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From Clinton Begin <clinton.be...@gmail.com>
Subject Re: ibatis 3 memory leak
Date Mon, 18 Jan 2010 16:40:13 GMT
That's exactly what should happen.  Here's the catch:  you should only
depend on lazy loading OR caching... not both.  I would suggest caching.

Try disabling lazy loading and use one sqlSession instance.


On Mon, Jan 18, 2010 at 9:37 AM, Dave Rafkind <dave.rafkind@gmail.com>wrote:

> Unfortunately the data model is very thin (it's a database representation
> of a very large series of XSD schema documents), with "types" that have
> "properties", where each "type" has a "parent", and each "property" has a
> "type" as well (among other things).
> Big objects and N+1 selects don't bother me too much, but since a given
> "type" or "property" may be involved in several relationships I was hoping
> that information about them could be pulled from the cache instead of
> re-selected, during lazy-load joins.
> On Mon, Jan 18, 2010 at 11:10 AM, Clinton Begin <clinton.begin@gmail.com>wrote:
>> Hmm... it looks to me like your situation is a perfect storm.
>>  * You're loading lots of objects.
>>  * You're loading big objects.
>>  * You are guaranteeing an N+1 selects problem with that for loop.
>> No one of these situations should create a major problem, but all three
>> together are a guaranteed disappointment.
>> That said, I'm surprised that 2000 rows would be that much of a problem...
>> how big are these objects really?
>> Also, memory wise, there should be almost no difference between using one
>> session or multiple.  And no, I would not open multiple as in your second
>> example.
>> As for the serialization error, that might be a bug if LoadPair wasn't
>> serializable.
>> Clinton
>> On Mon, Jan 18, 2010 at 8:24 AM, Dave Rafkind <dave.rafkind@gmail.com>wrote:
>>> Ok, that's some good information, I understand that you should marshall
>>> your objects with care. Unfortunately for me at some point I will have to
>>> marshall every single one of my database rows into an object graph. However
>>> what I took from your advice was not to be so stingy in re-using the session
>>> objects. So if I do something like this:
>>> for (id : ids) {
>>>   SqlSession session2 = sessionMapper.openSession();
>>>   MyObject o = session2.select("getOne", id.getActualId());
>>>   session2.close();
>>> }
>>> Now memory usage is pretty good! However now performance is terrible. In
>>> attempting to rectify this I tried to put in a custom cache implementation,
>>> logging cache access to see how it was performing. I noticed it was never
>>> actually using the cache (never doing cache.putObject).
>>> I noticed that according to this discussion:
>>> http://markmail.org/search/?q=select+commit+cache+list%3Aorg.apache.ibatis.user-java#query:select%20commit%20cache%20list%3Aorg.apache.ibatis.user-java+page:1+mid:4mmwki3dnp57gweu+state:results
>>> ...commits don't occur (and thus cache fill doesn't occur) unless a
>>> transaction is committed. So for a pure read-only use case of the db the
>>> cache won't be very useful, right?
>>> Additionally, putting in a "session2.commit()" before the call to close()
>>> causes the following error (which I assume, but don't know is caused by
>>> committing when nothing actually needs to be comitted:)
>>> ### Error committing transaction.  Cause:
>>> org.apache.ibatis.cache.CacheException: Error serializing object.  Cause:
>>> java.io.NotSerializableException:
>>> org.apache.ibatis.executor.loader.ResultLoaderRegistry$LoadPair
>>> On Thu, Jan 14, 2010 at 2:44 PM, Clinton Begin <clinton.begin@gmail.com>wrote:
>>>> By nested results, yes, I mean collections and associations.  And by
>>>> "flattening" I mean avoiding the use of those.
>>>> iBATIS exhibits this behavior, the same way any ORM would, because the
>>>> object instances need to be cached to preserve object identity.  So as
>>>> you're result set is being read through, each unique object is stored.
>>>> iBATIS isn't quite as effcient as something like Hibernate can be with
>>>> circular references though, in that depending on how you map it out, you
>>>> end up with multiple instances of the same data (parent/child relationships
>>>> mapped with resultMap).
>>>> So with iBATIS, the most memory efficient approach is to use nested
>>>> select associations/collections.  But for query performance, nested
>>>> resultMaps are ideal.  I often find I need to use a combination of both to
>>>> get the best optimization.  But if I often don't load lists of complete
>>>> objects either.  If I'm loading a large list, I'll use a lighter weight
>>>> representation, and then only load the complete object graph for individual
>>>> instances.  I've never found a case where this wasn't a good idea anyway.
>>>> Even when working with something like Rails, a rich domain ORM, I would
>>>> often write optimized lightweight queries for large lists of flatter
>>>> objects.
>>>> The memory should not be significantly more than will ultimately be
>>>> required to store your final result set.   And any additional memory used
>>>> should be released upon the closing of the SqlSession.
>>>> If the memory isn't being released at the end of the session, that's a
>>>> different story... but otherwise, this is normal behavior.
>>>> Clinton
>>>> On Thu, Jan 14, 2010 at 11:20 AM, Dave Rafkind <dave.rafkind@gmail.com>wrote:
>>>>> Thanks for the reply. What do you mean by nested result maps or
>>>>> selects? Do you mean collections or associations with their own selects
>>>>> result maps? Why would ibatis exhibit this behavior in that case?
>>>>>  And by flattening, you mean the same kind of stuff used to avoid the
>>>>> n+1 select problem?
>>>>> On Thu, Jan 14, 2010 at 1:02 PM, Clinton Begin <
>>>>> clinton.begin@gmail.com> wrote:
>>>>>> If it uses nested result maps or nested selects, I'm afraid you're
>>>>>> of luck.   You'll need to reduce the query results, or flatten out
>>>>>> results.
>>>>>> Clinton
>>>>>> On 2010-01-14, Dave Rafkind <dave.rafkind@gmail.com> wrote:
>>>>>> > Hi ibatis list, I'm new to ibatis so perhaps this is a noob
>>>>>> question. I'm
>>>>>> > using Ibatis 3 (ibatis-3-core- with a somewhat
>>>>>> complicated
>>>>>> > schema (plenty of circular links etc).
>>>>>> >
>>>>>> > I'm doing something like this:
>>>>>> >
>>>>>> > List<MyIdObject> ids = session.selectList("getAll");
>>>>>> >
>>>>>> > for (id : ids) {
>>>>>> >   MyObject o = session.select("getOne", id.getActualId());
>>>>>> > }
>>>>>> >
>>>>>> > The first query returns a list about 2k big, and the second
query in
>>>>>> the for
>>>>>> > loop returns objects that are somewhat large (have several
>>>>>> collections in
>>>>>> > them, a discriminator, etc).
>>>>>> >
>>>>>> > The problem I have is that as the for loop marches on it uses
>>>>>> > ever-increasing amount of memory. I would assuming that when
>>>>>> objects in
>>>>>> > the body of the for loop go out of scope they can get garbage
>>>>>> collected, but
>>>>>> > apparently that never happens; is there some weird interaction
>>>>>> the
>>>>>> > "first-level cache"? Should I be going about this a different
>>>>>> >
>>>>>> > Thanks!
>>>>>> > Dave
>>>>>> >
>>>>>> --
>>>>>> Sent from my mobile device
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