[Xen-devel] Xen Performance Results

[Sergej Proskurin <proskurin@xxxxxxxxxxxxx>]

Hi all,

I have a question concerning a 'correct' Xen configuration to measure
performance, as I am currently experiencing a quite unexpected behavior.

My overall setup comprises a Skylake micro-architecture based system
with a Debian Buster and Linux kernel 4.13.16 running on top of Xen
v4.8. For performance measurements, I make use of the Phoronix Test
Suite v7.6.0 and SPECINT 2017. I compare the results of the test suits
performed in a Xen domu with results performed natively (with the
"performance" CPU governor on bare metal). Since my test case requires
the performance measurements run on only one CPU, I limit the Linux
running on bare metal to using only one CPU (maxcpus=1). I do the same
with Xen and additionally pin domu to the same CPU that runs dom0, as to
measure the entire overhead that comes with Xen.

The odd thing is that the resulting cpu-intensive performance
measurements of Xen seem to be partially faster than on Bare Metal. The
affected measurements inside the domu seem to be between ~6% and ~8%
faster than the ones measured on bare metal. Normally, I would say this
is a caching issue but the results are quite stable. BTW: there is no
such issue when running KVM.

My first assumption was that the benchmak suits make use of the TSC that
might have been falsely adjusted by the hypervisor. Yet, after playing
around with the domain control "tsc_mode" (and also the options
"no_migrate" and "timer_mode"), I did not experience any changes.
Besides, since the performance benchmarks seemingly use the wallclock
time, the TSC would not affect the stated time issues anyway. Thus, I
have set up the domu to use the same localtime as the dom0
(localtime=1), while at the same time the dom0 uses NTP. Nevertheless,
the tests showed the same results as before.

In my oppinion, this is a rather odd behavior that (despite the
virtualization overhead of Xen) the domu appears to be faster than the
same Linux kernel running on bare metal. Thus, I wanted to ask you
whether you had an advice regarding the stated issue, as I am sure that
I miss a configuration option (also I can't be the only one experiencing
such behavior, yet I did not find any useful hints on the Internet).

It would be great if you could help me with my concern :) Thank you very
much in advance :)

Thanks and best regards,

~Sergej




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