Re: [kvm-devel] performance with guests running 2.4 kernels (specifically RHEL3)

David S. Ahern
2008-05-19T04:14:16Z

[resend to new list].

David S. Ahern wrote:
> I was just digging through the sysstat history files, and I was not
> imagining it: I did have an excellent overnight run on 5/13-5/14 with
> your patch and the standard RHEL3U8 smp kernel in the guest. I have no
> idea why I cannot get anywhere close to that again. I have updated quite
> a few variables since then (such as going from 2.6.25-rc8 to 2.6.25.3
> kernel in the host), but backing them out (i.e., resetting the test to
> my recollection of all the details of 5/14) has not helped. baffling and
> frustrating.
>
> more in-line below.
>
>
> Avi Kivity wrote:
>> David S. Ahern wrote:
>>> Avi Kivity wrote:
>>>
>>>> Okay, I committed the patch without the flood count == 5.
>>>>
>>>>
>>> I've continued testing the RHEL3 guests with the flood count at 3, and I
>>> am right back to where I started. With the patch and the flood count at
>>> 3, I had 2 runs totaling around 24 hours that looked really good. Now, I
>>> am back to square one. I guess the short of it is that I am not sure if
>>> the patch resolves this issue or not.
>>>
>>>
>> What about with the flood count at 5? Does it reliably improve
>> performance?
>>
>
> [dsa] No. I saw the same problem with the flood count at 5. The
> attachment in the last email shows kvm_stat data during a kscand event.
> The data was collected with the patch you posted. With the flood count
> at 3 the mmu cache/flood counters are in the 18,000/sec and pte updates
> at ~50,000/sec and writes at 70,000/sec. With the flood count at 5
> mmu_cache/flood drops to 0 and pte updates and writes both hit
> 180,000+/second. In both cases these last for 30 seconds or more. I only
> included data for the onset as it's pretty flat during the kscand activity.
>
>>> Also, in a prior e-mail I mentioned guest time advancing rapidly. I've
>>> noticed that with the -no-kvm-pit option the guest time is much better
>>> and typically stays within 3 seconds or so of the host, even through the
>>> high kscand activity which is one instance of when I've noticed time
>>> jumps with the kernel pit. Yes, this result has been repeatable through
>>> 6 or so runs. :-)
>>>
>> Strange. The in-kernel PIT was supposed to improve accuracy.
>>
>
> [dsa] I started a run with the RHEL4 guest 8 hours ago and it is showing
> the same kind of success. With the in-kernel PIT, time in the guest
> advanced ~120 seconds over real time after just 2 days of up time. With
> the userspace PIT, time in the guest is behind real time by only 1
> second after 8 hours of uptime. Note that I am running the RHEL4.6
> kernel recompiled with HZ at 250 instead of the usual 1000.
>
> david
>