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From Mike Matrigali <mikem_...@sbcglobal.net>
Subject Re: [jira] Commented: (DERBY-666) Enhance derby.locks.deadlockTrace to print stack traces for all threads involved in a deadlock
Date Thu, 17 Nov 2005 19:55:53 GMT
I tried your repro on a build from the trunk and got a deadlock message
from the embedded case, when I ran it on a clean database. I reran after
that and inconsistently got sometimes deadlock and sometimes timeout - 
note each run will add another uncommitted row to each of the tables in 
the db and thus change the timing.  As you say it seems like the amount
of time waited is the deadlock wait time, not the lock timeout time in
all cases.

This seems like a bug to me.  I know there are some "time" estimates 
used by the lock manager to avoid expensive time request calls in java
for every lock request.  Maybe this estimate is going wrong for some

Maybe Dan may have some advice here?


ps. I guess any bug numbered 666 is destined to cause problems :-)

Bryan Pendleton (JIRA) wrote:
>     [ http://issues.apache.org/jira/browse/DERBY-666?page=comments#action_12357762 ]

> Bryan Pendleton commented on DERBY-666:
> ---------------------------------------
> The strange thing is, when I run the program in Embedded mode, and I get the lock timeout,
the program *is* sensitive to the value of derby.locks.deadlockTimeout! If I set derby.locks.deadlockTimeout
to 5, the program gives the "A lock could not be obtained within the time requested." error
after 5 seconds, rather than after the standard 20 seconds. So it seems that the Embedded
configuration is detecting the problem as a deadlock, fundamentally; it's just reporting it
back out to my program as a lock timeout rather than as a deadlock.
> I think I'd better stop now. I've succeeded in getting myself thoroughly confused :)
>>Enhance derby.locks.deadlockTrace to print stack traces for all threads involved in
a deadlock
>>         Key: DERBY-666
>>         URL: http://issues.apache.org/jira/browse/DERBY-666
>>     Project: Derby
>>        Type: Improvement
>>  Components: Store
>>    Versions:
>>    Reporter: Bryan Pendleton
>>    Priority: Minor
>> Attachments: repro.java
>>I was reading http://www.linux-mag.com/content/view/2134/ (good article, btw!), and
it says:
>>>  The next two properties are needed to diagnose concurrency (locking and deadlock)
>>>     *derby.locks.monitor=true logs all deadlocks that occur in the system.
>>>     *derby.locks.deadlockTrace=true log a stack trace of all threads involved
in lock-related rollbacks.
>>It seems, that, in my environment, the deadlockTrace property does not log a stack
trace of *all* threads involved in the deadlock.
>>Instead, it only logs a stack trace of the *victim* thread involved in the deadlock.
>>I think it would be very useful if the derby.locks.deadlockTrace setting could in
fact log a stack trace of all involved threads.
>>In a posting to derby-dev, Mike Matrigali noted that an earlier implementation of
a similar feature had to be removed because it was too expensive in both time and space, but
he suggested that there might be several possible ways to implement this in an acceptably
efficient manner:
>>>A long time ago there use to be room in each lock to point at a
>>>stack trace for each lock, but that was removed to optimize the size
>>>of the lock data structure which can have many objects outstanding.
>>>And creating and storing the stack for every lock was incredibly slow
>>>and just was not very useful for any very active application.  I think
>>>I was the only one who ever used it.
>>>The plan was sometime to add a per user data structure which could be
>>>filled in when it was about to wait on a lock, which would give most of what is
interesting in a deadlock.
>>>The current deadlockTrace is meant to dump the lock table out to derby.log when
a deadlock is encountered.
>>>I agree getting a dump of all stack traces would be very useful, and
>>>with the later jvm debug interfaces may now be possible - in earlier
>>>JVM's there weren't any java interfaces to do so.  Does anyone have
>>>the code to donate to dump all thread stacks to a buffer?
>>Mike also suggested a manual technique as a workaround; it would be useful to put
this into the documentation somewhere, perhaps on the page which documents derby.locks.deadlockTrace?
Here's Mike's suggestion:
>>>What I do if I can reproduce easily is set try to catch the wait by
>>>hand and then depending on the environment either send the magic
>>>signal or hit ctrl-break in the server window which will send the JVM
>>>specific thread dumps to derby.log.
>>The magic signal, btw, is 'kill -QUIT', at least with Sun JVMs in my experience.

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