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From "Knut Anders Hatlen (JIRA)" <j...@apache.org>
Subject [jira] Created: (DERBY-3024) Validation of shared plans hurts scalability
Date Thu, 23 Aug 2007 10:26:30 GMT
Validation of shared plans hurts scalability
--------------------------------------------

                 Key: DERBY-3024
                 URL: https://issues.apache.org/jira/browse/DERBY-3024
             Project: Derby
          Issue Type: Improvement
          Components: Performance, SQL
    Affects Versions: 10.4.0.0
         Environment: Sun Java SE 6, Solaris 10, Sun Fire V880 (8 CPUs)
            Reporter: Knut Anders Hatlen
            Priority: Minor


To investigate whether there was anything in the SQL execution layer that prevented scaling
on a multi-CPU machine, I wrote a multi-threaded test which continuously executed "VALUES
1" using a PreparedStatement. I ran the test on a machine with 8 CPUs and expected the throughput
to be proportional to the number of concurrent clients up to 8 clients (the same as the number
of CPUs). However, the throughput only had a small increase from 1 to 2 clients, and adding
more clients did not increase the throughput. Looking at the test in a profiler, it seems
like the threads are spending a lot of time waiting to enter synchronization blocks in GenericPreparedStatement.upToDate()
and BaseActivation.checkStatementValidity() (both of which are synchronized on the a GenericPreparedStatement
object).

I then changed the test slightly, appending a comment with a unique thread id to the "VALUES
1" statement. That means the threads still did the same work, but each thread got its own
plan (GenericPreparedStatement object) since the statement cache didn't regard the SQL text
strings as identical. When I made that change, the test scaled more or less perfectly up to
8 concurrent threads.

We should try to find a way to make the scalability the same regardless of whether or not
the threads share the same plan.

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