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https://issues.apache.org/jira/browse/OAK-808?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13648235#comment-13648235
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Lukas Eder commented on OAK-808:
--------------------------------
I had thought about this change of semantics at first, but observe closely:
{{currentNamePathMapper()}} creates a new {{NamePathMapperImpl}} instance,
which _already_ acts as a "proxy" to a {{documentPrefixMap}}. The
{{documentPrefixMap}} is not materialised within the {{NamePathMapperImpl}},
but it is a reference to a map that is changed just as often as the original
{{documentContext}}
In other words, changing the semantics wouldn't change the behaviour. Instead
it would more clearly communicate it, as it already is today.
> 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
> Attachments: Screen Shot YourKit Profiling Session.png,
> TargetImportHandler.java.patch
>
>
> 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. Having a live view
> may of course create concurrency issues, etc.
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