[hibernate-dev] Memory consumption
golovnin at gmx.net
Sat May 12 16:48:58 EDT 2012
> 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.
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