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

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Message-ID: <4F314F87.60807@codemonkey.ws>
Date:	Tue, 07 Feb 2012 10:21:27 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	Jan Kiszka <jan.kiszka@...mens.com>
CC:	Avi Kivity <avi@...hat.com>, qemu-devel <qemu-devel@...gnu.org>,
	Rob Earhart <earhart@...gle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>
Subject: Re: [Qemu-devel] [RFC] Next gen kvm api

On 02/07/2012 10:18 AM, Jan Kiszka wrote:
> On 2012-02-07 17:02, Avi Kivity wrote:
>> 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.
>>
>
> Isn't there another level in between just scheduling and full syscall
> return if the user return notifier has some real work to do?

Depends on whether you're scheduling a kthread or a userspace process, no?  If 
you're eventually going to end up in userspace, you have to do the full heavy 
weight exit.

If you're scheduling to a kthread, it's better to do the type of trickery that 
ioeventfd does and just turn it into a function call.

Regards,

Anthony Liguori

>
> Jan
>

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