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https://issues.apache.org/jira/browse/NUTCH-356?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12502846
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Doğacan Güney commented on NUTCH-356:
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This problem exists with nutch's latest version as evidenced by these
discussions:
http://www.nabble.com/Plugins-initialized-all-the-time%21-tf3830461.html
http://www.nabble.com/Loading-mechnism-of-plugin-classes-and-singleton-objects-tf3869131.html
It seems that we only hit this bug in local environment. For each map task
hadoop creates a new Configuration object and each new configuration reloads
all plugins, causing a leak. (though I don't understand why garbage collector
doesn't collect unused plugins).
One solution suggested is to change CACHE from WeakHashMap to another Map and
add a equals method to Configuration that compares all key, value pairs.
> Plugin repository cache can lead to memory leak
> -----------------------------------------------
>
> Key: NUTCH-356
> URL: https://issues.apache.org/jira/browse/NUTCH-356
> Project: Nutch
> Issue Type: Bug
> Affects Versions: 0.8
> Reporter: Enrico Triolo
> Attachments: NutchTest.java, patch.txt
>
>
> While I was trying to solve a problem I reported a while ago (see Nutch-314),
> I found out that actually the problem was related to the plugin cache used in
> class PluginRepository.java.
> As I said in Nutch-314, I think I somehow 'force' the way nutch is meant to
> work, since I need to frequently submit new urls and append their contents to
> the index; I don't (and I can't) have an urls.txt file with all urls I'm
> going to fetch, but I recreate it each time a new url is submitted.
> Thus, I think in the majority of times you won't have problems using nutch
> as-is, since the problem I found occours only if nutch is used in a way
> similar to the one I use.
> To simplify your test I'm attaching a class that performs something similar
> to what I need. It fetches and index some sample urls; to avoid webmasters
> complaints I left the sample urls list empty, so you should modify the source
> code and add some urls.
> Then you only have to run it and watch your memory consumption with top. In
> my experience I get an OutOfMemoryException after a couple of minutes, but it
> clearly depends on your heap settings and on the plugins you are using (I'm
> using
> 'protocol-file|protocol-http|parse-(rss|html|msword|pdf|text)|language-identifier|index-(basic|more)|query-(basic|more|site|url)|urlfilter-regex|summary-basic|scoring-opic').
> The problem is bound to the PluginRepository 'singleton' instance, since it
> never get released. It seems that some class maintains a reference to it and
> this class is never released since it is cached somewhere in the
> configuration.
> So I modified the PluginRepository's 'get' method so that it never uses the
> cache and always returns a new instance (you can find the patch in
> attachment). This way the memory consumption is always stable and I get no
> OOM anymore.
> Clearly this is not the solution, since I guess there are many performance
> issues involved, but for the moment it works.
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