Jmeter __javascript() got blocked

One of my Friends are using JMeter 2.3.2 __javascript() func to generate Random number, but after using VisualVM to monitoring Jmeter, we found such a threaddump:

"Thread Group 1-18" prio=6 tid=0x379c3800 nid=0xf3c runnable [0x39f2f000..0x39f2fd18]
java.lang.Thread.State: RUNNABLE
at org.mozilla.javascript.gen.c123605._c0()
at org.mozilla.javascript.gen.c123605.call()
at org.mozilla.javascript.ContextFactory.doTopCall(ContextFactory.java:340)
at org.mozilla.javascript.ScriptRuntime.doTopCall(ScriptRuntime.java:2758)
at org.mozilla.javascript.gen.c123605.call()
at org.mozilla.javascript.gen.c123605.exec()
at org.mozilla.javascript.Context.evaluateString(Context.java:1132)
at org.apache.jmeter.functions.JavaScript.execute(JavaScript.java:94)
- locked <0x0b9991e0> (a org.apache.jmeter.functions.JavaScript)
at org.apache.jmeter.engine.util.CompoundVariable.execute(CompoundVariable.java:138)
at org.apache.jmeter.engine.util.CompoundVariable.execute(CompoundVariable.java:107)
....

"Thread Group 1-17" prio=6 tid=0x390f4c00 nid=0x288 waiting for monitor entry [0x39edf000..0x39edfd98]
java.lang.Thread.State: BLOCKED (on object monitor)
at org.apache.jmeter.functions.JavaScript.execute(JavaScript.java:73)
- waiting to lock <0x0b9994a8> (a org.apache.jmeter.functions.JavaScript)
at org.apache.jmeter.engine.util.CompoundVariable.execute(CompoundVariable.java:138)
at org.apache.jmeter.engine.util.CompoundVariable.execute(CompoundVariable.java:107)
at org.apache.jmeter.testelement.property.FunctionProperty.getStringValue(FunctionProperty.java:87)
at org.apache.jmeter.testelement.property.AbstractProperty.hashCode(AbstractProperty.java:221)
at java.util.HashMap.getEntry(Unknown Source)
at java.util.HashMap.containsKey(Unknown Source)
at java.util.HashSet.contains(Unknown Source)
at org.apache.jmeter.testelement.AbstractTestElement.isTemporary(AbstractTestElement.java:376)
at org.apache.jmeter.testelement.AbstractTestElement.recoverRunningVersion(AbstractTestElement.java:351)
at org.apache.jmeter.threads.JMeterThread.notifyTestListeners(JMeterThread.java:561)
at org.apache.jmeter.threads.JMeterThread.access$200(JMeterThread.java:60)
....

"Thread Group 1-15" prio=6 tid=0x37149000 nid=0x878 waiting for monitor entry [0x39e3f000..0x39e3fa98]
java.lang.Thread.State: BLOCKED (on object monitor)
at org.apache.jmeter.functions.JavaScript.execute(JavaScript.java:73)
- waiting to lock <0x0b998000> (a org.apache.jmeter.functions.JavaScript)
at org.apache.jmeter.engine.util.CompoundVariable.execute(CompoundVariable.java:138)
at org.apache.jmeter.engine.util.CompoundVariable.execute(CompoundVariable.java:107)
....

Here is the source code for your reference :)

public synchronized String execute(SampleResult previousResult, Sampler currentSampler)
throws InvalidVariableException {

JMeterContext jmctx = JMeterContextService.getContext();
JMeterVariables vars = jmctx.getVariables();

String script = ((CompoundVariable) values[0]).execute();
// Allow variable to be omitted
String varName = values.length < 2 ? null : ((CompoundVariable) values[1]).execute().trim();
String resultStr = "";

then you know why...
__javascript() itself got Performance problem, So be Careful to use __javascript when you perform load testing