Re: kvm-27 vs 28 I/O speed

[Avi Kivity <avi-atKUWr5tajBWk0Htik3J/w@public.gmane.org>]

Dave Hansen wrote:
> On Sun, 2007-07-08 at 11:14 +0300, Avi Kivity wrote:
>   
>> Dave Hansen wrote:
>>     
>>> I've noticed that some of my tests run *MUCH* slower in kvm-28 than in
>>> 27.  I'm sure that wall time is pretty wonky in the guests, but it is
>>> much slower in real-world time as well.
>>>
>>> Here's a little test to create a 32MB zeroed file with dd.  Here it is
>>> from kvm-27 (this took ~5.5 seconds on my wristwatch):
>>> 33554432 bytes transferred in 0.052050 seconds (644657845 bytes/sec)
>>> 33554432 bytes transferred in 0.062933 seconds (533176451 bytes/sec)
>>>
>>> Here's the same thing from kvm-28 (~80 seconds on my wristwatch):
>>> 33554432 bytes transferred in 38.607065 seconds (869127 bytes/sec)
>>> 33554432 bytes transferred in 22.274318 seconds (1506418 bytes/sec)
>>>
>>> Same host kernel, same kvm kernel modules (from kvm-28) same guest
>>> kernel, same command-line options, same disk image.
>>>
>>> Any ideas what is going on?
>>>   
>>>       
>> Is this repeatable?  I don't see anything in kvm-27..kvm-28 that 
>> warrants such a regression.
>>     
>
> Right now, it's completely repeatable
>
>   
>> Things to check:
>>
>> - effect of pinning the vm onto one cpu (with 'taskset')
>>     
>
> Tried this.  No effect that I can see at all.
>
>   
>> - does any counter in kvm_stat behave differently
>>     
>
> Nothing really stands out.  The most strange thing to me is that the
> counters are very stationary on the slow one.  It doesn't look like it
> is spinning on something, just that it is idle.  Its counters during the
> dd look very much like the guest does when it is idle:
>   

What does 'top' on the host show in both cases?

There was a change in kvm userspace that affected IDE.  Can you try
kvm-27 userspace with a kvm-28 kernel module?  If that's slow, then the
only recourse is bisecting.


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


-------------------------------------------------------------------------
This SF.net email is sponsored by DB2 Express
Download DB2 Express C - the FREE version of DB2 express and take
control of your XML. No limits. Just data. Click to get it now.
http://sourceforge.net/powerbar/db2/