CentOS virt - Apr 2014 - Pages In and Read Sectors in Guest Domain

Hello.

I have been doing some performance tests with Xen and CentOS and have found
some strange statistics when comparing the virtual memory (/proc/vmstat)
and I/O (/proc/diskstat) statistics between the Domain 0 and guests.

When I look at the "pgpgin" statistic it is exactly the same as the
"read
sectors" in Dom0.  However, when I look at the same statistics in the
guest, the "read sectors" is exactly 2x "pgpgin".

I have tried this on 2 different versions, each on a different physical
server. Here is an example of the statistics over a 30 second period (while
running a benchmark in Dom1) in the Dom0 and Guest Domain.
                       pgpgin       read sectors
                       =======     ========      Dom0        1698024     1698024
      Dom1          848412     1696824

I have looked at some various settings to see if I could explain it.  I
can't tell if the Dom1 guest kernel is reporting incorrectly, or there is
some other explanation.  If anyone has any ideas about this I would
appreciate any feedback.

Dom 0 kernels
3.10.29-11.el6.centos.alt.x86_64
3.4.54-8.el6.centos.alt.x86_64

Guest kernel
2.6.32-431.el6.x86_64

Thanks,

Deron
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On Sun, 6 Apr 2014, Deron wrote:
> Hello.
> 
> I have been doing some performance tests with Xen and CentOS and have found
some strange statistics when comparing the
> virtual memory (/proc/vmstat) and I/O (/proc/diskstat) statistics between
the Domain 0 and guests. ?
> 
> When I look at the "pgpgin" statistic it is exactly the same as
the "read sectors" in Dom0. ?However, when I look at
> the same statistics in the guest, the "read sectors" is exactly
2x "pgpgin".
> 
> I have tried this on 2 different versions, each on a different physical
server. Here is an example of the statistics
> over a 30 second period (while running a benchmark in Dom1) in the Dom0 and
Guest Domain.
> ? ? ? ? ? ? ? ? ? ? ? ?pgpgin ? ? ? read sectors
> ? ? ? ? ? ? ? ? ? ? ? ?======= ? ? ========> ? ? ? Dom0 ? ? ? ?1698024 ?
? 1698024
> ? ? ? Dom1 ? ? ? ? ?848412 ? ? 1696824
> 
> I have looked at some various settings to see if I could explain it. ?I
can't tell if the Dom1 guest kernel is
> reporting incorrectly, or there is some other explanation. ?If anyone has
any ideas about this I would appreciate any
> feedback.
> 
> Dom 0 kernels
> 3.10.29-11.el6.centos.alt.x86_64
> 3.4.54-8.el6.centos.alt.x86_64
> 
> Guest kernel
> 2.6.32-431.el6.x86_64
The numbers that you are seeing are probably wrong due to an accounting
issue in blkfront or blkback. It would be interesting to know whether
this happens even with the most recent kernels.