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    SubjectRe: [PATCH RESEND 5/5] vhost-blk: Add vhost-blk support
    On 07/18/2012 10:31 PM, Jeff Moyer wrote:
    > Asias He <asias@redhat.com> writes:
    >
    >> On 07/18/2012 03:10 AM, Jeff Moyer wrote:
    >>> Asias He <asias@redhat.com> writes:
    >>>
    >>>> vhost-blk is a in kernel virito-blk device accelerator.
    >>>>
    >>>> This patch is based on Liu Yuan's implementation with various
    >>>> improvements and bug fixes. Notably, this patch makes guest notify and
    >>>> host completion processing in parallel which gives about 60% performance
    >>>> improvement compared to Liu Yuan's implementation.
    >>>
    >>> So, first off, some basic questions. Is it correct to assume that you
    >>> tested this with buffered I/O (files opened *without* O_DIRECT)?
    >>> I'm pretty sure that if you used O_DIRECT, you'd run into problems (which
    >>> are solved by the patch set posted by Shaggy, based on Zach Brown's work
    >>> of many moons ago). Note that, with buffered I/O, the submission path
    >>> is NOT asynchronous. So, any speedups you've reported are extremely
    >>> suspect. ;-)
    >>
    >> I always used O_DIRECT to test this patchset. And I mostly used raw
    >> block device as guest image. Is this the reason why I did not hit the
    >> problem you mentioned. Btw, I do have run this patchset on image based
    >> file. I still do not see problems like IO hangs.
    >
    > Hmm, so do the iovec's passed in point to buffers in userspace? I
    > thought they were kernel buffers, which would have blown up in
    > get_user_pages_fast.

    Yes. The iovec's passed in point to userspace buffers. ;-)

    --
    Asias




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