We are using Drools compiler to compile DRL and create KieSession. Internally, when we try to compile large number of Spreadsheet (e.g. say 500), each containing about 30 rules, we get Out of memory error as shown in logs below:

java.lang.OutOfMemoryError: unable to create native thread: possibly out of memory or process/resource limits reached

at java.base/java.lang.Thread.start0(Native Method)
at java.base/java.lang.Thread.start(Thread.java:803)
at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.compileRulesLevel(KnowledgeBuilderImpl.java:1155)
at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.compileRules(KnowledgeBuilderImpl.java:1135)
at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.compileKnowledgePackages(KnowledgeBuilderImpl.java:973)
at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.buildRules(KnowledgeBuilderImpl.java:2554)
at org.drools.compiler.builder.impl.KnowledgeBuilderImpl.buildPackages(KnowledgeBuilderImpl.java:2489)
at org.drools.compiler.builder.impl.CompositeKnowledgeBuilderImpl.buildPackages(CompositeKnowledgeBuilderImpl.java:113)
at org.drools.compiler.builder.impl.CompositeKnowledgeBuilderImpl.build(CompositeKnowledgeBuilderImpl.java:103)
at org.drools.compiler.kie.builder.impl.AbstractKieProject.buildKnowledgePackages(AbstractKieProject.java:251)
at org.drools.compiler.kie.builder.impl.AbstractKieProject.verify(AbstractKieProject.java:74)
at org.drools.compiler.kie.builder.impl.KieBuilderImpl.buildKieProject(KieBuilderImpl.java:267)
at org.drools.compiler.kie.builder.impl.KieBuilderImpl.buildAll(KieBuilderImpl.java:235)
at org.drools.compiler.kie.builder.impl.KieBuilderImpl.buildAll(KieBuilderImpl.java:184)
at org.kie.internal.utils.KieHelper.verify(KieHelper.java:94)
at abc.xyz.KnowledgeBaseCreator.createKnowledgeBaseFromDrl(KnowledgeBaseCreator.java:319)

This issue can be traced to KnowledgeBuilderImpl.java as shown below:

Map<String, RuleBuildContext> ruleCxts = new ConcurrentHashMap<>();
ForkJoinPool pool = new ForkJoinPool(); // avoid common pool
try {
pool.submit(() ->
rules.stream().parallel()
.filter(ruleDescr -> filterAccepts(ResourceChange.Type.RULE, ruleDescr.getNamespace(), ruleDescr.getName()))
.forEach(ruleDescr -> {
initRuleDescr(packageDescr, pkgRegistry, ruleDescr);
RuleBuildContext context = buildRuleBuilderContext(pkgRegistry, ruleDescr);
ruleCxts.put(ruleDescr.getName(), context);
List<? extends KnowledgeBuilderResult> results = addRule(context);
if (!results.isEmpty()) {
synchronized (this.results)

{ this.results.addAll(results); }

}
})
).get();
} catch (InterruptedException | ExecutionException e)

{ throw new RuntimeException("Rules compilation failed or interrupted", e); }

for (RuleDescr ruleDescr : rules) {
RuleBuildContext context = ruleCxts.get(ruleDescr.getName());
if (context != null)

{ pkgRegistry.getPackage().addRule(context.getRule()); }

}

Here ForkJoinPool is created without specifying any parallelism limit as well as without shutting it down. Moreover, since its avoiding common pool, anyone who is using it in a loop will face the issue as discussed above.