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Re: [Xen-users] Is NPTL in PV Xen Possible in RHEL5/CentOS 5 Distros?

To: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Subject: Re: [Xen-users] Is NPTL in PV Xen Possible in RHEL5/CentOS 5 Distros?
From: Nico Kadel-Garcia <nkadel@xxxxxxxxx>
Date: Fri, 31 Aug 2007 06:46:40 +0100
Cc: xen-users@xxxxxxxxxxxxxxxxxxx, George Lenzer <George.Lenzer@xxxxxxx>
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Mark Williamson wrote:
I'm in a bit of a bind.  My organization is planning to move to a new
mail system and I really want this box to be up as close to 24x7 as
possible.  I am currently working on an HVM domain running RHEL on top
of CentOS.  What I'm concerned about is the issue of I/O (disk and
network) since a mail system will be very busy in that way.  The mail
app I'm planning on running needs NPTL.  When I tried a paravirtualized
Fedora Core a few years back, the installer for that app said it
wouldn't install since the kernel didn't have NPTL.  So this led me to
believe that I had to wait for HVM.

You're right that HVM ought to work for you.

Now, I'm not so sure... In my recent searches, I've seen that all that's really required is a glibc
that supports NPTL and that if one runs Xen this way, you wind up with
slow performance.  I suspect this has something to do with the TLS lib
that many Xen docs say to rename?

In the olden days, if you didn't rename the TLS lib (to disable it) then Xen would have to do a whole load of emulation. Ought to be correct, but would cause some slowdown.

RHEL5 and CentOS 5 ship a glibc setup which is Xen aware and will do the Right Thing when used on a Xenified kernel. It should Just Work for you.
Amusingly, it's actually not the kernel or glibc itself. They kernel RPM installs a configuration file in /etc/ld.so.conf.d that blocks access to the /lib/tls libraries. I haven't tried performance testing with this, but I'd be very interested to see results of trying this stunt with a Xen published or locally configured Xen kernel.

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