[hibernate-dev] Memory consumption

[Andrej Golovnin]

Hi Emmanuel,

> Andrej, I wonder if you could share with us these memory dumps privately so we could also analyze them.

I'm sorry but this is not possible. But I will give my best to find the root cause
of this problem.

Here are my findings so far:

- In the memory dump with Hibernate 4 there are a lot of small
  String objects. The content of these objects starts always with "$PlaceHolder$."
  followed by a column name. This was introduced by HHH-4440.
  Maybe interning of these strings would help. I will try it on monday.

- EntityPersisters contain SQL related strings which are never used.
  For example: AbstractEntityPersister#temporaryIdTableDDL and 
  #temporaryIdTableName. It exists also in Hibernate 3. I know for sure
  that this #temporaryIdTableDDL is never used in our application
  as the database user used by the application has no rights to create
  temporary tables. Hibernate generates always all SQL statements
  regardless whether they are required during the life time of an
  application or not. Because Hibernate 3 shows the same behavior
  in this case, I wouldn't touch it for now. But you should consider
  it as possible improvement for the future.

- We make use of UserTypes. Although the specification of UserType 
  requires custom implementations to be immutable, Hibernate creates
  multiple instances, to be exactly for every occurrence of the Type annotation,
  of the same implementation of UserType instead of reusing already
  created instances. Hibernate 3 and 4 share here the behavior.
  This could be also improved in the future. Personally I would prefer
  to not use the Type annotation at all. Users of Hibernate may
  publish they implementations of UserType by means of the ServiceLoader
  from JDK 6.

- There are a lot of empty ArrayLists, HashMaps and HashSets which
  are empty and have the default initial capacity (e.g. 10 by ArrayLists and 16
  by HashMaps). But those objects exists also in the memory dump
  with Hibernate 3. We may improve it when we change how collections
  are created and filled in Hibernate code. I have done small changes to
  org.hibernate.mapping.SimpleValue (see http://goo.gl/yKx5D). These
  changes reduce the size of Configuration in my case from 16187776 bytes
  to 15161672. The changes may look like an ugly hack and they have
  higher resource costs (CPU, memory) during creation of SessionFactoryImpl.
  But it should help to reduce memory if we use it wherever it makes sense.
  What do you think?

  BTW, what is the purpose of CollectionHelper.EMPTY_LIST?
  Why don't you use Collections.EMPTY_LIST?

That's all for now.

Best regards,
Andrej Golovnin