Jon Masamitsu Jon.Masamitsu at Sun.COM
Wed Jul 11 13:34:43 UTC 2007

Florian Weimer wrote:

>* Jon Masamitsu:
>>Comments on this proposal?
>In this mode, you really should mmap the heap PROT_NONE in the
>beginning (so that you reserve a continuous memory area, which is
>required AFAIK), and use mprotect to actually allocate backing store
>when the used portion of the heap grows.  Without this change, it's
>impossible to run more than two or three VMs on systems which do not
>support memory overcommitment (which is getting more and more common
>on Linux).

Yes, hotspot does do a reserve of a continuous address space for the 
entire heap at
initialization with an mmap call using


As the heap is actually expanded the space is mmap'ed with


so it can actually be used.  See the reserve_memory() and 
commit_memory() methods
in os/linux/vm/os_linux.cpp for the details.


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