Major Description
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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