On 02/07/2012 06:03 AM, Avi Kivity wrote:On 02/06/2012 09:11 PM, Anthony Liguori wrote:
I'm not so sure. ioeventfds and a future mmio-over-socketpair have to put the
kthread to sleep while it waits for the other end to process it. This is
effectively equivalent to a heavy weight exit. The difference in cost is
dropping to userspace which is really neglible these days (< 100 cycles).
On what machine did you measure these wonderful numbers?
A syscall is what I mean by "dropping to userspace", not the cost of a heavy weight exit.
I think a heavy weight exit is still around a few thousand cycles.
Any nehalem class or better processor should have a syscall cost of around that unless I'm wildly mistaken.
But I agree a heavyweight exit is probably faster than a double context switch
on a remote core.
I meant, if you already need to take a heavyweight exit (and you do to schedule something else on the core), than the only additional cost is taking a syscall return to userspace *first* before scheduling another process. That overhead is pretty low.