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From "stack (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-10320) Avoid ArrayList.iterator() in tight loops
Date Mon, 13 Jan 2014 05:48:19 GMT

    [ https://issues.apache.org/jira/browse/HBASE-10320?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13869306#comment-13869306
] 

stack commented on HBASE-10320:
-------------------------------

I'd go with what your benchmarks are telling you but came across this recently: "The server
compiler likes a loop with an int counter (int i = 0), a constant stride (i++), and loop-invariant
limit (i <= n)" and "Loops over arrays work especially well when the compiler can relate
the counter limit to the length of the array(s)." from https://wikis.oracle.com/display/HotSpotInternals/PerformanceTechniques

On commit, add comment on why you are doing the 'unusual' pre-sizing before the loop else
the next fellow through will think it an unnecessary extra line/task and put it back the way
it as previous.

Good one Lars.

> Avoid ArrayList.iterator() in tight loops
> -----------------------------------------
>
>                 Key: HBASE-10320
>                 URL: https://issues.apache.org/jira/browse/HBASE-10320
>             Project: HBase
>          Issue Type: Bug
>          Components: Performance
>            Reporter: Lars Hofhansl
>         Attachments: 10320-0.94-v2.txt, 10320-0.94.txt
>
>
> I noticed that in a profiler (sampler) run ScanQueryMatcher.setRow(...) showed up at
all.
> In turns out that the expensive part is iterating over the columns in ExcplicitColumnTracker.reset().
I did some microbenchmarks and found that
> {code}
> private ArrayList<X> l;
> ...
> for (int i=0; i<l.size(); i++) {
>    X = l.get(i);
>    ...
> }
> {code}
> Is twice as fast as:
> {code}
> private ArrayList<X> l;
> ...
> for (X : l) {
>    ...
> }
> {code}
> The indexed version asymptotically approaches the iterator version, but even at 1m entries
it is still faster.
> In my tight loop scans this provides for a 5% performance improvement overall when the
ExcplicitColumnTracker is used.



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