[jira] [Created] (WICKET-5527) Inefficient DefaultPageStore.SerializedPagesCache

["Martin Grigorov (JIRA)" <ji...@apache.org>]

Martin Grigorov created WICKET-5527:
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             Summary: Inefficient DefaultPageStore.SerializedPagesCache
                 Key: WICKET-5527
                 URL: https://issues.apache.org/jira/browse/WICKET-5527
             Project: Wicket
          Issue Type: Bug
          Components: wicket
    Affects Versions: 6.14.0
            Reporter: Martin Grigorov
            Assignee: Martin Grigorov


We have identified some problems in org.apache.wicket.pageStore.DefaultPageStore.SerializedPagesCache.

Some history first: 
At https://cwiki.apache.org/confluence/display/WICKET/Page+Storage I have explained how the page storage management works in Wicket 1.5+ 

In brief:
First level cache/store is the HttpSession - here Wicket saves the live instances of all touched pages in the last request cycle.
Second level cache/store is DefaultPageStore.SerializedPagesCache - here Wicket saves the last N (org.apache.wicket.settings.StoreSettings#getInmemoryCacheSize) used pages in the whole application (by default 40 pages)
Third level cache/store is DiskDataStore - here Wicket stores all pages and depending on org.apache.wicket.settings.StoreSettings#getMaxSizePerSession it will "recycle" the file contents

The identified problems:

- org.apache.wicket.pageStore.DefaultPageStore.SerializedPagesCache uses ArrayList as a data structure to keep SerializedPage instances. When the limit N (StoreSettings#getInmemoryCacheSize) is reached the ArrayList uses #remove() to remove the oldest entry. The #remove(0) operation internally uses System.arraycopy() to compact the internal array structure. As you already realize this ArrayList is constantly being recompacted in any application in production.

-  DefaultPageStore.SerializedPagesCache#cache (the same ArrayList) is used as synchronization monitor for every operation (read/write/remove). I.e. we have synchronization on application level !!

- at the moment DefaultPageStore.SerializedPagesCache stores org.apache.wicket.pageStore.DefaultPageStore.SerializedPage. This is a structure of {String sessionId, int pageId, byte[] data}. 
Since this data is stored in the application scope it is never replicated, so there is no need to serialize the live page instance to byte[] at all. Only the third level cache (IDataStore) should work with byte[]


A workaround to avoid the slowness caused by this is to set 0 or negative value to org.apache.wicket.settings.StoreSettings#setInmemoryCacheSize



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