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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2991) Index split deadlock
Date Tue, 24 Feb 2009 13:28:01 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12676266#action_12676266
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Knut Anders Hatlen commented on DERBY-2991:
-------------------------------------------

I've been trying to write test cases for the rest of the calls to BTreeScan.reposition(),
but I haven't managed to come up with any sensible test cases for IndexSplitDeadlockTest.
Some of the calls appear to be unreachable unless we use the internal API. Others are reachable,
but in order to test them better than they're already tested by other tests (for instance
testing that they work if there's been a split right before the repositioning) I think we'll
need to write tests against the internal API for those calls as well. I'll defer writing tests
against the internal API for now (of course, suggestions of how to test them via the public
API are welcome) and instead add more test cases for the places we are saving the position.
I added a comment to IndexSplitDeadlockTest describing which calls to reposition() that are
not explicitly tested (revision 747371).

> 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
>            Assignee: Knut Anders Hatlen
>         Attachments: d2991-preview-1a.diff, d2991-preview-1a.stat, d2991-preview-1b.diff,
d2991-preview-1b.stat, d2991-preview-1c.diff, d2991-preview-1c.stat, d2991-preview-1d.diff,
d2991-preview-1d.stat, d2991-preview-1e.diff, derby.log, InsertSelectDeadlock.java, perftest.diff,
Repro2991.java, stacktraces_during_deadlock.txt, test-1.diff, test-2.diff, test-3.diff
>
>
> 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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