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From "Ard Schrijvers (JIRA)" <j...@apache.org>
Subject [jira] Commented: (JCR-1213) UUIDDocId cache does not work properly because of weakReferences in combination with new instance for combined indexreader
Date Fri, 23 Nov 2007 13:43:43 GMT

    [ https://issues.apache.org/jira/browse/JCR-1213?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12545025
] 

Ard Schrijvers commented on JCR-1213:
-------------------------------------

Aaaah yes, you are right.

ATM I have a working test version, that seems to solve this issue,  keep consecutive DescendantSelfAxisWeight/ChildAxisQuery
queries fast when gc() has done its work (so the WeakReferences are correct now), and is also
fast when incremental nodes are added/deleted from the index. 

To test  the performance improvement, you need a *large* repository (for 1.000.000 nodes)
where parent nodes are frequently found in different indexes.  Then running queries like xpath
= "//documents//*[@caption]" , where many nodes have this property will be much faster in
consecutive runs. A query like  "//documents//*[@date]" that has many common parents with
@caption should run fast. The problem of inital 'slow' DescendantSelfAxisWeight/ChildAxisQuery
keeps being a problem. OTOH, we might do some cache warming up if we start the repository.
Since CachingIndexReader are kept during the live time of a persistent index, it might be
quite useful. Here is also what I think is confusing about  "http://jackrabbit.apache.org/doc/arch/operate/index-readers.html
"  :

There it says:  "A SharedIndexReader is kept open for the entire lifetime of a PersistentIndex"
but AFAIU, the CachingIndexReader which is wrapped by SharedIndexReader is already kept for
lifetime of a PersistentIndex, and the SharedIndexReader is merely kept for the lifetime of
all running requests by reference counting. (If I am correct I can change the documentation
slightly). 

Furthermore, I tested the impact of the step (1) --> step(2) check for the reference to
the MultiIndexReader  (if valid return docNumber instantly) or, when invalid but segment reader
is valid, recompute docNumber. If I remove step(1) I see no performance change, therefore,
will refactor to only have step(2), and I will always recompute the actual docNumber. 

I'll try to have a patch for testing ready today

I have not invested  "Thinking more about this issue it might be worth looking at an alternative.
There is a DocNumberCache, which maps a UUID to a CachingIndexReader with a document number.
This is exactly the information that is also present in a UUIDDocId. So we might just as well
not cache the result in UUIDDocId but always use the DocNumberCache to resolve it. However
I'm not sure how much overhead that adds. I'll have to investigate that first..."







> UUIDDocId cache does not work properly because of weakReferences in combination with
new instance for combined indexreader 
> ---------------------------------------------------------------------------------------------------------------------------
>
>                 Key: JCR-1213
>                 URL: https://issues.apache.org/jira/browse/JCR-1213
>             Project: Jackrabbit
>          Issue Type: Improvement
>          Components: query
>    Affects Versions: 1.3.3
>            Reporter: Ard Schrijvers
>             Fix For: 1.4
>
>
> Queries that use ChildAxisQuery or DescendantSelfAxisQuery make use of getParent() functions
to know wether the parents are correct and if the result is allowed. The getParent() is called
recursively for every hit, and can become very expensive. Hence, in DocId.UUIDDocId, the parents
are cached. 
> Currently,  docId.UUIDDocId's are cached by having a WeakRefence to the CombinedIndexReader,
but, this CombinedIndexReader is recreated all the time, implying that a gc() is allowed to
remove the 'expensive' cache.
> A much better solution is to not have a weakReference to the CombinedIndexReader, but
to a reference of each indexreader segment. This means, that in getParent(int n) in SearchIndex
the return 
> return id.getDocumentNumber(this) needs to be replaced by return id.getDocumentNumber(subReaders[i]);
and something similar in CachingMultiReader. 
> That is all. Obviously, when a node/property is added/removed/changed, some parts of
the cached DocId.UUIDDocId will be invalid, but mainly small indexes are updated frequently,
which obviously are less expensive to recompute.

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