How to manage the concurrency using JTA(entityManager) so that the application can scale at large number of concurrent users?

I am having a J2EE application that uses JPA as a persistence framework having transaction type JTA. I am using EntityManager annotated with PersistenceContext. My SessionBean looks like this :

package com.session;

import java.util.ArrayList;
import java.util.List;

import javax.annotation.Resource;
import javax.ejb.LocalBean;
import javax.ejb.SessionContext;
import javax.ejb.Stateless;
import javax.ejb.TransactionAttribute;
import javax.ejb.TransactionAttributeType;
import javax.ejb.TransactionManagement;
import javax.ejb.TransactionManagementType;
import javax.persistence.EntityManager;
import javax.persistence.PersistenceContext;
import javax.persistence.Query;
import javax.transaction.HeuristicMixedException;
import javax.transaction.HeuristicRollbackException;
import javax.transaction.NotSupportedException;
import javax.transaction.RollbackException;
import javax.transaction.SystemException;
import javax.transaction.UserTransaction;

import com.beans.Bean_B;

/**
 * Session Bean implementation class Bean_BSession
 */

@Stateless
@LocalBean
public class Bean_BSession implements Bean_BSessionLocal {

        @PersistenceContext(unitName = "bench")
        private EntityManager em;


        /**
         * Default constructor.
         */

        public Bean_BSession() {
                // TODO Auto-generated constructor stub
        }

        @Override
        public List<Bean_B> findAll(int A_Id) {
                // TODO Auto-generated method stub
                List<Bean_B> bList = null;
                        Query q = em
                                        .createQuery("Select b FROM Bean_B b where b.BI_AID = :id");
                        q.setParameter("id", A_Id);

                        bList = (List<Bean_B>) q.getResultList();

                return bList;
        }

        @Override
        public int delete(int BI_ID) {
                // TODO Auto-generated method stub
                        Query q = em
                                        .createQuery("delete from Bean_B b where b.BI_ID = :id");
                        q.setParameter("id", BI_ID);
        
                return q.executeUpdate();
        }
}

When I put it on a test of multiple user I don't find it scaling though I am having max-pol size of the datasource 200. My Persistence.xml looks like :

<persistence xmlns="http://ift.tt/1cKbVbQ"
        xmlns:xsi="http://ift.tt/ra1lAU"
        xsi:schemaLocation="http://ift.tt/1cKbVbQ

http://ift.tt/1kMb4sd"

        version="2.1">

        <persistence-unit name="bench">

                <jta-data-source>jdbc/benchDS</jta-data-source>

                <class>com.beans.Bean_A</class>
                <class>com.beans.bean_B</class>
                <class>com.beans.Bean_C</class>
                <class>com.beans.Bean_D</class>
                <exclude-unlisted-classes>false</exclude-unlisted-classes>
                <properties>
                        
                </properties>
        </persistence-unit>
</persistence>

Let me know what are the changes I need to incorporate. Thanks in advance!