Why does memory go down after I take a heap dump?

Asked May 22 '18 at 14:35

Active May 22 '18 at 14:35

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I am running a web application with WildFly 9 and I've allocated X GB of memory. Over a period of time the memory gets accumulated and I run the jmap utility to collect the heap dump for analysing the memory utilization.

When I do this, the memory consumed by the java.exe comes down from X to X-Y GB. Sometimes the Y amounts to GBs of memory.

Why does this happen? If this chunk of memory is marked to be cleared, why doesn't GC clear it?

edited May 22 '18 at 14:35

TylerH

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asked May 22 '18 at 14:35

Anjan Baradwaj

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It isn't "marked to be cleared" until GC happens. There is a full garbage collection before the heap dump is taken. – Michael May 22 '18 at 14:41
And why doesn't full GC happen when I don't take the heap dump? – Anjan Baradwaj May 22 '18 at 14:45
It depends on the garbage collector that you're using. It hasn't happened because the GC has decided that it doesn't need to happen. – Michael May 22 '18 at 14:49
Understood but without GC happening, the memory is going high and OOM occurs. Is it recommended / possible to initiate a full GC, manually? – Anjan Baradwaj May 22 '18 at 14:56
Which options you used with jmap command? – Alexey Ragozin May 23 '18 at 12:28
jmap -dump:file=FileName.hprof – Anjan Baradwaj May 23 '18 at 14:36

Why does memory go down after I take a heap dump?

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