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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-2877) Print the entire lock list when a deadlock occurs and deadlock tracing is on
Date Mon, 03 Sep 2007 07:49:19 GMT

    [ https://issues.apache.org/jira/browse/DERBY-2877?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12524458
] 

Knut Anders Hatlen commented on DERBY-2877:
-------------------------------------------

I looks to me that the deadlock in John's example is caused by an index split deadlock like
the one reported in DERBY-2991, only that it involves more transaction and therefore is detected
as a deadlock, not as a timeout. There is probably an index split waiting for an X lock on
(2,1), and that's what preventing the delete transaction from obtaining a shared lock. Perhaps
we would get enough debugging information if we printed all waiters for the locks in the cycle.

> Print the entire lock list when a deadlock occurs and deadlock tracing is on
> ----------------------------------------------------------------------------
>
>                 Key: DERBY-2877
>                 URL: https://issues.apache.org/jira/browse/DERBY-2877
>             Project: Derby
>          Issue Type: Improvement
>          Components: Services
>    Affects Versions: 10.2.2.0
>            Reporter: John H. Embretsen
>
> When a deadlock occurs, derby includes the cycle of locks which caused the deadlock in
the SQLException message. This is also printed to derby.log if the properties derby.locks.deadlockTrace
and derby.locks.monitor are set to true, or if debug code is being used (e.g. jars from lib-debug
distributions). It will be easier to debug deadlocks if the entire lock table is printed to
derby.log as well (alternatively to both derby.log and the exception message) in these cases.
An example of such a lock table is available at http://wiki.apache.org/db-derby/LockDebugging.
> For example, in a long-running test I have observed deadlocks with lock cycle messages
such as:
> Lock : ROW, DELETED, (2,1)
>   Waiting XID : {6241401573, S} , U1, DELETE FROM "U1"."DELETED" WHERE CURRENT OF "SQL_CURLH000C9"
>   Granted XID : {6241401662, S} 
> Lock : ROW, DELETED, (3,3523)
>   Waiting XID : {6241401662, U} , U1, SELECT ITEMID FROM DELETED
>   Granted XID : {6241401573, U} 
> . The selected victim is XID : 6241401573.
> It is not clear from this output why XID 6241401573 is waiting for a shared lock (S)
on row (2,1), as an S lock is compatible with other S locks [1]. 
> Having a snapshot of the contents of the lock table at the time of the deadlock would
probably help a great deal in the debugging process. 
> [1]: Lock compatibility: http://db.apache.org/derby/docs/dev/devguide/rdevconcepts2462.html

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