Improve os::attempt_reserve_memory_at() fallback coding on Linux, BSD, Solaris

In the Linux implementation of os::attempt_reserve_memory_at() we have a fallback implementation which, if the first mmap(req_addr) does not yield the requested address, repeatedly allocates memory using mmap(NULL) without freeing it, in the hope that at some point this reserved memory will intersect with the requested address:

os_linux.cpp:

3708 int i;
3709 for (i = 0; i < max_tries; ++i) {
3710 base[i] = reserve_memory(bytes);
3711
3712 if (base[i] != NULL) {
3713 // Is this the block we wanted?
3714 if (base[i] == requested_addr) {
3715 size[i] = bytes;
3716 break;
3717 }
3718
3719 // Does this overlap the block we wanted? Give back the overlapped
3720 // parts and try again.
3721
3722 ptrdiff_t top_overlap = requested_addr + (bytes + gap) - base[i];
3723 if (top_overlap >= 0 && (size_t)top_overlap < bytes) {
3724 unmap_memory(base[i], top_overlap);
3725 base[i] += top_overlap;
3726 size[i] = bytes - top_overlap;
3727 } else {
3728 ptrdiff_t bottom_overlap = base[i] + bytes - requested_addr;
3729 if (bottom_overlap >= 0 && (size_t)bottom_overlap < bytes) {
3730 unmap_memory(requested_addr, bottom_overlap);
3731 size[i] = bytes - bottom_overlap;
3732 } else {
3733 size[i] = bytes;
3734 }
3735 }
3736 }
3737 }
3738
3739 // Give back the unused reserved pieces.
3740
3741 for (int j = 0; j < i; ++j) {
3742 if (base[j] != NULL) {
3743 unmap_memory(base[j], size[j]);
3744 }
3745 }

It tries by default 10 times.

This code assumes that the requested address is somewhere in the vicinity of the memory mmap(NULL) returns by default. It also assumes that the memory returned by mmap(NULL) is attached at monotonously growing addresses.

Both assumptions should be checked and if wrong, the fallback should be skipped.

This is mainly a performance issue, as os::attempt_reserve_memory_at() is called a large number of times when allocating heap optimizes for compressed Oops.

See: http://mail.openjdk.java.net/pipermail/hotspot-runtime-dev/2015-April/014600.html