[
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.
--
This message is automatically generated by JIRA.
For more information on JIRA, see: http://www.atlassian.com/software/jira
