On Tue, Mar 1, 2011 at 6:41 AM, Amit Shah <amit.s...@redhat.com> wrote: > On (Mon) 28 Feb 2011 [15:28:49], Stefan Hajnoczi wrote: >> On Mon, Feb 28, 2011 at 11:12 AM, Amit Shah <amit.s...@redhat.com> 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
