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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-4437) Concurrent inserts into table with identity column perform poorly
Date Fri, 06 Nov 2009 11:05:32 GMT

    [ https://issues.apache.org/jira/browse/DERBY-4437?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12774270#action_12774270
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Knut Anders Hatlen commented on DERBY-4437:
-------------------------------------------

I haven't investigated this enough to say if (1) or (2) is the real problem. Since the nested
transaction has to commit before it can release the lock, and a commit may need to wait for
disk I/O operations, it may be that (2) is just a symptom, and the real problem is that all
the insert threads compete for the same row lock.

In my application, I could work around the problem by removing the identity column and instead
maintain a counter in an AtomicInteger that's initialized by a SELECT MAX(id) query on start-up.
This works because the application is one single process with multiple threads, so all threads
have access to the AtomicInteger. If the clients run in different processes, such a workaround
cannot be used, though.

> 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
>
> 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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