore nh; private ResearchStore rr; public boolean handleDataEvent (DataFeedEvent dfe, AlertList al) { boolean consumed = false; // possibly generate new alerts ... // ... annotated with relevant research reports... Thread annotationThread = new Thread (new StoreThreadAdapter (rr, alert)); annotationThread.setPriority (Thread.MIN_PRIORITY); annotationThread.start ();
return consumed; }}
Real-Time Java Programming
Christopher D. Gill
Java: Threading Issues// Analysis Tool Code, Revisited
// Separate low latency activitypublic class AlertThreadAdapter implements Runnable{ private AnalysisPipeline pipeline; private long timeout; public AlertThreadAdapter (AnalysisPipeline ap, long t) { pipeline = ap; timeout = t;} public void run () { for (;;) // in reality, could use more sophisticated { // loop control e.g., wait, notifyAll, etc. try { Thread.sleep (timeout); pipeline.sendAlerts (); } catch (java.lang.InterruptedException e) {/* ... */} } }}
Real-Time Java Programming
Christopher D. Gill
Java: Threading Issues// Analysis Pipeline Code, Revisited
// Separate medium latency activitypublic class AnalysisPipeline{ private CompositeFilter cf; // filters in the pipeline private DataFeed df; // paced data event feed private AlertList al; // list of alerts public void addFilter (AnalysisFilter af) {cf.addFilter (af);} public void sendAlerts () {/* Send all alerts in the list, reset alert list */} public void run () { for (;;) { DataFeedEvent dfe = df.pullDataFeedEvent (); cf.handleDataEvent (dfe, al); // possibly long latency } }}
Real-Time Java Programming
Christopher D. Gill
Java: Threading Issues// Analysis Tool Code, Revisitedpublic class AnalysisTool { public static void main (String [] args) { AnalysisPipeline ap = new AnalysisPipeline (); ap.addFilter (new PortfolioBalanceFilter ()); ap.addFilter (new SectorPEFilter ());
Thread alertThread = new Thread (new AlertThreadAdapter (ap, 1000)); alertThread.setPriority (Thread.MAX_PRIORITY); alertThread.start (); ap.run (); // run pipeline in the current thread }}
Real-Time Java Programming
Christopher D. Gill
Java: Synchronization Issues• But, before we go
further addressing liveness issues, need to address concurrency safety
• Shift to top-down safety-first design mode, using fine-grain synchronization (Lea, “Concurrent Programming in JavaTM”)
• We’ll combine two styles: block and method synchronization
// Concurrency safety additions// using method synchronizationpublic abstract class Alert{ /* ... */ public synchronized void addAnnotation (Annotation a) {/* ...*/}
public synchronized Annotation nextAnnotation (boolean restart) {/*...*/}}
Real-Time Java Programming
Christopher D. Gill
Java: Synchronization Issues// Concurrency safety additions using block synchronization public class AnalysisPipeline{ /* ... */ protected void sendAlerts () { synchronized (al) {/* Send all the alerts in the list, reset alert list */} } public void run () { for (;;) { DataFeedEvent dfe = df.pullDataFeedEvent ();
// spawns separate threads for long latency activities cf.handleDataEvent (dfe, al); }}
Real-Time Java Programming
Christopher D. Gill
Java: Synchronization Issues// Concurrency safety additions using block synchronization
public class PortfolioBalanceFilter extends AnalysisFilter { protected Portfolio p; public boolean handleDataEvent (DataFeedEvent dfe, AlertList al) { boolean consumed = false; synchronized (al) { /* add alerts based on data feed event and the projected risk and gain changes to portfolio */ } return consumed; }}
Real-Time Java Programming
Christopher D. Gill
Java: Synchronization Issues// Concurrency safety additions using block synchronization public class SectorPEFilter extends AnalysisFilter { private NasdaqStore nh; private ResearchStore rr; public boolean handleDataEvent (DataFeedEvent dfe, AlertList al) { boolean consumed = false; /* compare PE to the average for its sector */ synchronized (al) { /* look at existing alerts*/ } /* possibly generate new ones, annotated in a separate thread with relevant research reports... */ synchronized (al) { /* add any new alerts to the list */ } return consumed; }}
Real-Time Java Programming
Christopher D. Gill
Threads and Synch PointsAnalysisPipeline
SectorPEFilter PortfolioBalanceFilterCompositeFilter
MarketOrderAlert OptionAlert
CallAlert
DataFeed Alert
PutAlert
BuyAlert SellAlert
AnalysisFilter
AnalysisTool
AnnotationDataFeedEvent
NasdaqDataFeed
NasdaqAnnotation
ResearchAnnotation
DataStore
NasdaqStore ResearchStore
Portfolio
low latency
high latency
medium
latency
AlertList
AnnotationList
Synchronization points:
AlertList
AnnotationList
Real-Time Java Programming
Christopher D. Gill
The RTSJ and Real-Time Issues• Threads (revisited)• Release characteristics & failures• Scheduling• Synchronization (revisited)• Time and timers• Asynchronous event handling• Memory management• Asynchronous transfer of control• Exceptions• System-level options
Real-Time Java Programming
Christopher D. Gill
RT Issues: Threads• Multi-threading is useful to
decouple different activities– Active objects, request
queues, synch/asynch
• Must ensure resource usage by non-critical activities does not interfere with needs of critical activities
• However, work in different threads competes for CPU time and memory resources
Real-Time Java Programming
Christopher D. Gill
RTSJ: Threading Issues• Threads compete for
time on the CPU• Some activities are
higher priority than others
• Java thread priorities take us a step in the right direction, but…– garbage collector
thread priority and preemption issues
– Non-RT priority uniqueness is not ensured
// Solution: real-time threads
AlertThreadAdapter alertAdapter = new AlertThreadAdapter (ap, 1000);
javax.realtime.RealtimeThread alertThread = new javax.realtime.RealtimeThread (alertAdapter);
javax.realtime.RealtimeThread pipelineThread = new javax.realtime.RealtimeThread (ap);
alertThread.start ();pipelineThread.start ();
Real-Time Java Programming
Christopher D. Gill
RTSJ: Threading Issues// To run the pipeline in a Realtime thread, it could just
implement Runnable: for AnalysisPipeline this is not very invasive so we’ll skip writing a separate adapter
public class AnalysisPipeline{ /* ... */ protected void sendAlerts () { synchronized (al) {/* Send all the alerts in the list, reset alert list */} } public void run () { for (;;) { DataFeedEvent dfe = df.pullDataFeedEvent ();
// spawns separate threads for long latency activities cf.handleDataEvent (dfe, al); }}
implements Runnable
Real-Time Java Programming
Christopher D. Gill
RT Issues: Release Characteristics• To know whether threads
will interfere, need to characterize their temporal behavior
Time
execution cost
period
minimum inter-arrival spacing
deadline
• Can abstract out separate descriptors for canonical behavioral classes– I.e., periodic,
aperiodic, sporadic
• Need descriptors with key temporal attributes– E.g., execution cost,
deadline
Real-Time Java Programming
Christopher D. Gill
RTSJ: Release Characteristics Issues• While threading
allows priority partitioning, specific information and/or constraints on threads are needed
• Must ensure sufficient resources are available and correctly managed for desired behavior
javax.realtime.RelativeTime cost = new javax.realtime.RelativeTime (100, 0);
javax.realtime.RelativeTime period = new javax.realtime.RelativeTime (1000, 0);
javax.realtime.PeriodicParameters pp = newjavax.realtime.PeriodicParameters ( null, // start immediately,
period, cost, null, // deadline = period end
null, null);
alertThread.setReleaseParameters (pp);alertThread.start ();
Real-Time Java Programming
Christopher D. Gill
RTSJ: Release Characteristics Issues// Analysis Tool Code, Revisited
public class AlertThreadAdapter implements javax.realtime.Schedulable
{ /* we can & should get/set release parameters, scheduling parameters, memory parameters, ... */
public void run () {addToFeasibility (); javax.realtime.RealtimeThread t = (javax.realtime.RealtimeThread) Thread.currentThread (); for (;;) { t.waitForNextPeriod (); // respect advertised cost, period times pipeline.sendAlerts (); } }}
Real-Time Java Programming
Christopher D. Gill
• Release characteristics advertise how threads are projected to behave
Time
actual execution cost
RT Issues: Release Failures
deadline
execution finished (late)
projected execution cost • However, differences between projected and actual behavior can lead to unexpected failures• Need to be able to detect (and if possible handle) release failures– Cost overruns– Deadline misses
Real-Time Java Programming
Christopher D. Gill
RTSJ: Release Failure Issues• Differences between
projected and expected behavior result in release failures– Execution
overruns– Deadline misses
• Can install a handler for each release characteristics instance to at least record, and possibly correct, failures
public class CostOverrunEventHandler extends javax.realtime.AsyncEventHandler { public void handleAsyncEvent() {/* ... */}}public class DeadlineMissEventHandler extends javax.realtime.AsyncEventHandler { public void handleAsyncEvent() {/* ... */}}
javax.realtime.PeriodicParameters pp = new javax.realtime.PeriodicParameters (null, // start immediately,
period, cost, null, // deadline = period end
new CostOverrunEventHandler (), new DeadlineMissEventHandler ()); alertThread.setReleaseParameters (pp);alertAdapter.setReleaseParameters (pp);alertThread.start ();
Real-Time Java Programming
Christopher D. Gill
RT Issues: Scheduling• Priorities
– Need sufficient unique priority levels
• Preemptive scheduling– Need well defined and
appropriate semantics• Fairness among threads
is not usually a Real-Time concern (FIFO vs. RR)– But may be useful
• Feasibility– Admission control,
certification/testing
sche
dule
r
blocked
runnable
executing
Real-Time Java Programming
Christopher D. Gill
RTSJ: Scheduling Issues• Release
characteristics give control over threads
• Scheduling addresses how to manage those threads
• Priority, preemption
• Feasibility
// Analysis Tool Code, Revisitedjavax.realtime.PriorityScheduler psched = (javax.realtime.PriorityScheduler) javax.realtime.Scheduler.getDefaultScheduler ();javax.realtime.PriorityParameters high = new javax.realtime.PriorityParameters
(psched.getMaxPriority ());javax.realtime.PriorityParameters med = new javax.realtime.PriorityParameters
(psched.getNormPriority ());try { alertThread.setSchedulingParameters (high); pipelineThread. setSchedulingParameters (med);} catch (java.lang.IllegalArgumentException e) {/* ... */}alertThread.start ();pipelineThread.start ();
Real-Time Java Programming
Christopher D. Gill
RTSJ: Scheduling Issues// Analysis Tool Code, Revisitedpublic class StoreThreadAdapter implements javax.realtime.Schedulable{/* ... */ public void run () { javax.realtime.PriorityScheduler psched = (javax.realtime.PriorityScheduler) javax.realtime.Scheduler.getDefaultScheduler (); try { javax.realtime.PriorityParameters pp = new javax.realtime.PriorityParameters (psched.getMinPriority ()); setSchedulingParameters (pp); javax.realtime.RealtimeThread t = (javax.realtime.RealtimeThread) Thread.currentThread (); t.setSchedulingParameters (pp); } catch (java.lang.IllegalArgumentException e) {/* ... */} store.annotateAlert (alert); }}
Real-Time Java Programming
Christopher D. Gill
RT Issues: Synchronization• Risk of unbounded
priority inversions– Canonical high, low,
middle scenario
sync
hron
ized
blo
ck
blocked at guard
running outside block
waiting (blocked) ona condition variable
running inside block
priority key: high lowmiddle
• Priorities can uncover or exacerbate “bad” executions of existing race conditions– Horstmann & Cornell,
”Core Java 2”• Need well defined
thread and locking semantics
Real-Time Java Programming
Christopher D. Gill
RTSJ: Synchronization Issues• Real-time threads at
different priorities share resources
• However, this presents new real-time issues– Priority inversions
• Need additional mechanisms to ensure priority-safe sharing– Monitor Control
• Methods wait and notifyAll still work (avoid notify unless absolutely sure OK)– But, add overhead
• Non-blocking R/W queues: thread glue
// Solution: Monitor Controljavax.realtime.MonitorControl.setMonitorControl (new javax.realtime.PriorityInheritance ());
// Solution: wait-free queuespublic class StoreThreadAdapter implements javax.realtime.Schedulable{ /* ... */private javax.realtime.WaitFreeDequeue dequeue; /* ... */}
Real-Time Java Programming
Christopher D. Gill
RT Issues: Time and Timers• Time resolution needed
– Hours down to nsec
start expire
• Absolute time– Common temporal
reference, e.g., UTC
• Occurrences over time• Absolute clock• Timer mechanisms
– One-shot, periodic
• Relative Time– Since start of thread– Since last period
Real-Time Java Programming
Christopher D. Gill
RTSJ: Time and Timer Issues
• Threads offer a clean programming model
• However, many real-time systems benefit from asynchronous behavior
• Also, pacing is an effective/alternative way to reduce resource contention and improve resource utilization
// A needed solution: watchdog timerpublic class StoreTimeoutHandler extends javax.realtime.AsyncEventHandler{public void handleAsyncEvent() {/* ... */}}public class StoreThreadAdapter implements javax.realtime.Schedulable{ public void run () { // ... set up thread priorities ... long m = 60000; // one minute new javax.realtime.OneShotTimer (new javax.realtime.RelativeTime (m,0), new StoreTimeoutHandler ()); store.annotateAlert (alert); } // ... }
Real-Time Java Programming
Christopher D. Gill
RT Issues: Asynch Event Handling• Threads allow synchronous
programming styles• Sometimes, asynchronous
styles are more appropriate– Real-world timing issues– Decoupling processing
handler event
handlermethod
• Events-and-handlers model provides mechanisms for:– Synchronous –> threads– Asynchronous –> timers– Mixed –> half-synch /
half-asynch pattern
Real-Time Java Programming
Christopher D. Gill
RTSJ: Asynch Event Handling Issues• We saw an earlier
example of a one-shot timer used to determine when a long-running thread had been gone too long
• Could also use a periodic timer to re-implement the high priority alert transmission code
// Another way to implement periodicity
public class TransmitTimeoutHandler
extends javax.realtime.AsyncEventHandler
{public void handleAsyncEvent () {/*...*/}}
new javax.realtime.PeriodicTimer (null, new javax.realtime.RelativeTime (1000, 0), new TransmitTimeoutHandler ());
Real-Time Java Programming
Christopher D. Gill
RT Issues: Memory Management• Bounded allocation times• Managed vs. raw access
– Trade-off in control vs. responsibility
• Memory lifetimes– Program, local scope
• Resource use descriptors
memorymanager
• Application/manager interactions– Priority inversions– Memory contention
• Safety and liveness
Real-Time Java Programming
Christopher D. Gill
RTSJ: Memory Management Issues• Realtime threads get
higher priority than the garbage collector
• However, there is still a possibility of priority inversion– If GC is collecting
the heap, it must reach a “safe” state before RT threads can use the heap
• NoHeapRealtime threads avoid this
// Solution: separate memory areas and// no-heap real-time threads
javax.realtime.MemoryArea ma = new javax.realtime.LTMemory (initSize, maxSize);
javax.realtime.NoHeapRealtimeThread alertThread = new javax.realtime.NoHeapRealtimeThread (sp, // sched params rp, // release params mp, // memory params ma, // memory area pg, // processing group alertAdapter);
Real-Time Java Programming
Christopher D. Gill
RTSJ: Memory Management Issues// Immortal Memory is a Singleton
javax.realtime.MemoryArea im = javax.realtime.ImmortalMemory.instance ();
im.enter (this); // this must be Runnable // allocates memory on // the ImmortalMemory area // until another memory // area is entered, or // the Runnable run () // call exits and enter () // returns
• Scoped memory is key for no-heap real-time threads
• Other kinds of MemoryArea– Immortal
Memory: can improve GC performance
• Physical Memory– Immortal,
scoped, raw– Factory
Real-Time Java Programming
Christopher D. Gill
RT Issues: Asynch Transfer of Control• Want to provide real-
time behavior for long-running synchronous activities (e.g., searches)
• For fault-tolerance, some activities may need to be halted immediately
• However, standard threading and interrupt semantics can produce undefined/deadlock behavior in many common use-cases
• ATC refines semantics
Publisher
Exhaustive Lookup
Shipper
“findanythingrelevant”
“stop and giveme what you havefound so far”
searching
Real-Time Java Programming
Christopher D. Gill
RTSJ: ATC Issues• Even with the one-shot
timer, the long running-thread must be reigned in somehow
• Deprecated Thread stop, suspend calls are unsafe
• ATC defers exception as pending in synchronized methods – avoids problem w/deprecated Thread stop method
// Data Store Query Code, Revisited
public abstract class DataStore
{ /* ... */
public abstract void
annotateAlert (Alert a)
throws javax.realtime.AsynchronouslyInterruptedException;
}
// In timer handling for
// StoreThreadAdapter run ()
t.interrupt ();
Real-Time Java Programming
Christopher D. Gill
RT Issues: Exceptions• Additional special-
purpose exceptions w/ standard semantics for– Memory management– Synchronization– System resource
management
safe scope
unsafe scope“tunnels”
propagates
caught
(re)thrown• Special semantics for
ATC– When to throw (or not)– Deferred propagation
semantics (“exception tunneling”) - safety
– Nesting/replacement
raised
Real-Time Java Programming
Christopher D. Gill
RTSJ: Exceptions Issues• Semantics for AIE are different than others
– deferred in pending state until inside a safe scope, where it will be thrown
• Other new exceptions deal primarily with incompatibilities of memory areas– Trying to assign a reference to scoped memory to
a variable in immortal or heap memory– Setting up a WaitFreeQueue, exception
propagation, etc. in an incompatible memory area– Raw memory allocation errors (offset, size)– Raw memory access errors
Real-Time Java Programming
Christopher D. Gill
RT Issues: System-level Options• Although strict layering is
often desirable, platform-specific issues tend to peek through– E.g., signals, schedulers
• Collecting the system-wide constants, methods, etc. under one or more classes reduces pollution and improves the programming model
• May add points of configurability (I.e., various system-wide managers)
SIGKILLSIGINT
SIGABRT
securitymanager
getManager
setManager
Real-Time Java Programming
Christopher D. Gill
RTSJ: System-level Options Issues• javax.realtime.RealtimeSystem is analogous to
java.lang.System– Gives access to real-time system properties
•E.g., concurrent locks, endian properties– Allows a RealtimeSecurity manager to be set as
the system security manager– Gives access to the current garbage collector
• PosixSignalHandler– Required on platforms that provide POSIX
signals– Thus, can only be used portably among those
implementations
Real-Time Java Programming
Christopher D. Gill
Review: Time Scales
AlertList
MarketOrderAlert OptionAlert
CallAlert
Alert
PutAlert
BuyAlert SellAlert
AnalysisTool
DataFeedEvent
ResearchAnnotation
ResearchStore
AnnotationList
Portfolio
Low MediumLatency: High
AnalysisPipeline
SectorPEFilter PortfolioBalanceFilterCompositeFilter
DataFeed
AnalysisFilter
NasdaqDataFeed
NasdaqAnnotationDataStore
NasdaqStore
Annotation
AnalysisTool
ResearchAnnotation
ResearchStore
Real-Time Java Programming
Christopher D. Gill
Review: Java, RTSJ, Real-Time Issues• Threads (Java, revisited in RTSJ)• Release characteristics & failures• Scheduling• Synchronization (Java, revisited in RTSJ)• Time and timers• Asynchronous event handling• Memory management• Asynchronous transfer of control• Exceptions• System-level options
Real-Time Java Programming
Christopher D. Gill
Review: Java and RTSJAnalysisPipeline
SectorPEFilter PortfolioBalanceFilterCompositeFilter
MarketOrderAlert OptionAlert
CallAlert
DataFeed Alert
PutAlert
BuyAlert SellAlert
AnalysisFilter
AnalysisTool
AnnotationDataFeedEvent
NasdaqDataFeed
NasdaqAnnotation
ResearchAnnotation
DataStore
NasdaqStore ResearchStore
Portfolio
low latency
high latency
medium
latency
AlertList
AnnotationList
Synchronization points:
AlertList
AnnotationList
real-timeperiodicno heap
feasibile
scoped memory
priority inheritance
high priority
over-run handler
duration timer
aynch transfer of control
Real-Time Java Programming
Christopher D. Gill
Concluding Remarks• The RTSJ extends and/or refines existing Java
semantics to address issues of real-time concern– Priority control, memory management, release
parameters, feasibility, …• However, the RTSJ largely stays within the existing
programming model– Some new idioms to master, but much is
preserved– ATC in particular illustrates the trade-offs
• Stay tuned, more evolution is on the horizon– Reference implementations and benchmarking– New specification efforts, e.g., the DRTSJ (JSR 50)
Real-Time Java Programming