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From "David Van Couvering (JIRA)" <derby-...@db.apache.org>
Subject [jira] Commented: (DERBY-704) Large page cache kills initial performance
Date Mon, 14 Nov 2005 23:39:28 GMT
    [ http://issues.apache.org/jira/browse/DERBY-704?page=comments#action_12357639 ] 

David Van Couvering commented on DERBY-704:
-------------------------------------------

Hi, Knut.  Normally I wouldn't review a kernel change, but this is a very simple change, it
seems to have a lot of value, and it makes sense to me.  I'll do a sanity build and test and
then I will go ahead and check in.  If a kernel developer would like to review, please do
so.

> Large page cache kills initial performance
> ------------------------------------------
>
>          Key: DERBY-704
>          URL: http://issues.apache.org/jira/browse/DERBY-704
>      Project: Derby
>         Type: Bug
>   Components: Services, Performance
>     Versions: 10.1.1.0, 10.2.0.0, 10.1.2.0, 10.1.1.1, 10.1.1.2, 10.1.2.1, 10.1.3.0, 10.1.2.2
>  Environment: All platforms
>     Reporter: Knut Anders Hatlen
>     Assignee: Knut Anders Hatlen
>      Fix For: 10.2.0.0
>  Attachments: DERBY-704.diff, cpu-usage.png, derbyall_report.txt, throughput.png
>
> When the page cache is large the performance gets lower as the page
> cache is being filled. As soon as the page cache is filled, the
> throughput increases. In the period with low performance, the CPU
> usage is high, and when the performance increases the CPU usage is
> lower.
> This behaviour is caused by the algorithm for finding free slots in
> the page cache. If there are invalid pages in the page cache, it will
> be scanned to find one of those pages. However, when multiple clients
> access the database, the invalid pages are often already taken. This
> means that the entire page cache will be scanned, but no free invalid
> page is found. Since the scan of the page cache is synchronized on the
> cache manager, all other threads that want to access the page cache
> have to wait. When the page cache is large, this will kill the
> performance.
> When the page cache is full, this is not a problem, as there will be
> no invalid pages.

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