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Message-ID: <46A59145.3040705@qumranet.com> Date: Tue, 24 Jul 2007 08:42:29 +0300 From: Avi Kivity <avi@...ranet.com> To: Shaohua Li <shaohua.li@...el.com> CC: kvm-devel <kvm-devel@...ts.sourceforge.net>, lkml <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu> Subject: Re: [RFC 0/8]KVM: swap out guest pages Shaohua Li wrote: > On Mon, 2007-07-23 at 18:27 +0800, Avi Kivity wrote: > >> Shaohua Li wrote: >> >>> This patch series make kvm guest pages be able to be swapped out and >>> dynamically allocated. Without it, all guest memory is allocated at >>> guest start time. >>> >>> patches are against latest git, and you need first patch Avi's >>> >> kvm-sch >> >>> integration patch >>> >>> >> (http://sourceforge.net/mailarchive/forum.php?thread_name=11841693332609-git-send-email-avi%40qumranet.com&forum_name=kvm-devel ). >> >>> Patch is quite stable in my test. With the patch, I can run a 256M >>> memory guest in a 300M memory host. >>> >> What about the opposite? >> >> >>> If guest is idle, the memory it used >>> can be less than 10M. I did a simple performance test (measure >>> >> kernel >> >>> build time in guest), if there is few swap, the performance w/wo the >>> patch difference isn't significent. If you have better measurement >>> approach, please let me try. >>> >>> Unresolved issue: >>> 1. swapoff doesn't work, we need a hook. >>> 2. SMP guest might not work, as kvm doesn't support smp till now. >>> 3. better algorithm to select swaped out guest pages according to >>> guest's memory usage. >>> Maybe more. >>> >>> Any suggests and comments are appreciated. >>> >>> >> The big question is whether to have kvm's own address_space or not. >> >> Having an address_space (like your patch does) is remarkably simple, >> and >> requires few hooks from the current vm. However using existing vmas >> mapped by the user has many advantages: >> >> - compatible with s390 requirements >> - allows the user to use hugetlbfs pages, which have a performance >> advantage using ept/npt (but which are unswappable) >> - allows the user to map a file (which can be regarded as way to >> specify >> the swap device) >> - better ingration with the rest of the vm >> >> I am quite torn between the simplicity of your approach and the >> advantages of using generic vmas. However, s390 pretty much forces >> our >> hand. >> >> What is your opinion of extending generic vmas to back kvm guest >> memory? >> > several issues: > 1. vma is to manage usersapce address, kvm guest uses full address > space. > 2. qemu itself must use some address space. > My idea is to keep the current slot concept, but instead of having kvm allocate pages for a slot, it would call get_user_pages() for a virtual address range. Userspace doesn't directly talk about vmas, just virtual address ranges. > 3. kvm need special page fault for shadow page table. generic page table > operations can't be directly used for guest. > I have no idea if your idea is feasible. The s390 guys said their shadow > page table is the same as host, this is why they can easily implement > swap, x86 is hard. > No question that it is hard. I'd like to explore just how hard it is. -- Do not meddle in the internals of kernels, for they are subtle and quick to panic. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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