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Date: Tue, 1 Mar 2011 12:11:11 +0530
From: Amit Shah <amit.shah@redhat.com>
Subject: Re: [Qemu-devel] [PATCH (resend, rebase) 3/3] virtio-serial: Enable
	ioeventfd
Message-ID: <20110301063947.GH8326@amit-x200.redhat.com>
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: qemu list <qemu-devel@nongnu.org>

On (Mon) 28 Feb 2011 [15:28:49], Stefan Hajnoczi wrote:
> On Mon, Feb 28, 2011 at 11:12 AM, Amit Shah <amit.shah@redhat.com> wrot=
e:
> > Enable ioeventfd for virtio-serial devices by default. =A0Commit
> > 25db9ebe15125deb32958c6df74996f745edf1f9 lists the benefits of using
> > ioeventfd.
> >
> > Copying a file from guest to host over a virtio-serial channel didn't
> > show much difference in time or io_exit rate.
>=20
> The cost of enabling ioeventfd is one eventfd file descriptor and KVM
> in-kernel device slot per virtqueue.  The current maximum number per
> VM is 200, this is a kernel limit in
> include/linux/kvm_host.h:NR_IOBUS_DEVS.
>=20
> Do you really want to use ioeventfd for virtio-serial?  Perhaps this
> is more useful for high-frequency device interfaces.

I guess virtio-serial is being used heavily -- by almost all guest
agents nowadays.  The primary use-case, though, is not for
high-bandwidth communication.

This setting could be default off, it didn't show any difference in my
test run, but depends on what people who use it see and think.

		Amit