问题
I recently made a heapdump in a hprof format when my jboss server was running with a xms of 4096m and xmx of 4096m and a permsize of 512m.
The hprof file generated is over 5gb. When I load the heapdump in visualvm, mat analyzer or yourkit, I only see a total bytes of approximately 1gb. I've tried changed the reachability scope in yourkit but it does not show more than 1 gb.
Any idea what this big difference in filesize vs displayed heapdump size can cause?
ps: I'm using jdk1.6.0_23
Unfortunately I'm not allowed to submit screenshots here.
On the filesystem the hprof size is of 5.227.659 kb and in yourkit it states:
Objects: 9.738.282 / shallow size 740 mb / retained size: 740 mb String reachable among them: 6.652.515 (68%) / shallow size: 381 mb (51%) / retained size: 381 MB (51%)
The largest retained size is a byte[] of 206.810.176
回答1:
which command did you use to generate heap dump?
$JAVA_HOME/bin/jmap -dump:live,format=b,file=c:/tmp/heap_dump.bin PID
maybe you need to pass live option, according to spec
-dump:<dump-options> to dump java heap in hprof binary format
dump-options:
live dump only live objects; if not specified,
all objects in the heap are dumped.
回答2:
Did you try "Unreachable Objects Histogram" (you can find the link from the top of "Overview" page)? In one of my heapdumps sized 1509MB, mat shows only 454MB, but the rest is essentially garbage, and sure enough, the sum of "Shallow Heap" in unreachable objects histogram is 966MB.
回答3:
This just means that most likely your heap-dump consisted of a large amount of unreachable objects that would have been garbage collected, if a GC were to run. Now that does not mean that you don't still have a leak, it just means that in your 5 GB Hprof, 4 GB of objects were unreachable and hence were not interesting sources of a leak.
In Java a memory leak can only occur if Garbage Collection can't clean out an object because something is holding a reference to it (unexpectedly). So your leak (if any) is to be found in the 1 GB of objects that remained in your hprof.
来源：https://stackoverflow.com/questions/11666667/heapdump-size-vs-hprof-size