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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] [Updated] (DERBY-4437) Concurrent inserts into table with identity column perform poorly
Date Wed, 15 Jun 2011 16:13:47 GMT

     [ https://issues.apache.org/jira/browse/DERBY-4437?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Knut Anders Hatlen updated DERBY-4437:
--------------------------------------

    Attachment: D4437PerfTest.java
                insertperf.png

Thanks for working on this issue, Rick. I haven't looked at the code yet, but I wrote a small
performance test (see the attached Java class D4437PerfTest.java) and ran an experiment on
a Sun Fire T2000 machine with 32 cores.

The test runs multi-threaded inserts, each thread has its own table to avoid lock/latch conflicts.
I just now realized that one table per thread is probably not the ideal test, since the problem
reported here actually was lock contention... I'll update the test and rerun it, but I'm posting
the results from this first run anyways (see the graph in insertperf.png), as the results
are quite interesting. Even in this test with no contention, head of trunk is able to insert
rows almost twice as fast as 10.8.1.2 when the table has an identity column. Presumably this
is because we don't need to access the system tables so often?

> 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, derby-4437-01-aj-allTestsPass.diff, derby-4437-02-ac-alterTable-bulkImport-deferredInsert.diff,
derby-4437-03-aa-upgradeTest.diff, insertperf.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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