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Date: Thu, 30 Aug 2007 06:50:30 -0700 (PDT)
From: "Bogdan Calmac (JIRA)" <jira@apache.org>
To: derby-dev@db.apache.org
Subject: [jira] Commented: (DERBY-2991) Index split deadlock
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    [ https://issues.apache.org/jira/browse/DERBY-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12523850 ] 

Bogdan Calmac commented on DERBY-2991:
--------------------------------------

The first thread (http://www.nabble.com/lock-escalation-and-deadlocks-t4197785.html ) is just my wrong interpretation of the cause, so can be discarded.

> Index split deadlock
> --------------------
>
>                 Key: DERBY-2991
>                 URL: https://issues.apache.org/jira/browse/DERBY-2991
>             Project: Derby
>          Issue Type: Bug
>          Components: Store
>    Affects Versions: 10.2.2.0, 10.3.1.4
>         Environment: Windows XP, Java 6
>            Reporter: Bogdan Calmac
>         Attachments: derby.log, InsertSelectDeadlock.java, Repro2991.java, stacktraces_during_deadlock.txt
>
>
> 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 10.2.2.0 and 10.3.1.4 with identical behaviour.
> Thanks,
> Bogdan Calmac.

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