Re: Erratic gettimeofday() behavior with KVM and newer Linux kernels?

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

Anthony Liguori wrote:
> In debugging a virtbench bug on KVM, I seemed to uncover some weird 
> behaviors with gettimeofday() and KVM with newer kernels.
>
> The attached program will run a tight loop of code a certain number of 
> times and measure it's duration with gettimeofday().  With a stock 
> ubuntu 2.6.20-15 kernel, I see pretty reasonable behavior in the guest:
>
> ./loop 0
> 0
> ./loop 100
> 0.5
> ./loop 10000
> 0.150
> ./loop 1000000
> 0.15954
>
> Note the format is <secs>.<usecs> so it's behaving quite well.  Now, 
> with a 2.6.22-rc4 kernel, using the same kernel config (saying N to 
> all of the new features), we get:
>
> ./loop 0
> 0.16
> ./loop 100
> 0.0
> ./loop 10000
> 0.0
> ./loop 1000000
> 0.20008
>
> It's pretty erratic.  If I run if a few times at any rate, sometimes I 
> get 0.0 and sometimes I get a more sane result.

I expect the newer kernel is sometimes disqualifying the timestamp 
counter from being used as a clocksource.   Running pinned to a cpu may 
improve things.

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


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