presto-main/src/main/java/io/prestosql/execution/SplitConcurrencyController.java/SplitConcurrencyController/getTargetConcurrency
class SplitConcurrencyController:    public int getTargetConcurrency()
    {
        checkState(targetConcurrency > 0, "Possible deadlock detected. Target concurrency is zero");
        return targetConcurrency;
    }