Re: 50% slowdown on 2 processor XEN 4.1.3 vs 1 processor system

[Roger Pau Monné <roger.pau%citrix.com@localhost>]

Hello,

On 26/11/12 10:00, Harry Waddell wrote:
> 
> I just built a new server with 2 x E5-2630 processors and was comparing the 
> performance to a nearly identical xen server with 1 x E5-1660 processor and
> found that on a per core basis, instead of being about 50% faster ( as one 
> would expect from the clock speed given that the architecture is nearly 
> identical ), the E5-1660 system is %300 faster, so I ran some benchmarks and 
> started looking for a pattern. 
> 
> Both systems run NetBSD amd64 6.0-STABLE with xen 4.1.3. The 2
> processor system is running only 1/2 as fast with the XEN3_DOM0 kernel
> as with the GENERIC kernel, e.g. with simple single threaded benchmarks
> like dhrystone and whetstone. ( 22624434.0 vs 10416667.0 dhrystones -- It's 
> also clearly slower when compiling, etc... ) The one processor system works 
> just fine. 

I'm a little bit lost here, are you comparing the speed of the Dom0 vs a
baremetal install?

> So far, I've tried: 
> 
> 1. a netbsd 6.0 release XEN3_DOM0 kernel
> 
> 2. run the test in a netbsd domU
> 
> 3. disabled HT and NUMA
> 
> 4. used xl to create pools based on NUMA nodes and assigned/pinned dom0 to one
> 
> 5. compiled and installed xen 4.2.0rc4

Have you checked the output of xl info, to see if the number of CPUs,
NUMA nodes and clock speed is consistent?

Also, I would recommend disabling any kind of energy savings in the BIOS
and trying again.

> 
> and nothing seems to influence the disparity in the performance. 
> 
> Has anyone else seen similar behavior, or does anyone have any suggestions on 
> how to proceed? Removing a cpu is kind of dangerous, so I'd like to avoid 
> that, but if there is a good xen dom0 linux live CD, or something similar, I 
> could try booting and testing under linux? It was pretty difficult getting 
> netbsd to install and boot on my 6TB raid 5 that
> I'd hate to blow that away for such a test, but I do have usb keys I could 
> install into etc. I assume that if linux dom0's had this issue I would have 
> found something about it during my searches, so I'm guessing this is a BSD 
> issue, but that's still just an assumption, so if there's an easyish way to 
> test that theory, I'll do it. 

I've checked some time ago the performance of Linux vs NetBSD as a PV
guests on both NetBSD and Linux Dom0, and the difference was not that
big: http://www.slideshare.net/xen_com_mgr/free-and-net-bsd-xen-roadmap
(see the last part of the slides for the perf results)