LWN: Comments on "A look at Xen"

no nptl though.

rimuhosting — Wed, 03 Aug 2005 00:42:15 +0000

Xen runs NPTL code. There is a performance penalty (not sure how significant). There are glibc patches out to reduce that performance penalty.

--
Peter
RimuHosting - Xen VPS Hosting

A look at Xen

tsinclai — Fri, 01 Jul 2005 14:55:22 +0000

I *am* familiar with OS programming and the ring model, but I've never used VMWare and I wasn't
clear on how the Xen memory model worked.

However, you gave a good, clear explanation and I appreciate that.

no nptl though.

astrophoenix — Thu, 30 Jun 2005 19:15:15 +0000

the lack of nptl support was a killer for me. I wanted to run a virtual redhat advanced server 3 using gentoo as a host system. but the whole reason I wanted AS3 running virtually was so I could test production multi-threaded code using redhat's 2.4 kernel with back-ported nptl. in production, the code would be using nptl but on my virtual machine, nptl would have to be disabled to get xen to work.

A look at Xen

MarkWilliamson — Thu, 30 Jun 2005 14:32:23 +0000

For details of live migration see:
http://www.cl.cam.ac.uk/netos/papers/2005-migration-nsdi-...

Other papers under:
http://www.cl.cam.ac.uk/Research/SRG/netos/xen/architectu...

If you run software that dirties large amounts of memory then the precopy
approach doesn't work very well: in this case, the live migration code
will figure it out and cuts its losses by doing "stop and copy" -
incurring a longer stoppage but more efficient use of network bandwidth.

For real world server workloads it works well: Running Quake 3 servers
incurred a downtime of about 60ms (I think), whilst an Apache webserver
running SpecWeb incurred a downtime of 300ms and didn't drop any clients.

linux-vserver.org?

gvy — Thu, 30 Jun 2005 08:34:51 +0000

For "utility computing" everyone already uses VServer ;-) (which "only" allows for contexts within the same Linux kernel but that suffices for a wide range of applications, hosting/testing/resource provisioning being the most evident)

It's in a different weight but still the same category of virtualization software, and I wonder why mention UML (which is basically dead to me) and Virtuozzo (which is proprietary) and forget of VServer.

PS: shameless plug: yes, ALT Linux has the support out-of-box with 2.4 release. :-)

A look at Xen

csamuel — Sun, 26 Jun 2005 08:40:57 +0000

Both Intel and AMD have made public declarations of support for Xen with respect to their virtualisation technology.

It's this which will make it possible to run unmodified OS's under Xen in the future (fingers crossed)..

Chris

A look at Xen

erich — Fri, 24 Jun 2005 23:34:17 +0000

Do you know the basics of OS programming? About the ring model?
They are mentioned in http://en.wikipedia.org/wiki/X86

Xen runs in ring-0, requiring the OS to run in the lower rings.
Usually linux itself is in ring 0; userspace apps run in ring 2 or 3.
Basically, inner rings have more privileges, and are protected from outer rings.
Xen manages the memory on a high level, and linux instead of just handling memory itself, requests memory from xen via special calls. Basically thats the same way, the linux kernel is protected from userspace applications.

If Linux would try to do it directly, it would cause a segmentation fault; but having this protection, it cannot trash xen or the other vms.

VMWare probably detects these calls in the binary and replaces that code. VMware is MUCH closer to emulating a separate PC; Xen just takes away some privileges from the OS in order to separate them, and requires the OS to "politely ask" for the resources. Thats why you need a different kernel.
But since everything accessing the hardware is handled by the linux kernel, all changes needed for Xen are within the kernel.

A look at Xen

erich — Fri, 24 Jun 2005 22:24:25 +0000

Hi,
according to talk recently at BaLUG, they copy and note which pages have become "dirty" in the meantime, after a few cycles they then stop the vm and migrate the rest over, start it on the new one.
According to them, they've done that with a Quake3 server without the users noticing much. That sounds really impressive.
Apparently "downtimes" of ~200ms is typical when you have a 100Mbit link between the two hosting machines.

A look at Xen

tsinclai — Fri, 24 Jun 2005 17:21:11 +0000

I've been following this project for quite some time and am looking forward to trying it out.

As a technical instructor, this would seem to make setting up a virtual network in a classroom
much easier. Our Windows classes use Virtual PC for that purpose right now, but I prefer a
Free (as well as free) solution.

Am I right in thinking that Xen takes a different tack than VMWare?

A look at Xen

patha — Thu, 23 Jun 2005 18:04:23 +0000

> It only (usually) takes less than a second to stop the VM on the source host, transfer any remaining differences, [...]

Does this also holds for a machine with 64GB RAM running GUPS (http://www.dgate.org/~brg/files/dis/gups/) all over the memory? ;)

Seriously, were can I read about how the step "transfer any remaining differences" is implemented?

A look at Xen

maceto — Thu, 23 Jun 2005 15:32:10 +0000

wow Redhat needs yast, that`s way! easier on suse to setup, it`s like vmware vs dos...

A look at Xen

dcreemer — Thu, 23 Jun 2005 03:26:39 +0000

Xen guest VM migration between hosts completes very quickly. The actual transfer takes longer, but the VM stays running on the source machine as long as possible. It only (usually) takes less than a second to stop the VM on the source host, transfer any remaining differences, then bring up the VM on the target machine.

A look at Xen

yodermk — Thu, 23 Jun 2005 02:07:10 +0000

>>> Crosby said that it's possible to move a Xen virtual machine "so that the guest is only non-responsive to the outside world for tens of miliseconds."

That sounds great, but I'm not sure I believe it. Wouldn't it take at least a few seconds to transfer an image to another box over 100 megabit ethernet?

Also, I read very briefly in a comment somewhere else that next-gen chips from Intel and AMD would allow for full virtualization without a modified guest, just as mainframes have done for decades. Does anyone know more about this and when they will come out? Will Xen support them right off the bat?