Re: [Qemu-devel] [PATCH] Add option to mlock guest and qemu memory

[Anthony Liguori]

Jan Kiszka <address@hidden> writes:

> On 2012-09-28 14:33, Anthony Liguori wrote:
>> Jan Kiszka <address@hidden> writes:
>> 
>>> On 2012-09-28 01:21, Satoru Moriya wrote:
>>>> This is a first time for me to post a patch to qemu-devel.
>>>> If there is something missing/wrong, please let me know.
>>>>
>>>> We have some plans to migrate old enterprise systems which require
>>>> low latency (msec order) to kvm virtualized environment. Usually,
>>>> we uses mlock to preallocate and pin down process memory in order
>>>> to avoid page allocation in latency critical path. On the other
>>>> hand, in kvm environment, mlocking in guests is not effective
>>>> because it can't avoid page reclaim in host. Actually, to avoid
>>>> guest memory reclaim, qemu has "mem-path" option that is actually
>>>> for using hugepage. But a memory region of qemu is not allocated
>>>> on hugepage, so it may be reclaimed. That may cause a latency
>>>> problem.
>>>>
>>>> To avoid guest and qemu memory reclaim, this patch introduces
>>>> a new "mlock" option. With this option, we can preallocate and
>>>> pin down guest and qemu memory before booting guest OS.
>>>
>>> I guess this reduces the likeliness of multi-millisecond latencies for
>>> you but not eliminate them. Of course, mlockall is part of our local
>>> changes for real-time QEMU/KVM, but it is just one of the many pieces
>>> required. I'm wondering how the situation is on your side.
>>>
>>> I think mlockall should once be enabled automatically as soon as you ask
>>> for real-time support for QEMU guests. How that should be controlled is
>>> another question. I'm currently carrying a top-level switch "-rt
>>> maxprio=x[,policy=y]" here, likely not the final solution. I'm not
>>> really convinced we need to control memory locking separately. And as we
>>> are very reluctant to add new top-level switches, this is even more
>>> important.
>> 
>> I think you're right here although I'd suggest not abbreviating.
>
> You mean the sense of "-realtime" instead of "-rt"?

Yes.  Or any other word that makes sense.

Regards,

Anthony Liguori

>
> Jan
>
> -- 
> Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
> Corporate Competence Center Embedded Linux