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Date: Fri, 01 Feb 2013 18:03:43 +0100
From: Paolo Bonzini <pbonzini@redhat.com>
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Subject: Re: [Qemu-devel] RFC migration of zero pages
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To: Orit Wasserman <owasserm@redhat.com>
Cc: Peter Lieven <pl@dlhnet.de>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

Il 31/01/2013 09:33, Orit Wasserman ha scritto:
>> > If my above assumption that the guest reads unmapped memory as zeroes is right, this mapping
>> > is not necessary in the case of a zero dup page.
>> > 
>> > We just have to make sure that we are still in the very first round when deciding not to sent
>> > a zero page, because otherwise it could be a page that has become zero during migration and
>> > this of course has to be transferred.
> OK, so if we won't send the pages than it won't be allocate in the dst and it can improve both 
> memory usage and reduce cpu consumption on it.
> That can be good for over commit scenario.

We don't allocate zero pages in the destination:

#ifndef _WIN32
            if (ch == 0 &&
                (!kvm_enabled() || kvm_has_sync_mmu()) &&
                getpagesize() <= TARGET_PAGE_SIZE) {
                qemu_madvise(host, TARGET_PAGE_SIZE, QEMU_MADV_DONTNEED);
            }
#endif

Paolo