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

[Jan Kiszka]

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?

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux