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When you run 64 bit it assumes all the SSE extensions, the presence of the extra register sets, and everything between the 386 and 686 being available. It almost always performs better with software compiled to utilize the extra goods, even if it has slightly larger memory footprints.

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Generally yes, you almost always see a performance increase with all the things x86-64 brings. But the increased cache pressure from 64-bit pointers is a real downside and the reason that pretty much every other architecture suffers in performance when it goes to 64 bits. If something about his workload makes it especially sensetive to cache pressure he could very well see a performance penalty, and I'd say heavy virtualization would actually make that more likely.

you're absolutely correct there. it might be that the performance difference I'm seeing is caused by the hypervisor or something else.

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