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Apparently the way LMDB does I/O is exercising slower (more complex?) code paths in the virtualization implementation code compared to LevelDB, and virtualization overhead is significant in these benchmarks (actually slows down execution several times).

LMDB does mmap while LevelDB does plain file I/O.

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It depends on the hypervisor. XEN/AWS uses and older slower path for memory allocation then the modern KVM/Intel-EPT allows.

Every `malloc` under XEN (that can't be satisfied with unused memory already mapped into that processes userspace) requires a hyper call.

This VMM approach was _largely_ depreciated about 6 years ago when Intel created page table bits that let the bare metal OS give the VM a _range_ of memory it can virtually re-map, but AWS hasn't adopted this.

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