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From "Bogdan Calmac (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-2991) Index split deadlock
Date Fri, 03 Aug 2007 04:10:52 GMT
Index split deadlock

                 Key: DERBY-2991
                 URL: https://issues.apache.org/jira/browse/DERBY-2991
             Project: Derby
          Issue Type: Bug
          Components: Store
    Affects Versions:
         Environment: Windows XP, Java 6
            Reporter: Bogdan Calmac

After doing dome research on the mailing list, it appears that the index split deadlock is
a known behaviour, so I will start by describing the theoretical problem first and then follow
with the details of my test case.

If you have concurrent select and insert transactions on the same table, the observed locking
behaviour is as follows:
 - the select transaction acquires an S lock on the root block of the index and then waits
for an S lock on some uncommitted row of the insert transaction
 - the insert transaction acquires X locks on the inserted records and if it needs to do an
index split creates a sub-transaction that tries to acquire an X lock on the root block of
the index

In summary: INDEX LOCK followed by ROW LOCK + ROW LOCK followed by INDEX LOCK = deadlock

In the case of my project this is an important issue (lack of concurrency after being forced
to use table level locking) and I would like to contribute to the project and fix this issue
(if possible). I was wondering if someone that knows the code can give me a few pointers on
the implications of this issue:
 - Is this a limitation of the top-down algorithm used?
 - Would fixing it require to use a bottom up algorithm for better concurrency (which is certainly
non trivial)?
 - Trying to break the circular locking above, I would first question why does the select
transaction need to acquire (and hold) a lock on the root block of the index. Would it be
possible to ensure the consistency of the select without locking the index?


The attached test (InsertSelectDeadlock.java) tries to simulate a typical data collection
application, it consists of: 
 - an insert thread that inserts records in batch 
 - a select thread that 'processes' the records inserted by the other thread: 'select * from
table where id > ?' 

The derby log provides detail about the deadlock trace and stacktraces_during_deadlock.txt
shows that the inser thread is doing an index split.

The test was run on and with identical behaviour.


Bogdan Calmac.

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