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From "Tomohito Nakayama (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2991) Index split deadlock
Date Tue, 20 Nov 2007 11:48:43 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2991?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12543873
] 

Tomohito Nakayama commented on DERBY-2991:
------------------------------------------

I read current code (and Knuts comment) that shared lock is gotten instead of latch of page
and 
the chance to release the latch are given to escape deadlock through trial for lock
in order to escape deadlock.

If we don't get shared lock any more in program, we can do for escaping deadlock is just release
the latch.

Releasing the latch may results the StoredPage, which also means latch for Page, to be removed.

If the PageCache is not removed, we can use lastRowMove of StoredPage as above.
If the PageCache is removed, there are three cases.
1: Only scanning index operation held latch and PageCache was removed. The row is not moved.
2: Other operation also held latch and move the row and release the latch and PageCache was
removed.
3: Other operation also held latch and does not move the row but store something and release
the latch and PageCache was removed.

Seeing page version of loaded PageCache, we can distinguish 1 from others.
I think it would be not so awful to check rows in page to know 2 or 3.

> 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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