Back to back Full Garbage collections when conditions for Full GC do not seem met

[Jon Masamitsu]

Alex,

The log in ProblematicFullGC.zip is from the serial GC collector which
is subject to the "young generation guarantee". The log from OKGC.zip
is from the parallel (UseParallelGC) collector and that collector is
not subject to the "young generation guarantee".

I don't see yet why the collections are starting in ProblematicFullGC.zip.

Jon

Alex Aisinzon wrote:
>
> Hi all
>
> A customer of ours is experiencing instances whereby the Sun JDK 
> 1.4.2_16 on Windows goes through many Full GCs for no good reason.
>
> The Sun JDK parameters used are: -Xms1280m -Xmx1280m -XX:NewSize=500m 
> -XX:MaxNewSize=500m -XX:PermSize=150m -XX:MaxPermSize=150m -server 
> -XX:+PrintGCTimeStamps -XX:+PrintHeapAtGC
>
> I have enclosed a snippet of the garbage collection logs with minor GC 
> before and after (ProblematicFullGC.zip).
>
> From timestamp 21928.044 to 21958.226 + 5 seconds aka 35 seconds, the 
> JVM goes through 8 Full GCs with hardly any time in between.
>
> To the users, the JVM is frozen. The JVM needs to get restarted to get 
> back to a good state.
>
> The most surprising is that the new generation is almost empty (1 or 2 
> MB max used out of 460MB) in all but the first Full GC and that there 
> is a lot of free space in the tenured generation (more than 400MB).
>
> Our thinking is that the JVM may be triggering a Full GC because of 
> the “Young Generation Guarantee”.
>
> If such is the case, the behaviour is not consistent across platforms: 
> we have run a similar load test in house with Sun JDK 1.4.2_16 on 
> Linux with the identical parameters (“-server -Xms1280m -Xmx1280m 
> -XX:NewSize=500m -XX:MaxNewSize=500m -XX:PermSize=150m 
> -XX:MaxPermSize=150m -XX:+PrintGCDetails -XX:+PrintHeapAtGC 
> -XX:+PrintGCTimeStamps -verbose:gc”. In that case, the Full GCs occur 
> very normally though the tenured space is much fuller (140-180MB free 
> only). A snippet of the GC logs is enclosed at OKGC.zip. It shows 
> several Full GCs with a healthy dose of minor GC in between.
>
> So far, we have recommended to the customer that they use a smaller 
> New Generation, as this may make it less likely to get closer to the 
> “Young Generation Guarantee” threshold. Nevertheless, as the math is 
> not there for that “Young Generation Guarantee”, we want to dig 
> further into this.
>
> Thanks in advance for any help.
>
> Alex Aisinzon
>
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