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https://issues.apache.org/jira/browse/DERBY-4437?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13052538#comment-13052538
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Rick Hillegas commented on DERBY-4437:
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With the previous checkin, the behavior of this improvement has changed. Now
Derby no longer leaks unused sequence/identity values--provided that the
database is shutdown in an orderly fashion before the VM exits. However, holes
will still appear in sequences and identity columns if you don't park your
databases before the VM exits.
Is the behavioral change narrow enough now that we think this work can be
backported to the 10.8 branch?
Thanks,
-Rick
> 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, 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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