HP-UX Programmer's Guide for Java 2

The PA1.1 binary is not EXEC_MAGIC because the jit in Classic is not compatible with
EXEC_MAGIC. The PA2.0 launcher binary is EXEC_MAGIC. With EXEC_MAGIC, we have
all the space from around 0x00008000 until 0x80000000 as writeable data area. Now
the C heap will start way down there. This allows us to allocate a Java™ heap larger
than 1GB. However, you should be aware of certain considerations which might be
application dependent. These are described in the section “Application dependent
considerations when using large heap size HP-UX 11i PA-RISC”.
Additional memory available under HP-UX 11i (PA-RISC only)
With HP-UX 11i, Java™ supports a Java™ heap as large as 3.8GB. Space previously
reserved for shared memory can be used for process private writeable data. This should
be used with caution, however, as a Java™ heap this large is subject to lengthy pauses
for garbage collection. Due to the design of HP-UX, the heap is not one contiguous
mmap. When the heap is getting this large, either the new space, set with -Xmn, or old
space, set by the difference of Xmx and Xmn, must be 900MB or less.
-------------- 0x00000000
| text |
-------------- around 0x00008000
| data |
-------------- 0x80000000
| data |
-------------- 0xc0000000
| data+shmem |
-------------- 0xffffffff
When using a large Java™ heap, you should be aware of certain considerations which
might be application dependent. These are described in the section, “Application
dependent considerations when using large heap size HP-UX 11i PA-RISC”. In addition,
be aware that if you need to load extra native libraries at runtime, you need to carefully
test to ensure that you haven't used up all your address space for Java™ heap so that
there won't be enough shared memory left to load another shared library.
Allocating physical memory and swap in the Java™ heap
The Hotspot JVM uses mmap to reserve memory for the various spaces within the Java™
heaps. These memory mapped regions are created during initialization and are sized
so that they can hold the maximum size of heap specified with the -Xmx command-line
option.
Normally the HP-UX reserves swap space for the whole of these memory mapped heap
regions when they are first mapped. However, in order to conserve system swap
resources, the JVM, by default, maps these regions using the mmap MAP_NORESERVE
flag (see man 2 mmap for details). When using this option, no swap space is reserved
when the region is first created, swap will only be reserved for each page as and when
it is first used. In both cases physical memory pages will only be allocated when the
pages are first used.
64 Expanding Memory