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https://issues.apache.org/jira/browse/ARROW-1922?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16291341#comment-16291341
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ASF GitHub Bot commented on ARROW-1922:
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siddharthteotia commented on a change in pull request #1419: ARROW-1922: Blog
post on JAVA vector changes
URL: https://github.com/apache/arrow/pull/1419#discussion_r157028237
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File path: site/_posts/2017-12-13-java-vector-improvements.md
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@@ -0,0 +1,110 @@
+---
+layout: post
+title: "Improved JAVA Vector APIs"
+excerpt: "This post describes the recent improvements in JAVA Vector code"
+date: 2017-12-13 12:50:00
+author: Siddharth Teotia
+categories: [application]
+---
+
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+
+
+This post gives insight into the major improvements in the JAVA implementation
+of vectors.
+
+## Design Goals
+
+1. Improved Maintainability and Extensibility.
+2. Improved heap usage.
+3. No performance overhead on hot code paths.
+
+## Background
+
+**Improved Maintainability and Extensibility**
+
+We use templates in several places for compile time JAVA code generation for
+different vector classes, readers, writers etc. Templates are helpful as the
+developers don't have to write a lot of duplicate code.
+
+However, we realized that over a period of time some specific JAVA
+templates became extremely complex with giant if-else blocks, poor code
indentation
+and documentation. All this impacted the ability to easily extend these
templates
+for adding new functionality or improving the existing infrastructure.
+
+So we evaluated the usage of templates for compile time code generation and
+decided not to use complex templates in some places by writing small amount of
+duplicate code which is elegant, well documented and extensible.
+
+**Improved Heap Usage**
+
+We did extensive memory analysis downstream in Dremio where Arrow is used
+heavily for in-memory query execution on columnar data. The general conclusion
+was that Arrow JAVA Vectors have non-negligible heap overhead and volume of
+objects was too high. There were places in code where we were creating objects
+unnecessarily and using structures that could be substituted with better
+alternatives.
+
+**No performance overhead on hot code paths**
+
+JAVA Vectors used delegation and abstraction heavily throughout the object
+hierarchy. The performance critical get/set methods of vectors went through
+a chain of function calls back and forth between different objects before
+doing meaningful work. We also evaluated the usage of branches in vector
+APIs and reimplemented some of them by avoiding branches completely.
+
+We took inspiration from how the JAVA memory code in ArrowBuf works. For
+all the performance critical methods, ArrowBuf bypasses all the netty object
+hierarchy, grabs the target virtual address and directly interacts with
+the memory.
+
+There were cases where branches could be avoided all together.
+
+In case of Nullable vectors, we were doing multiple checks to confirm if
+the value at a given position in the vector is NULL or not.
+
+## Our Implementation Approach
+
+- For scalars, the inheritance tree was simplified by writing different
+abstract base classes for fixed and variable width scalars.
+- The base classes contained all the common functionality across different
+types.
+- The individual subclasses implemented type specific APIs for fixed and
+variable width scalar vectors.
+- For the performance critical methods, all the work is done either in
+the vector class or corresponding ArrowBuf. There is no delegation to any
+internal object.
+- The mutator and accessor based access to vector APIs is removed. These
+objects led to unnecessary heap overhead and complicated the use of APIs.
+- Both scalar and complex vectors directly interact with underlying buffers
+that manage the offsets, data and validity. Earlier we were creating different
+inner vectors for each vector and delegating all the functionality to inner
+vectors. This introduced a lot of bugs in memory management, excessive heap
+overhead and performance penalty due to chain of delegations.
+- We reduced the number of vector classes by removing non-nullable vectors.
+In the new implementation, all vectors in JAVA are nullable in nature.
+
+## Performance Testing
+
+We did performance testing downstream in Dremio and saw a 5% average
improvement
+in Tpch queries.
Review comment:
@BryanCutler , Yes, that's correct. Let me see if I can provide some
graphical comparison of performance numbers from Tpch queries before and after
using new Java implementation.
W.r.t memory analysis, I don't think I have enough information to elaborate
on the results because the work done as part of refactoring project is a
precursor to a project in the pipeline --
https://issues.apache.org/jira/browse/ARROW-1807
This has the potential to reduce the heap usage substantially. Without the
work done as part of refactoring (ARROW-1463), ARROW-1807 would not be possible.
Having said that, the new implementation has reduced the heap usage to some
extent simply because there are less number of objects floating around in
vectors.
Let me see what I can do here to elaborate on memory usage.
@pcmoritz , there was no particular reason to use JAVA instead of Java. I
will correct it.
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> Blog post on recent improvements/changes in JAVA Vectors
> --------------------------------------------------------
>
> Key: ARROW-1922
> URL: https://issues.apache.org/jira/browse/ARROW-1922
> Project: Apache Arrow
> Issue Type: Task
> Components: Java - Vectors
> Reporter: Siddharth Teotia
> Assignee: Siddharth Teotia
> Labels: pull-request-available
>
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