[mod_cluster-issues] [JBoss JIRA] (MODCLUSTER-288) SystemMemoryUsageLoadMetric is not correct on Linux/Unix

[Radoslav Husar (JIRA)]

     [ https://issues.jboss.org/browse/MODCLUSTER-288?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Radoslav Husar updated MODCLUSTER-288:
--------------------------------------

              Status: Pull Request Sent  (was: Reopened)
    Git Pull Request: https://github.com/modcluster/mod_cluster/pull/52

    
> SystemMemoryUsageLoadMetric is not correct on Linux/Unix
> --------------------------------------------------------
>
>                 Key: MODCLUSTER-288
>                 URL: https://issues.jboss.org/browse/MODCLUSTER-288
>             Project: mod_cluster
>          Issue Type: Bug
>    Affects Versions: 1.0.10, 1.1.0.Final, 1.1.1.Final, 1.2.0.Final
>         Environment: Linux/Unix operating systems with standard memory management
>            Reporter: Michal Babacek
>            Assignee: Radoslav Husar
>
> I am convinced that the [SystemMemoryUsageLoadMetric.java|https://github.com/modcluster/mod_cluster/blob/master/core/src/main/java/org/jboss/modcluster/load/metric/impl/SystemMemoryUsageLoadMetric.java] is of no use to anyone.
> I have tested it by allocating some memory outside the JVM control with [NativeMemoryStress.c|https://github.com/Karm/clusterbench/blob/system-memory-load/clusterbench-native/src/main/native/NativeMemoryStress.c].
> The results from Mod_cluster are such that the system is almost fully loaded all the time despite the fact that I work with 8GB RAM box with almost nothing running on it.
> This [picture|http://hudson.qa.jboss.com/hudson/view/mod_cluster-QE/view/mod_cluster-QE-eap-6x/job/eap-6x-mod_cluster-rhel6-x86_64-load-mem/4/artifact/report/load.png] displays time on X axis and (100-(Load from Mod_cluster)) on Y axis. As you may observe, all 4 nodes appear to be under full load all the time and there is a spike on each of them. This spike represents a period of time when the aforementioned [NativeMemoryStress.c|https://github.com/Karm/clusterbench/blob/system-memory-load/clusterbench-native/src/main/native/NativeMemoryStress.c] was called with the following setting:
> {noformat}
>    megabytes    500;
>    milliseconds 60000;
> {noformat}
> What had happened is that kernel has cleared some cache and garbage data from the RAM in order to accommodate new demands, so by the time I freed the memory, there was all of sudden actually more free RAM then before.
> The actual problem with the current [SystemMemoryUsageLoadMetric.java|https://github.com/modcluster/mod_cluster/blob/master/core/src/main/java/org/jboss/modcluster/load/metric/impl/SystemMemoryUsageLoadMetric.java] implementation using [OperatingSystemMXBean|http://docs.oracle.com/javase/6/docs/jre/api/management/extension/com/sun/management/OperatingSystemMXBean.html] is that this code:
> {code:lang=java}
> +++
>   public static final String FREE_MEMORY = "FreePhysicalMemorySize";
>   public static final String TOTAL_MEMORY = "TotalPhysicalMemorySize";
> +++
>   double free = ((Number) this.server.getAttribute(name, FREE_MEMORY)).doubleValue();
>   double total = ((Number) this.server.getAttribute(name, TOTAL_MEMORY)).doubleValue();
>   return (total - free) / total;
> +++
> {code}
> Is somewhat similar to this (on 8GB RAM machine with almost nothing running on at the moment):
> {noformat}
> [karm at killerbox karm]$ free -m
>              total       used       free     shared    buffers     cached
> Mem:          7933       6755       1177          0         44        141
> -/+ buffers/cache:       6569       1363
> Swap:        16383         58      16325
> {noformat}
> Obviously, this does not mean that I could get only 1177MB of RAM if I want to. The "free" actually means "wasted". The majority of RAM is being used as cache. If I ask for e.g. 2GB, I am gonna get them, because kernel will simply sacrifice some cache, clears some garbage from RAM and free up space so as to accommodate my demands.
> So, according to my opinion, on a sane, well-behaving caching operating system, the [SystemMemoryUsageLoadMetric.java|https://github.com/modcluster/mod_cluster/blob/master/core/src/main/java/org/jboss/modcluster/load/metric/impl/SystemMemoryUsageLoadMetric.java] will always show ~ 98% RAM usage.
> Due to this fact, this metric is unlikely to be of some use to anyone on Linux/UNIX. What do you think?

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