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https://issues.apache.org/jira/browse/JENA-813?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14206321#comment-14206321
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Andy Seaborne commented on JENA-813:
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There are a couple of things I don't understand here.
If I use {{Iter.concat}} over {{Iter.append}} in the same style in
{{DatasetGraphCollection}}, {{Iter.concat}} is slower (118s vs 86s at the 4000
stage of the test programme. {{IteratorConcat}} allocates an array which for 2
items is an overhead while {{IteratorCons}} is an object of 2 slots.
The important step is using creating a single {{IteratorConcat}} as an
accumulator and calling {{.add}} on a single {{IteratorConcat}} object. That
way, there isn't a chain of objects (especially bad for {{IteratorCons}}, still
there for {{IteratorConcat}}). As used by {{Iter#concat}}, there is a list for
each iterator added (the other size being the accumulating iterator chain).
If we allowed mutable argument, it could test the class of argument {{iter1}}
but that will lead to confusing behaviour.
Something to watch out for: {{ExtendedIterator.andThen}} (as implemented by
{{NiceIterator.andThen}}) is in the style of {{IteratorCons}}.
Java8 fixes all this!
Can I clean this up to avoid the {{Iter}} interface change?
> In-memory DatasetGraph is extremely slow to iterate over when lots of named
> graphs are used
> -------------------------------------------------------------------------------------------
>
> Key: JENA-813
> URL: https://issues.apache.org/jira/browse/JENA-813
> Project: Apache Jena
> Issue Type: Bug
> Components: ARQ
> Affects Versions: Jena 2.12.1
> Reporter: Rob Vesse
> Assignee: Rob Vesse
> Attachments: SlowDatasetGraph.java, SlowDatasetGraph.java
>
>
> Discovered this accidentally in the course of debugging a test case which
> seemed to running exceptionally slow. Basically the in-memory
> {{DatasetGraph}} implementation will perform extremely poorly when trying to
> iterate all quads it contains.
> This problem manifests at relatively small scales and shows a marked
> difference between small numbers of quads in a single graph versus the quads
> in individual graphs. In my testing with only a few thousand quads placing
> them in a single graph versus many individual graphs can see 5/6 orders of
> magnitude performance difference.
> I will attach code to reproduce and show my log output from running this on
> my system (Mac OS X Mavericks - Quad Core - 8GB RAM):
> {noformat}
> 0 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Testing with
> 1000 quads
> 57 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 90 [main] INFO org.apache.jena.playground.SlowDatasetGraph - DatasetGraph
> implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has 1000 quads
> in 1 graphs
> 123 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took 0.021191
> seconds to iterate over 1000 quads in 1 graphs
> 123 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 144 [main] INFO org.apache.jena.playground.SlowDatasetGraph - DatasetGraph
> implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has 1000 quads
> in 1000 graphs
> 861 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took 0.716750
> seconds to iterate over 1000 quads in 1000 graphs
> 861 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Testing with
> 2000 quads
> 861 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 885 [main] INFO org.apache.jena.playground.SlowDatasetGraph - DatasetGraph
> implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has 2000 quads
> in 1 graphs
> 890 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took 0.005400
> seconds to iterate over 2000 quads in 1 graphs
> 890 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 922 [main] INFO org.apache.jena.playground.SlowDatasetGraph - DatasetGraph
> implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has 2000 quads
> in 2000 graphs
> 7925 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took 7.002919
> seconds to iterate over 2000 quads in 2000 graphs
> 7925 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Testing with
> 3000 quads
> 7925 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 7944 [main] INFO org.apache.jena.playground.SlowDatasetGraph - DatasetGraph
> implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has 3000 quads
> in 1 graphs
> 7948 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took 0.003924
> seconds to iterate over 3000 quads in 1 graphs
> 7948 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 7973 [main] INFO org.apache.jena.playground.SlowDatasetGraph - DatasetGraph
> implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has 3000 quads
> in 3000 graphs
> 36723 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took
> 28.749688 seconds to iterate over 3000 quads in 3000 graphs
> 36723 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Testing with
> 4000 quads
> 36723 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 36727 [main] INFO org.apache.jena.playground.SlowDatasetGraph - DatasetGraph
> implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has 4000 quads
> in 1 graphs
> 36730 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took
> 0.000773 seconds to iterate over 4000 quads in 1 graphs
> 36730 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 36741 [main] INFO org.apache.jena.playground.SlowDatasetGraph - DatasetGraph
> implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has 4000 quads
> in 4000 graphs
> 111638 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took
> 74.896949 seconds to iterate over 4000 quads in 4000 graphs
> 111638 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Testing
> with 5000 quads
> 111638 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 111644 [main] INFO org.apache.jena.playground.SlowDatasetGraph -
> DatasetGraph implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has
> 5000 quads in 1 graphs
> 111645 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took
> 0.000989 seconds to iterate over 5000 quads in 1 graphs
> 111645 [main] DEBUG com.hp.hpl.jena.shared.LockMRSW - Lock : main
> 111655 [main] INFO org.apache.jena.playground.SlowDatasetGraph -
> DatasetGraph implementation com.hp.hpl.jena.sparql.core.DatasetGraphMaker has
> 5000 quads in 5000 graphs
> 285026 [main] INFO org.apache.jena.playground.SlowDatasetGraph - Took
> 173.370614 seconds to iterate over 5000 quads in 5000 graphs
> {noformat}
> From debugging my best guess is this is down to how
> {{DatasetGraphCollection}} (and it's hierarchy) build the quads iterator by
> iterating over the graph names and then iterating over the individual graphs
> appending the iterators together. Something in the structuring means
> accessing and appending all those individual iterators is drastically slow
> compared to iterating a single graph.
> A related bug is that given a sufficiently large number of graphs attempts to
> iterate will fail with a {{StackOverflowError}}, the culprit in this case is
> {{IteratorCons}}.
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