[Bug 703811] Re: kvm guest's cpu usage with virtio storage driver goes up to 100% because of flush process

[Serge Hallyn]

The latest upstream version has too many changes to backport to a stable
release, but if we can identify one or a few patches which fix the
problem, we can cherrypick those.  In particular, I'm wondering whether
the one below is part of the problem.  I'll push a package to a ppa for
you to test with hopefully later today.

commit de6c8042ec55da18702fa51f09072fcaa315edc3
Author: Stefan Hajnoczi <stefanha at linux.vnet.ibm.com>
Date:   Fri May 14 22:52:30 2010 +0100

    virtio-blk: Avoid zeroing every request structure
    
    The VirtIOBlockRequest structure is about 40 KB in size.  This patch
    avoids zeroing every request by only initializing fields that are read.
    The other fields are either written to or may not be used at all.
    
    Oprofile shows about 10% of CPU samples in memset called by
    virtio_blk_alloc_request().  The workload is
    dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
    times.  This patch makes memset disappear to the bottom of the profile.
    
    Signed-off-by: Stefan Hajnoczi <stefanha at linux.vnet.ibm.com>
    Signed-off-by: Kevin Wolf <kwolf at redhat.com>

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https://bugs.launchpad.net/bugs/703811

Title:
  kvm guest's cpu usage with virtio storage driver goes up to 100%
  because of flush process