Spring, JPA (Hibernate) & Ehcache Poor Performance

Question

I have what I would assume is a pretty standard setup...spring, jpa (hibernate) and I'm trying to add ehcache.

I have it all configured and verified that it's working...I'm logging the hibernate sql statements and cache statements and can see that the cache is getting invoked and that once the object is in the cache, when I try to load it again, the sql statement is not written out (ie, it's not hitting the database).

However, the performance is terrible...it's pretty much the same performance if I get the object from the cache or from the db. I suspect that perhaps the problem is to do with spring automatically flushing the hibernate session, or maybe the overhead is due to transactions (which I have tried to turn off for that method), or maybe my connection pooling using c3p0 isn't working well enough. At any rate, I'm at a bit of a loss..

I'll post all the relevant information that I have and hopefully some genius here can help me out otherwise I'm stuck.

Firstly, my spring config:

<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:tx="http://www.springframework.org/schema/tx"
xmlns:hprof="http://www.nhprof.com/schema/hprof"
xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.0.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-2.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.0.xsd
http://www.nhprof.com/schema/hprof http://www.nhprof.com/schema/hprof/hprof.xsd">


<bean class="org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor" />

<bean id="systemPropertyManager" class="com.service.impl.SystemPropertyManagerImpl" />
<bean id="entityManagerFactory" class="org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean">
     <property name="dataSource" ref="mysql" /> 
    <property name="jpaVendorAdapter">
        <bean
            class="org.springframework.orm.jpa.vendor.HibernateJpaVendorAdapter">
            <property name="database" value="MYSQL" />
            <property name="showSql" value="true" />
        </bean>
    </property>
    <property name="jpaProperties">
        <props>

           <prop key="hibernate.dialect">org.hibernate.dialect.MySQLDialect</prop>
           <!-- always puts logging out to the console...we want it in the log file -->
           <prop key="hibernate.connection.show_sql">false</prop>
           <prop key="hibernate.hbm2ddl.auto">update</prop>
       <prop key="hibernate.cache.provider_class">net.sf.ehcache.hibernate.SingletonEhCacheProvider</prop>
       <prop key="net.sf.ehcache.configurationResourceName">ehcache.xml</prop>
           <prop key="hibernate.cache.use_second_level_cache">true</prop>
           <prop key="hibernate.cache.use_structured_entries">true</prop>
           <prop key="hibernate.cache.use_query_cache">true</prop>
           <prop key="hibernate.generate_statistics">true</prop>
           <prop key="hibernate.default_batch_fetch_size">500</prop>
           <prop key="hibernate.max_fetch_depth">5</prop>
           <prop key="hibernate.jdbc.batch_size">1000</prop>
           <prop key="hibernate.use_outer_join">true</prop>
        </props>
    </property> 
 <!--   <hprof:profiler /> -->
</bean>


 <bean id="mysql" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
  <property name="driverClass" value="com.mysql.jdbc.Driver" />
    <property name="jdbcUrl" value="jdbc:mysql://localhost/daily?relaxAutoCommit=true&amp;useUnicode=true&amp;characterEncoding=UTF-8&amp;jdbcCompliantTruncation=false&amp;emulateLocators=true" />
    <property name="user" value="root" />
    <property name="password" value="" />
    <property name="initialPoolSize"><value>5</value></property>
    <property name="minPoolSize"><value>5</value></property>
    <property name="maxPoolSize"><value>50</value></property>
    <property name="idleConnectionTestPeriod"><value>200</value></property>
    <property name="acquireIncrement"><value>3</value></property>
    <property name="numHelperThreads"><value>3</value></property>
</bean>

<bean id="transactionManager" class="org.springframework.orm.jpa.JpaTransactionManager">
    <property name="entityManagerFactory" ref="entityManagerFactory" />
</bean>

<tx:annotation-driven transaction-manager="transactionManager" />

My ehcache file is as follows

<ehcache>
<diskStore path="c:/cache"/>

    <defaultCache eternal="false" 
            overflowToDisk="false"
            maxElementsInMemory="50000" 
            timeToIdleSeconds="30" 
            timeToLiveSeconds="6000"
            memoryStoreEvictionPolicy="LRU"
            />
    <cache name="com.model.SystemProperty"
            maxElementsInMemory="5000"
            eternal="true"
            overflowToDisk="false"
            memoryStoreEvictionPolicy="LFU" />              

My cached object which is annotated in order to invoke the cache is as follows:

package com.model;

import javax.persistence.*;

import org.hibernate.annotations.Cache;
import org.hibernate.annotations.CacheConcurrencyStrategy;

@Entity
@Cache(usage = CacheConcurrencyStrategy.READ_ONLY, region="com.model.SystemProperty", include="non-lazy")
@EntityListeners(com.util.GenericEntityLogger.class)
public class SystemProperty extends BaseObject{

    private String name;
    private String value;

    // default constructor
    public SystemProperty(){

    }
    public SystemProperty(String name, String value){
        this.name = name;
        this.value = value;
    }

    @Id
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    public String getValue() {
        return value;
    }
    public void setValue(String value) {
        this.value = value;
    }

    @Override
    public boolean equals(Object o) {
        // TODO Auto-generated method stub
        return false;
    }
    @Override
    public int hashCode() {
        // TODO Auto-generated method stub
        return 0;
    }
    @Override
    public String toString() {
        // TODO Auto-generated method stub
        return null;
    }


}

and my implementation of my interface used to save and get the SystemProperty object with the spring @Transactional method is as follows:

package com.service.impl;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.springframework.transaction.annotation.Transactional;

import com.model.SystemProperty;
import com.service.SystemPropertyManager;


public class SystemPropertyManagerImpl extends BaseManagerImpl implements SystemPropertyManager {

    // logging
    protected final Log log = LogFactory.getLog(getClass());

    public SystemProperty find(String name){
        return (SystemProperty) super.entityManager.find(SystemProperty.class, name);
    }

    @Transactional
    public SystemProperty save(SystemProperty prop){
        return (SystemProperty) super.save(prop);
    }

}

Note that @Transactional is only on the save method. Finally, I have populated the mysql database with 3000 system property objects and then have written a little test file, that loads them up twice.

The first time it loads them, I can see the sql being generated and that the cache is not getting hit. The second time, the cache does get hit and the sql isn't generated.

Unit Test:

    @Test
public void testGetCachedUser1(){
    log.debug("starting testGetCachedUser");
    SystemPropertyManager mgr = ManagerFactory.getSystemPropertyManager();
    EntityManager em = mgr.getEntityManager();

    log.debug("start timing 1");
    for(int i = 0; i<3000; i ++){
        mgr.find("name_"+i);
    }
    log.debug("end timing 1");
    log.debug("start timing 2");
    for(int i = 0; i<3000; i ++){
        mgr.find("name_"+i);
    }
    log.debug("end timing 2");
}   

The log file has lots of info. I'll post the section from the log file in the "start timing 1" area. This corresponds to what happens only for the very first object that is loaded when it's not in the cache:

DEBUG 2010-10-22 11:57:49,533 [main][] org.springframework.transaction.annotation.AnnotationTransactionAttributeSource - Adding transactional method [find] with attribute [PROPAGATION_REQUIRED,ISOLATION_DEFAULT]
DEBUG 2010-10-22 11:57:49,533 [main][] org.hibernate.impl.SessionImpl - opened session at timestamp: 5274603804807168
DEBUG 2010-10-22 11:57:49,533 [main][] org.hibernate.transaction.JDBCTransaction - begin
DEBUG 2010-10-22 11:57:49,533 [main][] org.hibernate.jdbc.ConnectionManager - opening JDBC connection
DEBUG 2010-10-22 11:57:49,533 [main][] com.mchange.v2.resourcepool.BasicResourcePool - trace com.mchange.v2.resourcepool.BasicResourcePool@4e2f0a [managed: 5, unused: 4, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@1631573)
DEBUG 2010-10-22 11:57:49,533 [main][] org.hibernate.transaction.JDBCTransaction - current autocommit status: true
DEBUG 2010-10-22 11:57:49,533 [main][] org.hibernate.transaction.JDBCTransaction - disabling autocommit
DEBUG 2010-10-22 11:57:49,533 [main][] org.hibernate.jdbc.JDBCContext - after transaction begin
DEBUG 2010-10-22 11:57:49,549 [main][] org.springframework.transaction.support.TransactionSynchronizationManager - Bound value [org.springframework.jdbc.datasource.ConnectionHolder@5fbbf3] for key [com.mchange.v2.c3p0.ComboPooledDataSource [ acquireIncrement -> 3, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, dataSourceName -> 31ghzi8b1mphf1c19l14qo|149105b, debugUnreturnedConnectionStackTraces -> false, description -> null, driverClass -> com.p6spy.engine.spy.P6SpyDriver, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 31ghzi8b1mphf1c19l14qo|149105b, idleConnectionTestPeriod -> 200, initialPoolSize -> 5, jdbcUrl -> jdbc:mysql://localhost/daily?relaxAutoCommit=true&useUnicode=true&characterEncoding=UTF-8&jdbcCompliantTruncation=false&emulateLocators=true, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 0, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 50, maxStatements -> 0, maxStatementsPerConnection -> 0, minPoolSize -> 5, numHelperThreads -> 3, numThreadsAwaitingCheckoutDefaultUser -> 0, preferredTestQuery -> null, properties -> {user=******, password=******}, propertyCycle -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false ]] to thread [main]
DEBUG 2010-10-22 11:57:49,549 [main][] org.springframework.transaction.support.TransactionSynchronizationManager - Bound value [org.springframework.orm.jpa.EntityManagerHolder@1bb03ee] for key [org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean@11b99c4] to thread [main]
DEBUG 2010-10-22 11:57:49,549 [main][] org.springframework.transaction.support.TransactionSynchronizationManager - Initializing transaction synchronization
DEBUG 2010-10-22 11:57:49,549 [main][] org.springframework.transaction.interceptor.TransactionInterceptor - Getting transaction for [com.service.SystemPropertyManager.find]
DEBUG 2010-10-22 11:57:49,549 [main][] org.springframework.transaction.support.TransactionSynchronizationManager - Retrieved value [org.springframework.orm.jpa.EntityManagerHolder@1bb03ee] for key [org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean@11b99c4] bound to thread [main]
DEBUG 2010-10-22 11:57:49,549 [main][] org.hibernate.event.def.DefaultLoadEventListener - loading entity: [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,549 [main][] org.hibernate.event.def.DefaultLoadEventListener - attempting to resolve: [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,549 [main][] org.hibernate.event.def.DefaultLoadEventListener - object not resolved in any cache: [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,549 [main][] org.hibernate.persister.entity.AbstractEntityPersister - Fetching entity: [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,549 [main][] org.hibernate.loader.Loader - loading entity: [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,549 [main][] org.hibernate.jdbc.AbstractBatcher - about to open PreparedStatement (open PreparedStatements: 0, globally: 0)
DEBUG 2010-10-22 11:57:49,549 [main][] org.hibernate.SQL - select systemprop0_.name as name11_0_, systemprop0_.value as value11_0_ from SystemProperty systemprop0_ where systemprop0_.name=?
DEBUG 2010-10-22 11:57:49,549 [main][] org.hibernate.jdbc.AbstractBatcher - preparing statement
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.type.StringType - binding 'name_0' to parameter: 1
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.jdbc.AbstractBatcher - about to open ResultSet (open ResultSets: 0, globally: 0)
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.loader.Loader - processing result set
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.loader.Loader - result set row: 0
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.loader.Loader - result row: EntityKey[com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.loader.Loader - Initializing object from ResultSet: [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.persister.entity.AbstractEntityPersister - Hydrating entity: [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.type.StringType - returning 'value_0' as column: value11_0_
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.loader.Loader - done processing result set (1 rows)
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.jdbc.AbstractBatcher - about to close ResultSet (open ResultSets: 1, globally: 1)
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.jdbc.AbstractBatcher - about to close PreparedStatement (open PreparedStatements: 1, globally: 1)
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.jdbc.AbstractBatcher - closing statement
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.loader.Loader - total objects hydrated: 1
DEBUG 2010-10-22 11:57:49,580 [main][] org.hibernate.engine.TwoPhaseLoad - resolving associations for [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.engine.TwoPhaseLoad - adding entity to second-level cache: [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.cache.ReadOnlyCache - Caching: com.model.SystemProperty#name_0
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.engine.TwoPhaseLoad - done materializing entity [com.model.SystemProperty#name_0]
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.engine.StatefulPersistenceContext - initializing non-lazy collections
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.loader.Loader - done entity load
DEBUG 2010-10-22 11:57:49,596 [main][] org.springframework.transaction.interceptor.TransactionInterceptor - Completing transaction for [com.service.SystemPropertyManager.find]
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.transaction.JDBCTransaction - commit
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.impl.SessionImpl - automatically flushing session
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - flushing session
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - processing flush-time cascades
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.engine.Cascade - processing cascade ACTION_PERSIST_ON_FLUSH for: com.model.SystemProperty
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.engine.Cascade - done processing cascade ACTION_PERSIST_ON_FLUSH for: com.model.SystemProperty
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - dirty checking collections
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - Flushing entities and processing referenced collections
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - Processing unreferenced collections
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - Scheduling collection removes/(re)creates/updates
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - Flushed: 0 insertions, 0 updates, 0 deletions to 1 objects
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - Flushed: 0 (re)creations, 0 updates, 0 removals to 0 collections
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.pretty.Printer - listing entities:
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.pretty.Printer - com.model.SystemProperty{name=name_0, value=value_0}
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - executing flush
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.jdbc.ConnectionManager - registering flush begin
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.jdbc.ConnectionManager - registering flush end
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.event.def.AbstractFlushingEventListener - post flush
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.jdbc.JDBCContext - before transaction completion
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.impl.SessionImpl - before transaction completion
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.transaction.JDBCTransaction - re-enabling autocommit
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.transaction.JDBCTransaction - committed JDBC Connection
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.jdbc.JDBCContext - after transaction completion
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.jdbc.ConnectionManager - aggressively releasing JDBC connection
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.jdbc.ConnectionManager - releasing JDBC connection [ (open PreparedStatements: 0, globally: 0) (open ResultSets: 0, globally: 0)]
DEBUG 2010-10-22 11:57:49,596 [main][] com.mchange.v2.resourcepool.BasicResourcePool - trace com.mchange.v2.resourcepool.BasicResourcePool@4e2f0a [managed: 5, unused: 4, excluded: 0] (e.g. com.mchange.v2.c3p0.impl.NewPooledConnection@1631573)
DEBUG 2010-10-22 11:57:49,596 [main][] org.hibernate.impl.SessionImpl - after transaction completion
DEBUG 2010-10-22 11:57:49,596 [main][] org.springframework.transaction.support.TransactionSynchronizationManager - Clearing transaction synchronization
DEBUG 2010-10-22 11:57:49,596 [main][] org.springframework.transaction.support.TransactionSynchronizationManager - Removed value [org.springframework.orm.jpa.EntityManagerHolder@1bb03ee] for key [org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean@11b99c4] from thread [main]
DEBUG 2010-10-22 11:57:49,612 [main][] org.springframework.transaction.support.TransactionSynchronizationManager - Removed value [org.springframework.jdbc.datasource.ConnectionHolder@5fbbf3] for key [com.mchange.v2.c3p0.ComboPooledDataSource [ acquireIncrement -> 3, acquireRetryAttempts -> 30, acquireRetryDelay -> 1000, autoCommitOnClose -> false, automaticTestTable -> null, breakAfterAcquireFailure -> false, checkoutTimeout -> 0, connectionCustomizerClassName -> null, connectionTesterClassName -> com.mchange.v2.c3p0.impl.DefaultConnectionTester, dataSourceName -> 31ghzi8b1mphf1c19l14qo|149105b, debugUnreturnedConnectionStackTraces -> false, description -> null, driverClass -> com.p6spy.engine.spy.P6SpyDriver, factoryClassLocation -> null, forceIgnoreUnresolvedTransactions -> false, identityToken -> 31ghzi8b1mphf1c19l14qo|149105b, idleConnectionTestPeriod -> 200, initialPoolSize -> 5, jdbcUrl -> jdbc:mysql://localhost/daily?relaxAutoCommit=true&useUnicode=true&characterEncoding=UTF-8&jdbcCompliantTruncation=false&emulateLocators=true, maxAdministrativeTaskTime -> 0, maxConnectionAge -> 0, maxIdleTime -> 0, maxIdleTimeExcessConnections -> 0, maxPoolSize -> 50, maxStatements -> 0, maxStatementsPerConnection -> 0, minPoolSize -> 5, numHelperThreads -> 3, numThreadsAwaitingCheckoutDefaultUser -> 0, preferredTestQuery -> null, properties -> {user=******, password=******}, propertyCycle -> 0, testConnectionOnCheckin -> false, testConnectionOnCheckout -> false, unreturnedConnectionTimeout -> 0, usesTraditionalReflectiveProxies -> false ]] from thread [main]
DEBUG 2010-10-22 11:57:49,612 [main][] org.hibernate.impl.SessionImpl - closing session
DEBUG 2010-10-22 11:57:49,612 [main][] org.hibernate.jdbc.ConnectionManager - connection already null in cleanup : no action

You can see that it's getting loaded into the cache, the hibernate session is opened and closed and flushed.

I'll post my second log file in a comment below as I've hit the max length already of this question. Main point is, it all looks ok, the only real problem is that it's sooo damn slow.

When I load 3000 objects the first time (ie, hitting the db) it takes pretty much exactly the same as teh second time (ie, hitting the cache). Like I said, is it cos the overhead is opening and closing the connection, flushing the session, dealing with transactions? I'm not sure. But it's pretty brutal at any rate. Help me stackoverflow...you're my only hope :)

Answer 1

I would suggest trying to replace that 2nd level cache usage with plain applicative cache. When I tested Hibernate's 2nd level cache performance in the past, I noticed it wasn't very effective. This is due to the way the cached data is structured - similar to a result set - that is tabular data is being store, and not objects. This means many many cache requests, which turns out to be expensive.

In order to separate the cache code from the persistence logic, I use AOP. Since you use Spring this should be easy.

Answer 2

Main point is, it all looks ok, the only real problem is that it's sooo damn slow.

Damn slow is a subjective measurement unit :)

When I load 3000 objects the first time (ie, hitting the db) it takes pretty much exactly the same as the second time (ie, hitting the cache).

Show metrics, without all the logging overhead (and also without P6Spy), using System.nanoTime()

long startTime = System.nanoTime();
// ... the code being measured ...
long estimatedTime = System.nanoTime() - startTime;

Maybe also try to perform your test inside a single transaction, using a single EntityManager, just to see how things behave in these conditions.

Like I said, is it cos the overhead is opening and closing the connection, flushing the session, dealing with transactions? I'm not sure. But it's pretty brutal at any rate.

Can't say, you should profile your code to see where the time is spent (and also monitor the GC activity).

But in theory:

• The connection in a Session is lazy (will get acquired only when required)
• Acquiring a connection from C3P0 should be pretty fast (negligible)
• Since your entities are cached as read-only, the flush should be pretty fast

In other words, I wouldn't expect the above operations to be expensive (so I'd focus on the surrounding parts). But as I said, profile (and monitor the GC activity).