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https://issues.apache.org/jira/browse/SOLR-14586?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17144949#comment-17144949
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Jason Gerlowski commented on SOLR-14586:
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bq. Now, do you expect a developer to run a perf benchmark every time he has to
use a Map#computeIfAbsent() or we just create some best practices and people
just follow them?
Worth pointing out that we'd be in a much better place if Solr had some sort of
nightly or other performance benchmark that caught perf regressions in
particular areas. We could catch-and-correct when we see the graphs dip from a
particular commit, rather than trying to prematurely optimize things ourselves
- which takes effort and produces less-than-intuitive code. I know Ishan's
mentioned a few times that he's got something in the works to help with this,
very curious to see it.
> replace the second function parameter in Map#computeIfAbsent with static vars
> -----------------------------------------------------------------------------
>
> Key: SOLR-14586
> URL: https://issues.apache.org/jira/browse/SOLR-14586
> Project: Solr
> Issue Type: Bug
> Security Level: Public(Default Security Level. Issues are Public)
> Reporter: Noble Paul
> Assignee: Noble Paul
> Priority: Minor
> Time Spent: 10m
> Remaining Estimate: 0h
>
> continuing discussion from SOLR-14579
> consider the following
> {code:java}
> mapObject.computeIfAbsent(key, o -> new HashMap<>());
> {code}
> vs.
> {code:java}
> mapObject.computeIfAbsent(key, Utils.NEW_HASHMAP_FUN)
> {code}
>
> The first code fragment is executed as following
> {code:java}
> s.computeIfAbsent(key, new Function() {
> @Override
> public Object apply(String key) {
> return new HashMap<>();
> }
> }
> {code}
> So, there are two problems with this
> * A new anonymous inner class is created for that lambda. This one extra
> class becomes a part of your binary
> * a new instance of that class is created everytime the
> {{computeIfAbsent()}} method is invoked, irrespective of whether the value is
> absent for that key or not. Now imagine that method getting called millions
> of times and creating millions of such objects for no reason
> OTOH
> when I use {{Utils.NEW_HASHMAP_FUN}}
> * Only a single anonymous class is created for the entire codebase
> * Only single instance of that object is created in the VM
> Ideally, we should go all over the codebase and remove such lambdas
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