[fedora-virt] slightly OT: Comparing apples with apples?

Bill Davidsen davidsen at tmr.com
Fri Apr 24 16:39:39 UTC 2009

Gianluca Sforna wrote:
> Hi all,
> sorry for the slightly off topic post, but I'm looking for some
> definitive answers here.
> A couple days ago, I performed a "boot race" with Fedora 11 and Ubuntu
> 9.04, thinking it would be the easiest way to visualize which is
> faster to boot in a default installation.
> So basically, I created two identical VMs with virt manager and
> installed both from the liveCD, then fully updated them.
> The recorded boot sequence was repeated multiple times to see if
> results were consistent.
> Now, of course I hit a nerve there, because I had many complaints,
> ranging from the video being a fake, to personal attacks, to more or
> less weird attempts to explain why Fedora booted faster.
> The most common complain was that competing for the host's resources
> was not fair, something along the line: Fedora starts first becasue
> has no grub, grabs some critical resource and Ubuntu has to wait for
> it before continuing.
> So basically the question is: do you think there could be any reason
> why such a test can be unfair to one of the VMs?
Not unfair, but meaningless as a way of comparing which would boot 
faster on real hardware. Since disk writes seriously slow reads (per 
recent kernel list discussion) they clearly will interact, making the 
pattern of reads and writes in the startup script a factor in their own 
performance and definitely in the way they effect each other.

I would boot one, then the other, multiple times. And given the speed of 
the boots, I bet I could hold my breath while either one booted, so it 
won't take long. Then repeat with two CPUs configured, and produce 
another data point, possibly more relevant.

berrange at redhat.com noted:

I'd recommend running with cache=off for the -driver parameters to
ensure they a guarenteed to be using Direct IO,avoiding any cache
on the host OS.

At the least I would echo 1 >/proc/sys/vm/drop_caches before starting, 
I'm not sure how that would compare with the -direct option, but it's 
good practice with any benchmark.

bill davidsen <davidsen at tmr.com>
  CTO TMR Associates, Inc

"You are disgraced professional losers. And by the way, give us our money back."
    - Representative Earl Pomeroy,  Democrat of North Dakota
on the A.I.G. executives who were paid bonuses  after a federal bailout.

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