Re: [Qemu-devel] [RFC] Next gen kvm api

From: Avi Kivity
Date: Tue Feb 07 2012 - 11:02:53 EST

On 02/07/2012 05:17 PM, Anthony Liguori wrote:
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.

Ah. But then ioeventfd has that as well, unless the other end is in the kernel too.

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.

That's what I remember too.

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.


I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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