[ http://issues.apache.org/jira/browse/DERBY-704?page=all ]
Knut Anders Hatlen updated DERBY-704:
-------------------------------------
Attachment: DERBY-704-extra-comments.diff
Patch DERBY-704-extra-comments.diff adds the following comments
to the code:
// 1) find out how many invalid items there are in the
// cache
// 2) search for a free invalid item
// 3) stop searching when there are no more invalid
// items to find
and
// Invalid items might occur in the cache when
// a) a new item is created in growCache(), but it
// is not in use yet, or
// b) an item is deleted (usually when a table is
// dropped)
// It is critical to break out of the loop as soon as
// possible since we are blocking others trying to
// access the page cache. New items are added to the
// end of the page cache, so the search for invalid
// items should start from the end.
If the comments are OK, could a committer please commit this change?
Thanks.
svn stat:
M java/engine/org/apache/derby/impl/services/cache/Clock.java
> 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: 10.1.2.2, 10.1.3.0, 10.2.0.0, 10.1.2.1, 10.1.2.0, 10.1.1.2,
> 10.1.1.1, 10.1.1.0
> Environment: All platforms
> Reporter: Knut Anders Hatlen
> Assignee: Knut Anders Hatlen
> Fix For: 10.2.0.0
> Attachments: DERBY-704-extra-comments.diff, 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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