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https://issues.apache.org/jira/browse/DRILL-5125?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15768855#comment-15768855
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ASF GitHub Bot commented on DRILL-5125:
---------------------------------------
GitHub user paul-rogers opened a pull request:
https://github.com/apache/drill/pull/704
DRILL-5125: Provide option to use generic code for sv remover
Performance tests showed that, for queries with a large number of
columns, it is faster to use a “generic” implementation of the
selection vector remover “copier” than to use a generated version.
This PR provides "generic" versions of the SV2 and SV4 copiers
used by the selection vector remover. The generic forms are
enabled using a new boot-time config parameter that is disabled
by default (preserving the traditional generated code.)
The generic form relies on a "virtual function" (really, just a
plain Java function) defined in the base ValueVector class and
implemented by each concrete vector: both the pre-defined and
generated forms. This form "does the right thing" for the copy
operation so that we don't need to generate code just to handle
the method dispatch operation (which Java does quite well on its
own.)
A unit tests validates that the generic form works by runing
the existing SV remover tests with the generic option turned on.
See the DRILL-5125 for details.
Add test
You can merge this pull request into a Git repository by running:
$ git pull https://github.com/paul-rogers/drill DRILL-5125
Alternatively you can review and apply these changes as the patch at:
https://github.com/apache/drill/pull/704.patch
To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:
This closes #704
----
commit ba3a38a403b140149d1605decefae088765ead56
Author: Paul Rogers <[email protected]>
Date: 2016-12-12T18:06:43Z
DRILL-5125: Provide option to use generic code for sv remover
Performance tests showed that, for queries with a large number of
columns, it is faster to use a “generic” implementation of the
selection vector remover “copier” than to use a generated version.
This PR provides "generic" versions of the SV2 and SV4 copiers
used by the selection vector remover. The generic forms are
enabled using a new boot-time config parameter that is disabled
by default (preserving the traditional generated code.)
The generic form relies on a "virtual function" (really, just a
plain Java function) defined in the base ValueVector class and
implemented by each concrete vector: both the pre-defined and
generated forms. This form "does the right thing" for the copy
operation so that we don't need to generate code just to handle
the method dispatch operation (which Java does quite well on its
own.)
A unit tests validates that the generic form works by runing
the existing SV remover tests with the generic option turned on.
See the DRILL-5125 for details.
Add test
----
> Provide option to use generic code for sv remover
> -------------------------------------------------
>
> Key: DRILL-5125
> URL: https://issues.apache.org/jira/browse/DRILL-5125
> Project: Apache Drill
> Issue Type: Improvement
> Affects Versions: 1.8.0
> Reporter: Paul Rogers
> Assignee: Paul Rogers
> Priority: Minor
>
> Consider a non-typical Drill query: one with 6000 rows but 243 fields.
> Consider this query:
> {code}
> select * from (select *, row_number() over(order by somedate) as rn from
> dfs.`/some/path/data.json`) where rn=10
> {code}
> This produces a query with the following structure:
> {code}
> 00-00 Screen
> 00-01 ProjectAllowDup(*=[$0], rn=[$1])
> 00-02 Project(T0¦¦*=[$0], w0$o0=[$2])
> 00-03 SelectionVectorRemover
> 00-04 Filter(condition=[=($2, 10)])
> 00-05 Window(window#0=[window(partition {} order by [1] rows
> between UNBOUNDED PRECEDING and CURRENT ROW aggs [ROW_NUMBER()])])
> 00-06 SelectionVectorRemover
> 00-07 Sort(sort0=[$1], dir0=[ASC])
> 00-08 Project(T0¦¦*=[$0], validitydate=[$1])
> 00-09 Scan(groupscan=...)
> {code}
> Instrumenting, the code to measure compile time, two “long poles” stood out:
> {code}
> Compile Time for org.apache.drill.exec.test.generated.CopierGen3: 500
> Compile Time for org.apache.drill.exec.test.generated.CopierGen8: 1659
> {code}
> Much of the initial run time of 5578 ms is taken up in compiling two classes
> (2159 ms).
> The classes themselves are very simple: create member variables for 486
> vectors (2 x column count), and call a method on each to do the copy. The
> only type-specific work is the member variable and call to the (non-virtual)
> CopyFrom or CopyFromSafe methods. The generated class can easily be replaced
> by a “generic” class and virtual functions in the vector classes to choose
> the correct copy method.
> Clearly, avoiding code gen means avoiding the compile times with a first-run
> savings. Here are the last 8 runs (out of 10), with code cached turned off
> (forcing a compile on each query run), with and without the generic versions:
> * Original (no code cache): 1832 ms / run
> * Generic (no code cache): 1317 ms / run
> This demonstrates the expected outcome: avoiding compilation of generated
> code saves ~500 ms per run (or 28%). (Note: the numbers above were obtained
> on a version of the code that already had various optimizations described in
> other JIRA entries.)
> The reason, for generating code is that one would expect that 243 in-line
> statements (an unwound loop) to be faster than a loop with 243 iterations. In
> addition, the generic version uses an array in place of ~500 variables, and a
> virtual function call rather than in-line, type-specific calls. One would
> expect the unrolled loop to be faster.
> Repeat the exercise, this time with the code cache turned on so that no
> compile cost is payed for either code path (because the test excludes the
> first two runs in which the generated code is compiled.)
> * Original: 1302 ms / run
> * Generic version: 1040 ms / run
> Contrary to expectations, the loop is faster than the in-line statements. In
> this instance, the array/loop/virtual function version is ~260 ms faster
> (20%).
> The test shows that the code can be simplified, a costly costly code-gen and
> compile step can be skipped, and this query will go faster. Plus, since the
> change removes generated classes from the code cache, there is more room for
> the remaining classes, which may improve the hit rate.
> This ticket offers the performance improvement as an option, described in
> comments.
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