Stupid question

[Greg Bowyer]

Not sure if this is the right venue for this, or if I am insane

I have been playing with the openjdk code with a view to lock the heap 
in memory (or at least suggest to the OS that it wants to be locked in 
memory).

My use case is for java processes that are greedy in memory and 
typically have a virtual size larger than physical ram. This is not 
where an end user allocates a java heap beyond ram but rather things 
like Lucene / Cassandra, where typically the JVM heap is large but 
limited to say 1/4 of the total physical ram and the rest of the process 
virtual size is taken up with mmap()'d files.

There are java projects that currently do this with a call out via JNA / 
JNI to mlockall()

Asking the OS to use the MAP_LOCKED flag in the mmap calls in 
os_linux.cpp effectively does an mlock / mlockall which I think means 
that when the OS chooses pages to page out; then it should (for some 
measure of should) avoid paging out the JVM heap.

This means that horrors between CMS and paging do not cause hateful 
pauses (hopefully)

The thing I cant understand is that when I start a new VM with this code 
(attached) it appears to lock the full size of the heap (-Xmx) (even 
though the given memory is not used by the VM), this seems to make the 
entire space resident.

any ideas ?
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