Reported on the mailing lists:
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In a customer error file I found the following output:

Heap:
 garbage-first heap total 2592768K, used 1914672K [0x0000000679000000, 0x0000000679104f20, 0x00000007f0000000).

I wonder about the middle address value (0x0000000679104f20). I was assuming this to be the commit watermark. But the value seemed awfully low - just ~1MB beyond the start address of the reserved area. Given that about 4/5th of the heap is in-use, this seemed odd.

Looking at the code:

void G1CollectedHeap::print_on(outputStream* st) const {
....
  st->print(" [" PTR_FORMAT ", " PTR_FORMAT ", " PTR_FORMAT ")",
            p2i(_hrm.reserved().start()),
        >> p2i(_hrm.reserved().start() + _hrm.length() + HeapRegion::GrainWords), <<
            p2i(_hrm.reserved().end()));
...
}
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One problem here is that we don't multiply the number of regions with the region size, but just add them together. Another is that the top/commit watermark doesn't really make sense for G1 since there might be uncommitted regions in the heap.