presto-main/src/main/java/io/prestosql/operator/Driver.java/Driver/handleMemoryRevoke
class Driver:    @GuardedBy("exclusiveLock")
    private void handleMemoryRevoke()
    {
        for (int i = 0; i < activeOperators.size() && !driverContext.isDone(); i++) {
            Operator operator = activeOperators.get(i);

            if (revokingOperators.containsKey(operator)) {
                checkOperatorFinishedRevoking(operator);
            }
            else if (operator.getOperatorContext().isMemoryRevokingRequested()) {
                ListenableFuture<?> future = operator.startMemoryRevoke();
                revokingOperators.put(operator, future);
                checkOperatorFinishedRevoking(operator);
            }
        }
    }