Hi All, Native memory allocation for DBBs is tracked in java.nio.Bits, but that only includes what the user thinks they are allocating.
When the VM adds extra memory to the allocation amount this extra bit is not represented in the Bits total. A cursory glance shows, minimum, that we round the requested memory quantity up to the heap word size in the Unsafe.allocateMemory code, and something to do with nmt_header_size in os:malloc() (os.cpp) too. On its own, and in small quantities, align_up(sz, HeapWordSize) isn't that big of an issue. But when you allocate a lot of DBBs, and coupled with the nmt_header_size business, it makes the Bits values wrong. The more DBB allocations, the more inaccurate those numbers will be. To get the "+X", it seems to me that the best option would be to introduce an native method in Bits that fetches "X" directly from Hotspot, using the same code that Hotspot uses (so we'd have to abstract-out the Hotspot logic that adds X to the memory quantity). This way, anyone modifying the Hotspot logic won't risk rendering the Bits logic wrong again. That's only one way to fix the accuracy problem here though. Suggestions welcome. Best Regards Adam Farley Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
