[jira] [Created] (OAK-808) Significant performance issue in TargetImportHandler's prefix mapping handling

["Lukas Eder (JIRA)" <ji...@apache.org>]

Lukas Eder created OAK-808:
------------------------------

             Summary: Significant performance issue in TargetImportHandler's prefix mapping handling
                 Key: OAK-808
                 URL: https://issues.apache.org/jira/browse/OAK-808
             Project: Jackrabbit Oak
          Issue Type: Bug
          Components: jcr
    Affects Versions: 0.7
            Reporter: Lukas Eder


While playing around with Oak, I have observed a significant performance bottleneck in 

{code}
org.apache.jackrabbit.oak.jcr.xml.TargetImportHandler
{code}

The problem can be seen in the attached Screenshot taken from a Yourkit profiling session. The TargetImportHandler contains a reference to a "documentContext" ListMultimap, which acts like a stack of prefix <-> namespace mappings. To the outside, however, it is exposed as "documentPrefixMap", a regular prefix <-> namespace mapping map, which only references the last namespace for a prefix.

What's problematic here: "documentPrefixMap" is eagerly copied from "documentContext" every time the latter is modified, which may happen a lot in upstream products, such as Adobe CQ5. Instead, "documentPrefixMap" should be a Map view exposing the desired data. An example implementation using Guava:

{code}
// Create this view once, in the constructor
this.documentPrefixMap = Maps.transformValues(documentContext.asMap(),
    new Function<Collection<String>, String>() {
        @Override
        public String apply(Collection<String> input) {
            return Iterables.getLast(input, null);
        }
    });
{code}

The above implementation might work, as "documentPrefixMap" is a read-only view of the "documentContext" map. More explicit, verbose implementations may prove to be even faster.

Before I write a patch, please let me know what you think.

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