[jira] Commented: (NUTCH-356) Plugin repository cache can lead to memory leak

["Doğacan Güney (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/NUTCH-356?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12502846 ] 

Doğacan Güney commented on NUTCH-356:
-------------------------------------

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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