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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2012-09-28 14:33, Anthony Liguori wrote:
> Jan Kiszka <jan.kiszka@siemens.com> 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"?

Jan

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