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From aledsage <...@git.apache.org>
Subject [GitHub] brooklyn-docs pull request #122: info on debugging memory usage
Date Mon, 07 Nov 2016 12:18:44 GMT
Github user aledsage commented on a diff in the pull request:

    https://github.com/apache/brooklyn-docs/pull/122#discussion_r86764334
  
    --- Diff: guide/ops/troubleshooting/memory-usage.md ---
    @@ -0,0 +1,94 @@
    +---
    +layout: website-normal
    +title: "Troubleshooting: Monitoring Memory Usage"
    +toc: /guide/toc.json
    +---
    +
    +## Memory Usage
    +
    +Brooklyn tries to keep in memory as much history of its activity as possible,
    +for displaying through the UI, so it is normal for it to consume as much memory
    +as it can.  It uses "soft references" so these objects will be cleared if needed,
    +but **it is not a sign of anything unusual if Brooklyn is using all its available memory**.
    +
    +The number of active tasks, CPU usage, thread counts, and 
    +retention of soft reference objects are a much better indication of load.
    +This information can be found by looking in the log for lines containing
    +`brooklyn gc`, such as:
    +
    +    2016-09-16 16:19:43,337 DEBUG o.a.b.c.m.i.BrooklynGarbageCollector [brooklyn-gc]:
brooklyn gc (before) - using 910 MB / 3.76 GB memory; 98% soft-reference maybe retention (of
362); 35 threads; tasks: 0 active, 2 unfinished; 31 remembered, 1013 total submitted) 
    +
    +The soft-reference figure is indicative, but the lower this is, the more
    +the JVM has decided to get rid of items that were desired to be kept but optional.
    +It only tracks some soft-references (those wrapped in `Maybe`),
    +and of course if there are many many such items the JVM will have to get rid
    +of some, so a lower figure does not necessarily mean a problem.
    +Typically however if there's no `OutOfMemoryError` (OOME) reported,
    +there's no problem.
    +
    +If you are concerned about memory usage, or doing evaluation on test environments, 
    +the following method (in the Groovy console) can be invoked to force the system to
    +reclaim as much memory as possible, including *all* soft references:
    +
    +    org.apache.brooklyn.util.javalang.MemoryUsageTracker.forceClearSoftReferences()
    +
    +If things are happy usage should return to a small level.  This is quite disruptive
    +to the system however so use with care.
    +
    +The above method can also be configured to run automatically when memory usage 
    +is detected to hit a certain level.  That can be useful if external policies are
    +being used to warn on high memory usage, and you want to keep some headroom.
    +Many JVM references discourage interfering with its garbage collector, however,
    +so use with care and study the particular JVM you are using.
    +See the class `BrooklynGarbageCollector` for more information.
    +
    +
    +## Investigation of Memory Leaks
    +
    +Design problems of course can cause memory leaks, and due to the nature of the
    +soft references these can be difficult to notice until they are advanced.
    +If the "soft-reference maybe retention" starts to decrease, that can be
    +an early warning.
    +
    +Common problems such as runaway tasks and cyclic dependent configuration will often
    +show their own log errors, so also look for these if there is a performance or memory
problem.
    +
    +You should also note the task counts in the `brooklyn gc` messages described above,
    +and if there are an exceptional number of tasks or tasks are not clearing,
    +other log messages will describe what is happening, and the in-product task
    +view can indicate issues.  `jstack` can also be useful if it is a task problem.
    --- End diff --
    
    Also worth recommending the use of `jmap -histo:live <pid>` to see how many live
instances of each type there are.


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