On Tue, Jan 25, 2011 at 7:12 AM, Stefan Hajnoczi<address@hidden> wrote:
On Mon, Jan 24, 2011 at 8:05 PM, Kevin Wolf<address@hidden> wrote:
Am 24.01.2011 20:47, schrieb Michael S. Tsirkin:
On Mon, Jan 24, 2011 at 08:48:05PM +0100, Kevin Wolf wrote:
Am 24.01.2011 20:36, schrieb Michael S. Tsirkin:
On Mon, Jan 24, 2011 at 07:54:20PM +0100, Kevin Wolf wrote:
Am 12.12.2010 16:02, schrieb Stefan Hajnoczi:
Virtqueue notify is currently handled synchronously in userspace virtio. This
prevents the vcpu from executing guest code while hardware emulation code
handles the notify.
On systems that support KVM, the ioeventfd mechanism can be used to make
virtqueue notify a lightweight exit by deferring hardware emulation to the
iothread and allowing the VM to continue execution. This model is similar to
how vhost receives virtqueue notifies.
The result of this change is improved performance for userspace virtio devices.
Virtio-blk throughput increases especially for multithreaded scenarios and
virtio-net transmit throughput increases substantially.
Some virtio devices are known to have guest drivers which expect a notify to be
processed synchronously and spin waiting for completion. Only enable ioeventfd
for virtio-blk and virtio-net for now.
Care must be taken not to interfere with vhost-net, which uses host
notifiers. If the set_host_notifier() API is used by a device
virtio-pci will disable virtio-ioeventfd and let the device deal with
host notifiers as it wishes.
After migration and on VM change state (running/paused) virtio-ioeventfd
will enable/disable itself.
* VIRTIO_CONFIG_S_DRIVER_OK -> enable virtio-ioeventfd
* !VIRTIO_CONFIG_S_DRIVER_OK -> disable virtio-ioeventfd
* virtio_pci_set_host_notifier() -> disable virtio-ioeventfd
* vm_change_state(running=0) -> disable virtio-ioeventfd
* vm_change_state(running=1) -> enable virtio-ioeventfd
Signed-off-by: Stefan Hajnoczi<address@hidden>
On current git master I'm getting hangs when running iozone on a
virtio-blk disk. "Hang" means that it's not responsive any more and has
100% CPU consumption.
I bisected the problem to this patch. Any ideas?
Kevin
Does it help if you set ioeventfd=off on command line?
Yes, with ioeventfd=off it seems to work fine.
Kevin
Then it's the ioeventfd that is to blame.
Is it the io thread that consumes 100% CPU?
Or the vcpu thread?
I was building with the default options, i.e. there is no IO thread.
Now I'm just running the test with IO threads enabled, and so far
everything looks good. So I can only reproduce the problem with IO
threads disabled.
Hrm...aio uses SIGUSR2 to force the vcpu to process aio completions
(relevant when --enable-io-thread is not used). I will take a look at
that again and see why we're spinning without checking for ioeventfd
completion.
Here's my understanding of --disable-io-thread. Added Anthony on CC,
please correct me.
When I/O thread is disabled our only thread runs guest code until an
exit request is made. There are synchronous exit cases like a halt
instruction or single step. There are also asynchronous exit cases
when signal handlers use qemu_notify_event(), which does cpu_exit(),
to set env->exit_request = 1 and unlink the current tb.