NPTL Support?

Posted Jul 24, 2005 13:21 UTC (Sun) by cmeerw (guest, #555)
Parent article: OLS: Xen and UML

What about NPTL support? AFAIK UML doesn't support NPTL at all, does it work with Xen? Or will we have to wait for VT/Pacifica until NPTL is fully supported in the guest?

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NPTL Support?

Posted Jul 24, 2005 16:11 UTC (Sun) by copsewood (subscriber, #199) [Link] (4 responses)

Is that Native Posix Threading Library ? If so, how would this usefully interact with UML/Xen ?

NPTL Support?

Posted Jul 24, 2005 21:39 UTC (Sun) by jreiser (subscriber, #11027) [Link] (3 responses)

Yes, NPTL is Native Posix Thread Library. Supporting "full Linux [user-mode] processes" implies supporting the kernel pieces that support NPTL. The major pieces are the syscalls clone, set_thread_area, and set_tid_address; the support for segment %gs [on x86 and amd64]; etc. Both UML and Xen must do this in order to run an arbitrary user process. NPTL might be used as the thread model for a multithreaded single process web server, mail server, etc., which runs under UML or Xen in order to increase isolation, security, management flexibility, etc. On x86, glibc has a special compiletime option to support Xen by avoiding "negative" offsets to %gs, which have significantly higher implementation costs than "positive" offsets.

NPTL Support?

Posted Jul 24, 2005 22:37 UTC (Sun) by jwb (guest, #15467) [Link] (2 responses)

To answer the original question, Xen does not yet support NPTL.

NPTL Support?

Posted Jul 28, 2005 13:02 UTC (Thu) by demon (guest, #31340) [Link] (1 responses)

Actually, Xen works just fine with NPTL. The unmodified glibc will have a minor performance cost, due to the Xen hypervisor needing to enable a few workarounds to deal with the negative offsets of the GS register that were mentioned above, but a patched glibc makes this go away. NPTL-threaded apps work just as they do on a full-blown Linux machine.

As I don't use UML, I can't say if it does or not, though there might be similar concerns with it as to what had been cropping up with NPTL and Xen.

NPTL Support?

Posted Aug 3, 2005 0:13 UTC (Wed) by rimuhosting (guest, #31061) [Link]

As the parent says, Xen runs NPTL code OK - but a bit slower than other code. Apparently there is a patch to glibc for Xen guests that can improve the performance.

UML does not support NPTL right now. You typically need to just mv /lib/tls /lib/tls.broken to prevent apps using it. There is talk on the mailing lists of some modifications so that it will support NPTL in the future.

The main limitation that this all seems to create is with db4 which depends on NPTL for interprocess locking. e.g. this can affect subversion users, and the workaround is to use a file system backed repository instead.

--
Peter
RimuHosting - Xen/UML VPS Hosting