Re: [Xen-users] Re: [Xen-devel] Release 0.8.5 of GPL PV drivers forWindows

[Pasi Kärkkäinen <pasik@xxxxxx>]

On Tue, Mar 18, 2008 at 12:02:52PM -0700, Tom Brown wrote:
> >
> >Isn't these 100 mbps or 1000 mbps speeds funny numbers for today's CPU
> >power? I mean, somewhere in the design, there's something wrong that forces
> >us to make possibly too many context switches between DomU, Hypervisor and
> >Dom0. ???
> >
> >Emre
> 
> what, something like the 1500 byte maximum transmission unit (MTU) from 
> back in the days when 10 MILLION bits per second was so insanely fast we 
> connected everything to the same cable!? (remember 1200 baud modems?) Yes, 
> there might be some "design" decisions that don't work all that well 
> today.
> 
> AFAIK, XEN can't do oversize (jumbo) frames, that would be a big help for 
> a lot of things (iSCSI, ATAoE, local network )... but even so, AFAIK it 
> would only be a relatively small improvement (jumbo frames only going up 
> to about 8k AFAIK).
> 

Afaik Xen itself supports jumbo frames as long as everything in both dom0
and domU is configured correctly. Do you have more information about the 
opposite? 

"Standard" jumbo frames are 9000 bytes.. 

Something that might be interesting: 
http://www.vmware.com/pdf/hypervisor_performance.pdf

Especially the "Netperf" section..

"VMware ESX Server delivers near native performance for both one- and
two-client tests. The Xen hypervisor, on the other hand, is extremely slow,
performing at only 3.6 percent of the native performance."

"VMware ESX Server does very well, too: the throughput for two-client tests
goes up 1.9-.2 times compared to the one-client tests. Xen is almost CPU
saturated for the one-client case, hence it does not get much scaling and
even slows down for the send case."

"The Netperf results prove that by using its direct I/O architecture
together with the paravirtualized vmxnet network driver approach, VMware ESX
Server can successfully virtualize network I/O intensive datacenter
applications such as Web servers, file servers, and mail servers. The very
poor network performance makes the Xen hypervisor less suitable for any such
applications."

It seems VMware used Xen 3.0.3 _without_ paravirtualized drivers (using QEMU
emulated NIC), so that explains the poor result for Xen.. 


Another test, this time with Xen Enterprise 3.2: 
http://www.vmware.com/pdf/Multi-NIC_Performance.pdf

"With one NIC configured, the two hypervisors were each within a fraction of
one percent of native throughput for both cases. Virtualization overhead had no 
effect for this
lightly-loaded configuration."

"With two NICs, ESX301 had essentially the same throughput as native, but
XE320 was slower by 10% (send) and 12% (receive), showing the effect of CPU 
overhead."

"With three NICs, ESX301 is close to its limit for a uniprocessor virtual
machine, with a degradation compared to native of 4% for send and 3% for 
receive. XE320 is able to
achieve some additional throughput using three NICs instead of two, but the 
performance degradation
compared to native is substantial: 30% for send, 34% for receive."


So using paravirtualized network drivers with Xen should make a huge 
difference, but
there still seems to be something to optimize.. to catch up with VMware ESX. 


And some more bencnmark results by xensource: 
http://www.citrixxenserver.com/Documents/hypervisor_performance_comparison_1_0_5_with_esx-data.pdf

Something I noticed about the benchmark configuration:

"XenEnterprise 3.2 - Windows: Virtual Network adapters: XenSource Xen Tools
Ethernet Adapter RTL8139 Family PCI Fast Ethernet NIC, Receive Buffer Size=64KB"

Receive buffer size=64KB.. is that something that needs to be tweaked in the
drivers for better performance? Or is that just some benchmarking tool
related setting.. 

-- Pasi

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