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Re: [Qemu-devel] [PATCH (resend, rebase) 3/3] virtio-serial: Enable ioev
From: |
Amit Shah |
Subject: |
Re: [Qemu-devel] [PATCH (resend, rebase) 3/3] virtio-serial: Enable ioeventfd |
Date: |
Tue, 1 Mar 2011 12:11:11 +0530 |
User-agent: |
Mutt/1.5.21 (2010-09-15) |
On (Mon) 28 Feb 2011 [15:28:49], Stefan Hajnoczi wrote:
> On Mon, Feb 28, 2011 at 11:12 AM, Amit Shah <address@hidden> wrote:
> > Enable ioeventfd for virtio-serial devices by default. Commit
> > 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.
>
> 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.
>
> 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
- Re: [Qemu-devel] [PATCH (resend, rebase) 3/3] virtio-serial: Enable ioeventfd,
Amit Shah <=