Re: Workload spikes on KVM host when doing IO on a guest...

[Avi Kivity <avi@redhat.com>]

On 05/20/2012 08:29 PM, Erik Brakkee wrote:
> Avi Kivity wrote:
>> On 05/20/2012 08:02 PM, Erik Brakkee wrote:
>>> [...]
>>> Thanks for this information. Unfortunately, io="native" in domain.xml
>>> is not supported by opensuse 11.3. It is supported in 12.1 so it
>>> appears that the version of KVM I have on the server is too old. I
>>> tried it on a system running the newer version and indeed, as you say
>>> the load disappears completely when using io="native".
>>>
>>> I am going to update the host now (probably to centos 6.2) to get rid
>>> of this problem.
>> To be clear: it's not a problem.  It's completely normal, and doesn't
>> affect anything.
> The only problem with it is that it leads to high workload spikes,
> which is normally a reason to have a good look at what is going on. In
> this case, the newer version of KVM should help eliminate these
> spikes, so that the next time I see a spike in the workload I know
> that I have to look into something.

Problem is, it doesn't mean anything important.  It's the count of
running threads plus the count of threads uninterruptibly waiting on a
mutex.  It's absolutely meaningless.

> I noticed the issue after I started monitoring the server and all VMs
> using zabbix (www.zabbix.com) and made a graph showing the workload of
> the hosts and that of all guests. See below. Falcon is the host and
> sparrow is a continuous integration server which is creating an
> updated RPM repository and writing a lot of files.
>
>
>
> Still the whole area of workload is a bit confusing to me. Is the
> effect of native IO simply that some of the IO work is not being
> counted anymore as part of the workload because the work is no longer
> done in user space?

No, it no longer holds a mutex. Yet it does exactly the same thing. 
That's an indication that the counter is meaningless.

(If the counter doesn't drop on an idle machine, that usually indicates
trouble; but that's not the case)

-- 
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