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

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Date:	Tue, 07 Feb 2012 18:02:50 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Rob Earhart <earhart@...gle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	qemu-devel <qemu-devel@...gnu.org>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api

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.

Yeah.

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

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