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From "Kathey Marsden (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (DERBY-4437) Concurrent inserts into table with identity column perform poorly
Date Thu, 23 Jun 2011 17:51:47 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4437?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13054003#comment-13054003
] 

Kathey Marsden commented on DERBY-4437:
---------------------------------------

 In general I don't think people shut down on exiting the vm (although I wish they would).
 So what we are offering is change in behavior but a workaround for it which at the same time
encourages good Derby practices. That is tempting but I have heard quite  a few embedded products
say that they do not have control of the vm at exit time, so can't shutdown.  Also I think
we need to consider surprises that comes up on upgrade in the field we need a non-code solution,
so I think we should backport and leave it on by default,  but add a property to turn it off.
The more conservative  option might be to have it it off by default on 10.8, but I think the
fix is  valuable enough to keep it on and take the risk.



> Concurrent inserts into table with identity column perform poorly
> -----------------------------------------------------------------
>
>                 Key: DERBY-4437
>                 URL: https://issues.apache.org/jira/browse/DERBY-4437
>             Project: Derby
>          Issue Type: Improvement
>          Components: SQL
>    Affects Versions: 10.5.3.0
>            Reporter: Knut Anders Hatlen
>            Assignee: Rick Hillegas
>         Attachments: D4437PerfTest.java, D4437PerfTest2.java, derby-4437-01-aj-allTestsPass.diff,
derby-4437-02-ac-alterTable-bulkImport-deferredInsert.diff, derby-4437-03-aa-upgradeTest.diff,
derby-4437-04-aa-reclaimUnusedValuesOnShutdown.diff, derby-4437-05-aa-pluggablePreallocation.diff,
insertperf.png, insertperf2.png, prealloc.png
>
>
> I have a multi-threaded application which is very insert-intensive. I've noticed that
it sometimes can come into a state where it slows down considerably and basically becomes
single-threaded. This is especially harmful on modern multi-core machines since most of the
available resources are left idle.
> The problematic tables contain identity columns, and here's my understanding of what
happens:
> 1) Identity columns are generated from a counter that's stored in a row in SYS.SYSCOLUMNS.
During normal operation, the counter is maintained in a nested transaction within the transaction
that performs the insert. This allows the nested transaction to commit the changes to SYS.SYSCOLUMN
separately from the main transaction, and the exclusive lock that it needs to obtain on the
row holding the counter, can be releases after a relatively short time. Concurrent transactions
can therefore insert into the same table at the same time, without needing to wait for the
others to commit or abort.
> 2) However, if the nested transaction cannot lock the row in SYS.SYSCOLUMNS immediately,
it will give up and retry the operation in the main transaction. This prevents self-deadlocks
in the case where the main transaction already owns a lock on SYS.SYSCOLUMNS. Unfortunately,
this also increases the time the row is locked, since the exclusive lock cannot be released
until the main transaction commits. So as soon as there is one lock collision, the waiting
transaction changes to a locking mode that increases the chances of others having to wait,
which seems to result in all insert threads having to obtain the SYSCOLUMNS locks in the main
transaction. The end result is that only one of the insert threads can execute at any given
time as long as the application is in this state.

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