Re: kvm-27 vs 28 I/O speed

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

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.

Things to check:

- effect of pinning the vm onto one cpu (with 'taskset')
- does any counter in kvm_stat behave differently

If you are using qcow, maybe the effect is due to the first test hitting 
a hole and the second being forced to read from disk.  I recommend doing 
performance tests from a real partition or volume.

-- 
error compiling committee.c: too many arguments to function


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