Re: [Qemu-devel] [PATCH (resend, rebase) 3/3] virtio-serial: Enable ioeventfd

[Stefan Hajnoczi]

On Tue, Mar 1, 2011 at 6:41 AM, Amit Shah <address@hidden> wrote:
> 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.

I don't have strong opinions about this but wanted to make you aware
that there is a limited number of ioeventfds to go around.

Stefan