On a multi-processor machine you may wish to evaluate the same expression, or set of expressions, in multiple threads. There are two main ways of evaluation in multiple threads:
ThreadSafeEvaluator
which can use the same expression in multiple threads.ImportationVisitor
to give each thread its own copy of an expression.In both cases each thread will need its own Jep instance. Each Jep instance will have its own Evaluator
and VaraibleTable
but may share other components. The
LightweightComponentSet
is an easy way
to create a Jep instance with minimal memory footprint. The second technique is slightly faster.
ThreadSafeEvaluator
Normally variables are evaluated by using a direct reference from a Node
to a Variable
object.
This would not be thread safe as one thread might change the value of a variable.
With the ThreadSafeEvaluator
each thread has its own VariableTable
and when the evaluator encounters a variable node in an expression
it looks up its name in the VariableTable
,
effectively performing a hashtable lookup and preserving thread independence.
The Jep instance needs to be set up with
// create a Jep instance with the ThreadSafeEvaluator Jep baseJep = new Jep(new ThreadSafeEvaluator()); // use thread-safe versions of the assignment and element-of operators baseJep.getOperatorTable().getAssign().setPFMC(new ThreadSafeAssign()); baseJep.getOperatorTable().getEle().setPFMC(new ThreadSafeEle()); // use thread optimized version of the rand function baseJep.addFunction("rand", new ThreadSafeRandom());
Each child thread would be set up with
// create a child Jep instance Jep childJep = new Jep(new LightWeightComponentSet(baseJep));
A full example is
import com.singularsys.jep.EvaluationException; import com.singularsys.jep.Jep; import com.singularsys.jep.JepException; import com.singularsys.jep.Variable; import com.singularsys.jep.misc.LightWeightComponentSet; import com.singularsys.jep.misc.threadsafeeval.ThreadSafeAssign; import com.singularsys.jep.misc.threadsafeeval.ThreadSafeEle; import com.singularsys.jep.misc.threadsafeeval.ThreadSafeEvaluator; import com.singularsys.jep.parser.Node; public class ThreadRunner { // Setup and run multiple threads using the same expression public void go(String expression, int nThreads) throws JepException { // create a Jep instance with the ThreadSafeEvaluator Jep baseJep = new Jep(new ThreadSafeEvaluator()); // use thread-safe versions of the assignment and element-of operators baseJep.getOperatorTable().getAssign().setPFMC(new ThreadSafeAssign()); baseJep.getOperatorTable().getEle().setPFMC(new ThreadSafeEle()); // Parse a node in the base Jep instance Node baseNode = baseJep.parse(expression); // create a number of threads each with a different value for x EvaluationThread threads[] = new EvaluationThread[nThreads]; for(int i=0; i<nThreads; ++i) { threads[i] = new EvaluationThread(baseJep,baseNode,"x", Math.PI * i / nThreads ); } // run the threads each with a different value for x for(int i=0; i<nThreads; ++i) { threads[i].start(); } // wait for all threads to finish and print results for(int i=0; i<nThreads; ++i) { try { threads[i].join(); } catch (InterruptedException e) { } System.out.println("Thread "+i+" value "+ threads[i].varValue+" result "+threads[i].result); } } // Class to evaluate an expression in a thread class EvaluationThread extends Thread { Jep childJep; Node childNode; Variable childVar; double varValue; double result; // set up the tread before running EvaluationThread(Jep baseJep, Node baseNode, String varName, double value) throws JepException { // create a child Jep instance childJep = new Jep(new LightWeightComponentSet(baseJep)); // just use the baseNode node childNode = baseNode; // child copy of variable childVar = childJep.addVariable(varName); varValue = value; } // Run the thread @Override public void run() { try { // set variable value childVar.setValue(varValue); // Evaluate the expression Object res = childJep.evaluate(childNode); result = ((Double) res); } catch (JepException e) { System.out.println(e.getMessage()); } } threads[i].start(); } }
ImportationVisitor
The ImportationVisitor can import an expression from one jep instance to another
ImportationVisitor iv = new ImportationVisitor(Jep childJep); Node childNode = iv.deepCopy(baseNode);
This makes a copy of the expression changing references from one VariableTable
to another.
The new expression in childNode
can then be evaluated using any evaluator.
The code to use this is very similar to the above, apart from simpler Jep construction, and the line to import the node.
import com.singularsys.jep.EvaluationException; import com.singularsys.jep.Jep; import com.singularsys.jep.JepException; import com.singularsys.jep.Variable; import com.singularsys.jep.misc.LightWeightComponentSet; import com.singularsys.jep.parser.Node; import com.singularsys.jep.walkers.ImportationVisitor; public class ThreadRunner { // Setup and run multiple threads using the same expression public void go(String expression, int nThreads) throws JepException { // create a standard Jep Jep baseJep = new Jep(); // use thread optimized version of the rand function baseJep.addFunction("rand", new ThreadSafeRandom()); // Parse a node in the base Jep instance Node baseNode = baseJep.parse(expression); // create a number of threads each with a different value for x EvaluationThread threads[] = new EvaluationThread[nThreads]; for(int i=0; i<nThreads; ++i) { threads[i] = new EvaluationThread(baseJep,baseNode,"x", Math.PI * i / nThreads); } // run the threads each with a different value for x for(int i=0; i<nThreads; ++i) { threads[i].start(); } // wait for all threads to finish and print results for(int i=0; i<nThreads; ++i) { try { threads[i].join(); } catch (InterruptedException e) { } System.out.println("Thread "+i+" result "+threads[i].result); } } // Class to evaluate an expression in a thread class EvaluationThread extends Thread { Jep childJep; Node childNode; Variable childVar; double varValue; double result; // set up the tread before running EvaluationThread(Jep baseJep, Node baseNode, String varName, double value) throws JepException { // create a child Jep instance childJep = new Jep(new LightWeightComponentSet(baseJep)); // use a child copy of expression childNode = (new ImportationVisitor(childJep)).deepCopy(baseNode); // child copy of variable childVar = childJep.addVariable(varName); varValue = value; } // Run the thread @Override public void run() { try { // set the variable value childVar.setValue(varValue); // Evaluate the expression Object res = childJep.evaluate(childNode); result = ((Double) res); } catch (EvaluationException e) { System.out.println(e.getMessage()); } } } }
This method does not require special versions of the assignment and element of operators.
If the rand()
function is used then performance is improved by using the
ThreadSafeRandom.
A slight variation of the above technique is to use a
SerializableExpression.
This can handle much longer expressions than ImportationVisitor
. To create a child copy of a node use
SerializableExpression se = new SerializableExpression(baseNode); childNode = se.toNode(childJep);
See the Serialization help page for more details.
Creation of new Jep instances can have a considerable memory footprint, a Jep instance with a StandardParser takes about 56kB bytes for a Jep instance with a configurable parser takes about 14kB bytes. Its possible to create a light-weight Jep instance which reuses components from an existing Jep instance, such instances only take 1kB.
Light-weight instances have no parsing facility or printing, and their own copies of the VariableTable and Evaluator so are safe to uses in multiple threads.
Jep j = new Jep(); ComponentSet cs = new LightWeightComponentSet(j); Jep lwj = new Jep(cs);
Note that the new instance will share the FunctionTable and its associated
PostfixMathCommands. See the
LightWeightComponentSet
LightWeightComponentSet
documentation for issues which mary arise and workarounds.
The above code will create copies of all variables. The public LightWeightComponentSet(Jep jep,boolean copyConstants)
constructor
can be used to just copy constants or leave the table empty.
The
JepComponent interface defines a
getLightWeightInstance()
method which is used to create a light weight version of the component
suitable for multiple thread use. It is typical for JepComponents to either return this
, null
as appropriate. Some JepComponents have more complex implementation.
There are two special classes providing do-nothing implementations with minimal footprint. These are both accessed by singleton static fields: NullParser.NULL_PARSER and PrintVisitor.NULL_PRINT_VISITOR.
Two diagnostic applications
com.singularsys.jepexamples.diagnostics.ThreadSafeSpeedTest
com.singularsys.jepexamples.diagnostics.ThreadSpeedTest
are available for testing the two different approaches. The first uses the ThreadSafeEvaluator
and the second
uses the ImportationVisitor
. Both evaluate the same expression with half a million different values and compare the
results when the work is split over multiple threads. Results will depend on the number of processors available and other tasks running on the system.
The com.singularsys.jeptests.system.ThreadTest runs a number of JUnit tests on the system, including the examples in this page.