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From "Daniel John Debrunner (JIRA)" <j...@apache.org>
Subject [jira] Commented: (DERBY-3024) Validation of shared plans hurts scalability
Date Sat, 27 Oct 2007 00:07:51 GMT

    [ https://issues.apache.org/jira/browse/DERBY-3024?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12538150

Daniel John Debrunner commented on DERBY-3024:

GPS#getActivation & GPS#finish will not be called per execution (except when using a Statement).

The upToDate() check interacts with the table locking of any DDL that lead to the invalidation.

When a table T  is modified via DDL there is an exclusive lock held on T.
This lock is obtained and then plans dependent on that table are modified.

Thus if a statement has obtained an intent lock on T and it is valid (upToDate()) then it
can complete its execution knowing that no DDL can proceed and invalidate it since it holds
an intent table lock that will block any DDL's exclusive lock.

So ideally a plan will check that it's up to date once all of its table locks are obtained,
in Derby this is not centralized. Some DBMS's as part of their compilation setup a list of
table intent locks and obtain them at the start of execution. In Derby this is handled by
calling checkStatementValdity() in *each* open of a ResultSet (possibly regardless of it it
obtains a table lock or not).

Ideally this would be in one place, maybe after the open of the top level (language) ResultSet
and thus executed once per-plan. I'm not sure though if the top-level open is guaranteed to
open all the tables that the plan requires.

There's room for improvement here, not least by writing up & understanding all the interactions.


> 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:
>         Environment: Sun Java SE 6, Solaris 10, Sun Fire V880 (8 CPUs)
>            Reporter: Knut Anders Hatlen
>            Priority: Minor
>         Attachments: Values.java, values1.png
> 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
> 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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