Re: [Xen-users] Is Xen only in guests slow (tls.disabled)

["Daniel P. Berrange" <berrange@xxxxxxxxxx>]

On Wed, Apr 11, 2007 at 07:09:53PM +0100, Mark Williamson wrote:
> > Is Xen 32-bit non-pae not affected of the slow gests problem?
> > Sorry a lot of questions, I am xen newbi...
> 
> Non-PAE is also affected: any 32-bit paravirtualised Linux on an x86_32 
> hypervisor will be affected.
> 
> 64-bit fixes the problem for both 64-bit and 32-bit paravirt guests.  Other 
> architectures didn't have this problem in the first place.
> 
> It originally occurred because 32-bit x86 Xen and 32-bit x86 glibc both do 
> weird things with segmentation - and they don't coexist well as a result.  
> SuSE just remove the segmentation trick from glibc, as it has negligible 
> performance impact.
> 
> Alternative packages are available for other distros.  I think RedHat's glibc 
> might be able to switch modes for optimum performance on Xen or bare metal, 
> but I may be misremembering...

Our  glibc  RPM installs two different builds on 32-bit - one in /lib and
a second in /lib/i686/nosegneg.  The ELF dynamic loader can automatically
detect the correct version to use based on kernel capabilities. So if you're
running bare metal it'll use the default glibc, and if you then reboot into
Xen it'll transparently switch to the nosegneg version. That way both kernels
get to use the optimum version of glibc.

Regards,
Dan.
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