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From "Mike Matrigali (JIRA)" <j...@apache.org>
Subject [jira] Updated: (DERBY-2991) Index split deadlock
Date Wed, 04 Feb 2009 19:29:59 GMT

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

Mike Matrigali updated DERBY-2991:
----------------------------------


I believe we should go forward with this change in the trunk.  I also am ok with the tested

performance trade off's.  I don't think we should backport a change of this magnitude, I think
it is appropriate for a new feature release, hopefully 10.5.  I especially like that after
this change
the code is simpler, the scan locking stuff was complicated and it is great that it can be
removed
with performance sometimes increased and mostly not too affected.

Once the change goes in there may be more work possible to improve performance, but I think
it is fine to get the basic stuff in now.  One thing that comes to mind is to change the default
group
fetch size from a fixed size to a page worth.  That had always been a future direction and
I think the
interfaces are there (ie. allow store to set the size of the fetch, and passing variable size
groups back
to caller).  This seems even more important as once the latch is given up repositioning costs
may
be higher than before.  

Knut, I would be happy to review the entire package one more time, but would rather wait until
you do
the cleanup you mention.  Just post a comment when you are ready.

> 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
>
>
> 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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