Nolan wrote:
Windows does zero all memory at boot, and also runs a idle-priority thread in the background to zero memory as it is freed. This way it is far less likely to need to zero a page to satisfy a memory allocation request. Whether or not this is still a win now that people care about power consumption is an open question. I suspect the difference of behavior between KVM and VMware is related to VMware's page sharing. All those zeroed pages can be collapsed into one COW zero page. I wouldn't be surprised to learn that VMware has heuristics in the page sharing code specifically for windows guests. Perhaps KSM would help you? Alternately, a heuristic that scanned for (and collapsed) fully zeroed pages when a page is faulted in for the first time could catch these.
ksm will indeed collapse these pages. Lighter-weight alternatives exist -- ballooning (need a Windows driver), or, like you mention, a simple scanner that looks for zero pages and drops them. That could be implemented within qemu (with some simple kernel support for dropping zero pages atomically, say madvise(MADV_DROP_IFZERO).
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