Xen devel - Mar 2006 - Re: [Devel] Re: [RFC] Virtualization steps

Kirill Korotaev wrote:
>> Oh, after you come to an agreement and start posting patches, can you
>> also outline why we want this in the kernel (what it does that low
>> level virtualization doesn''t, etc, etc), and how and why
you''ve agreed
>> to implement it. Basically, some background and a summary of your
>> discussions for those who can''t follow everything. Or is that
a faq
>> item?
> Nick, will be glad to shed some light on it.
> 
> First of all, what it does which low level virtualization can''t:
> - it allows to run 100 containers on 1GB RAM
>   (it is called containers, VE - Virtual Environments,
>    VPS - Virtual Private Servers).
> - it has no much overhead (<1-2%), which is unavoidable with hardware
>   virtualization. For example, Xen has >20% overhead on disk I/O.
I think the Xen guys would disagree with you on this.  Xen claims <3% 
overhead on the XenSource site.

Where did you get these figures from?  What Xen version did you test? 
What was your configuration? Did you have kernel debugging enabled? You 
can''t just post numbers without the data to back it up, especially when
it conflicts greatly with the Xen developers statements.  AFAIK Xen is 
well on it''s way to inclusion into the mainstream kernel.

Thank you,
Matt Ayres

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Matt Ayres <matta@tektonic.net> writes:
> I think the Xen guys would disagree with you on this.  Xen claims <3%
overhead
> on the XenSource site.
>
> Where did you get these figures from?  What Xen version did you test? What
was
> your configuration? Did you have kernel debugging enabled? You
can''t just post
> numbers without the data to back it up, especially when it conflicts
greatly
> with the Xen developers statements.  AFAIK Xen is well on it''s way
to inclusion
> into the mainstream kernel.
It doesn''t matter.  The proof that Xen has more overhead is trivial
Xen does more, and Xen clients don''t share resources well.

Nor is this about Xen vs what we are doing.  These are different
non conflicting approaches that operating in completely different
ways and solve a different set of problems.

Xen is about multiple kernels.

The alternative is a supped of chroot.

Eric

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Eric W. Biederman wrote:
> Matt Ayres <matta@tektonic.net> writes:
> 
>> I think the Xen guys would disagree with you on this.  Xen claims
<3% overhead
>> on the XenSource site.
>>
>> Where did you get these figures from?  What Xen version did you test?
What was
>> your configuration? Did you have kernel debugging enabled? You
can''t just post
>> numbers without the data to back it up, especially when it conflicts
greatly
>> with the Xen developers statements.  AFAIK Xen is well on it''s
way to inclusion
>> into the mainstream kernel.
> 
> It doesn''t matter.  The proof that Xen has more overhead is
trivial
> Xen does more, and Xen clients don''t share resources well.
> 
I understand the difference. It was more about Kirill grabbing numbers 
out of the air.  I actually think the containers and Xen complement each 
other very well.  As Xen is now based on 2.6.16 (as are both VServer and 
OVZ) it makes sense to run a few Xen domains that then in turn run 
containers in some scenarios.  As far as the last part, Xen doesn''t 
share resources at all :)

Thank you,
Matt Ayres

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> Kirill Korotaev wrote:
>>> Oh, after you come to an agreement and start posting patches, can
you
>>> also outline why we want this in the kernel (what it does that low
>>> level virtualization doesn''t, etc, etc), and how and why
you''ve agreed
>>> to implement it. Basically, some background and a summary of your
>>> discussions for those who can''t follow everything. Or is
that a faq
>>> item?
>> Nick, will be glad to shed some light on it.
>>
>> First of all, what it does which low level virtualization
can''t:
>> - it allows to run 100 containers on 1GB RAM
>>   (it is called containers, VE - Virtual Environments,
>>    VPS - Virtual Private Servers).
>> - it has no much overhead (<1-2%), which is unavoidable with
hardware
>>   virtualization. For example, Xen has >20% overhead on disk I/O.
> 
> I think the Xen guys would disagree with you on this.  Xen claims <3% 
> overhead on the XenSource site.
> 
> Where did you get these figures from?  What Xen version did you test? 
> What was your configuration? Did you have kernel debugging enabled? You 
> can''t just post numbers without the data to back it up, especially
when
> it conflicts greatly with the Xen developers statements.  AFAIK Xen is 
> well on it''s way to inclusion into the mainstream kernel.
I have no exact numbers in the hands as I''m in another country right
now.
But! We tested Xen not long ago with iozone test suite and it gave 
~20-30% disk I/O overhead. Recently we were testing CPU scheduler and 
EDF scheduler gave me 33% overhead on some very simple loads with almost 
busy loops inside VMs. It also was not providing any good fairness on 
2CPU SMP system to my suprise. You can object to me, but better simply 
retest it if interested yourself. There were other tests as well, which 
reported very different overheads on Xen 3. I suppose Xen guys do such 
measurements themself, no?
And I''m sure, they are constantly improving it, they are doing a good 
work on it.

Thanks,
Kirill

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