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