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Date: Tue, 07 Feb 2012 09:17:35 -0600
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [RFC] Next gen kvm api
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To: Avi Kivity <avi@redhat.com>
Cc: Rob Earhart <earhart@google.com>, linux-kernel <linux-kernel@vger.kernel.org>, KVM list <kvm@vger.kernel.org>, qemu-devel <qemu-devel@nongnu.org>

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

>
>