Re: [Qemu-devel] is there a limit on the number of in-flight I/O operations?

[Chris Friesen <chris.friesen@windriver.com>]

On 07/18/2014 02:13 PM, Paolo Bonzini wrote:
> Il 18/07/2014 18:22, Chris Friesen ha scritto:
>> On 07/18/2014 09:24 AM, Paolo Bonzini wrote:
>>> Il 18/07/2014 16:58, Chris Friesen ha scritto:
>>>>
>>>> I've recently run up against an interesting issue where I had a number
>>>> of guests running and when I started doing heavy disk I/O on a virtio
>>>> disk (backed via ceph rbd) the memory consumption spiked and triggered
>>>> the OOM-killer.
>>>>
>>>> I want to reserve some memory for I/O, but I don't know how much it can
>>>> use in the worst-case.
>>>>
>>>> Is there a limit on the number of in-flight I/O operations?  (Preferably
>>>> as a configurable option, but even hard-coded would be good to know as
>>>> well.)
>>>
>>> For rbd, there is no such limit in QEMU except the size of the virtio
>>> ring buffer, but librbd may add limits of its own.
>>>
>>> For files, there's no limit if you use aio=threads, but the more I/O
>>> operations you trigger the more threads QEMU will create.  After 10
>>> seconds of idle the threads will be destroyed.
>>>
>>> Also for files, the limit is 128 per disk if you use aio=native.  You
>>> can only change it by recompilation only.
>>
>> Has anyone looked at enforcing some limits?  I'm okay with throttling
>> performance if necessary, but I really don't want to trigger the
>> OOM-killer.
>
> I forgot about "-drive ...,iops_max=NNN". :)

I'm not sure it's actually useful though, since it specifies the max IO 
operations per second, not the maximum number of in-flight operations.

If the far end can't keep up with the requested rate of operations, then 
the number of in-flight operations is going to grow even if you throttle 
the number of operations per second.

Chris