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From: | Anthony Liguori |
Subject: | Re: [Qemu-devel] [RFC] Next gen kvm api |
Date: | Tue, 07 Feb 2012 09:17:35 -0600 |
User-agent: | Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9.2.23) Gecko/20110922 Lightning/1.0b2 Thunderbird/3.1.15 |
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.
Regards, Anthony Liguori
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