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

[Jan Kiszka]

On 2012-02-07 17:21, Anthony Liguori wrote:
> 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 

Kthreads can't return, of course. User space threads /may/ do so. And
then there needs to be a differences between host and guest in the
tracked MSRs. I think to recall it's a question of another few hundred
cycles.

Jan

> 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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