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Posted Nov 13, 2008 10:56 UTC (Thu) by rvfh (subscriber, #31018)
Parent article: /dev/ksm: dynamic memory sharing

"Windows apparently zeroes all freed memory"

Could not the system just release zero'd pages altogether until someone writes something in them, rather sharing them amongst KVM instances? Seems easier and less costly to do than the SHA1 thing and seems (in Windows' case) to yield some memory economies?

Especially if it's freed memory...

Or did I miss something?


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Posted Nov 13, 2008 14:20 UTC (Thu) by avik (guest, #704) [Link]

There are also non-zero pages which can be shared, such as text and read-only data.

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Posted Nov 14, 2008 16:34 UTC (Fri) by jzbiciak (subscriber, #5246) [Link]

I'm pretty sure Linux has hypervisor hooks to allow the hypervisor to tell a hosted OS to give back pages, and for a hosted OS to tell the hypervisor when it may have back pages. I don't know what Windows offers here.

I get the sense though that such changes in page ownership are coarse grain, not fine grain, since there's cost in passing the ownership back and forth.

(Note that I'm not actually super familiar with the paravirt_ops interface. I just remember reading about it here.)


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