On 03/16/2011 04:20 AM, Stefan Hajnoczi wrote:
On Tue, Mar 15, 2011 at 1:13 PM, Anthony Liguori<aligu...@us.ibm.com> wrote:
Why even bothering signaling for completion with the virtio-9p threadpool?
There's no sane guest that's going to poll on virtio-9p completion with
interrupts disabled and no timer. Once we enable the I/O thread by default,
it won't even be necessary for the paio layer.
It's not just about preventing livelock under extreme cases. If you
omit the signal then !CONFIG_IOTHREAD builds will see increased
latency because virtfs request completion will piggy-back on other
events that *do* interrupt the vcpu.
But realistically, the timer is firing at a high enough frequency that I
doubt you'd even observe the latency.
There's an easy solution here, just do some sniff testing to see if you
can tell the difference. I bet you can't.
I'm no fan of !CONFIG_IOTHREAD
but skipping signals is currently bad practice and will continue to be
until CONFIG_IOTHREAD is removed entirely.
The proper solution would be a thin abstraction for thread-safe
notification that compiles out signals when CONFIG_IOTHREAD is used.
Then we have one place in QEMU that does signalling correctly and we
can optimize it or remove CONFIG_IOTHREAD completely without having
the burden of duplicating this code in several places.
We have probably 5 different ways to wake up a CPU. I don't think we
should add a new one just for this.
!CONFIG_IOTHREAD needs to go away in the very near future. I'd rather
focus on that.
Regards,
Anthony Liguori
Stefan
