Removal of avahi-daemon from Ubuntu 14.04 systems

It appears that the avahi-daemon causes the gtk+ library on Ubuntu 14.04 systems used by many GUIs  (e.g. firefox and chrome) to hammer the print spooler by opening hundreds of simultaneous connections, causing the GUI to crash. To avoid this, we are removing the avahi-daemon on all Ubuntu 14.04 systems. If you manage a machine which needs it, simply add ‘avahi-daemon’ to /etc/user-config/bundles as usual. and it will be auto-installed.

In lots more detail: About a year ago we updated our CUPS print server to a version which supported Bonjour/Avahi export of printer information. This was mainly to make life easier for Mac/Windows users, Linux users continued as before as Linux systems hadn’t added Avahi support at that time. Avahi support was added in Ubuntu 13 through the gtk+ v3 library. From that point on we started seeing problems as it turned out that Avahi printer discovery told gtk+ about our printers several times over. So applications using gtk+ (such as evince/Document Viewer, browsers etc) were frequently polling the CUPS server about each printer discovered by Avahi, resulting in several hundred simultaneous connections from the application to the CUPS server. Not surprisingly the CUPS server ground to a halt for about 15 mins whenever anybody using a Ubuntu 13/14 system tried to print. This caused a problem for everyone in the Lab, not just Ubuntu 13/14 users. As we hadn’t discovered the cause at that point we had no choice but to implement throttling on the CUPS server so that it would only accept up to 50 simultaneous connections from any one host. This prevented the CUPS server from dying, at the expense of printing problems for the (at that time small number of) Ubuntu 13/14 users – if using a printer at the top of the list printing would succeed but be quite slow, if using a printer at the bottom of the list then your application would probably crash. Our only solution was to suggest applications that don’t use gtk+, such as okular which is a PDF viewer superior in every way to evince/Document Viewer, and which we still recommend. We haven’t discovered a way to “fix” Avahi, but we have now discovered that removing the Avahi daemon altogether solves the problem completely – gtk+ backs off to a less harmful means of printer discovery, and applications don’t notice. We are not aware of any other use for Avahi on our systems, so we are simply removing it.

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Internal Lab facilities may not be available

Certain commands used to do “privileged” things within the Lab may not be working for a while. If some feature you particularly need does not work, please email details to sys-admin

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condor pool moved to NFS sec=krb5

As emailed to all recent users of condor, the execute machines have now moved to NFS sec-krb5. The documentation has been updated. If anyone has problems, please email sys-admin.

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Lab PhD wins Ubicomp 2014 Best Paper Award

Chloe Brown has received the Best Paper Award at the 2014 ACM International Joint Conference on Pervasive and Ubiquitous Computing (Ubicomp 2014).

The paper “The architecture of innovation: Tracking face-to-face interactions with ubicomp technologies” is joint work with Christos Efstratiou, Ilias Leontiadis, Daniele Quercia, Cecilia Mascolo, James Scott and Peter Key

Chloe Brown is a member of the Network and Operating Systems group under the supervision of Professor Cecilia Mascolo.

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September edition of ‘The Ring’ now online

The September edition of ‘The Ring‘ is now available.

Please contact for more information on the Computer Laboratory’s graduate association.

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network and Xen problems

A misconfiguration in the networking kit caused an outage yesterday afternoon. This caused many Linux Xen VMs to loose access to their root filesystems, so they mounted them read-only. After rebooting some of them, one of the Xen Servers lost the ability to reboot VMs, so itself had to be rebooted, causing all its VMs to crash. Many of the filesystems were deemed “unfixable” by fsck, so fsck had to be manually run before the systems rebooted.
Please let sys-admin know of any remaining problems.

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Java 8 in Action, co-authored by the Lab’s Prof Alan Mycroft and Raoul-Gabriel Urma, released

Java 8 in Action, co-authored by the Computer Laboratory’s Professor Alan Mycroft and PhD student Raoul-Gabriel Urma, along with Mario Fusco, has been released.

The book is a clearly written guide to the new features of Java 8. The book covers lambdas, streams, and functional-style programming.

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new slogin-otpw server – generate a new pad

mgk25 has just released version 1.5 of OTPW which does not need to use $HOME for the pad. This allows per-host local pads, removing the problems on NFS sec=krb5 systems. The DNS name slogin-otpw now points at the testing slogin server.
Any users need to generate a new pad by logging in and running cl-otpw-gen.

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Ubuntu 10.04 lucid libc bug

A libc bug involving nscd may cause some Ubuntu 10.04 systems to fail in various ways. The quick fix is to run “cl-asuser service nscd stop” and “cl-update-system” to install a fixed libc. Ubuntu 10.04 is getting a bit old now, and 14.04 LTS is stable – it is straightforward to install a fresh Ubuntu 14.04 onto a Lab system while it is in use, and the reboot into the new system – email sys-admin if you would like your machine upgraded.

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Computer Laboratory hosts 2014 GNU Tools Cauldron

The 2014 GNU Tools Cauldron was recently held at the Computer Laboratory (July 18-20). The event brought together researchers and developers working on the GNU Tool Chain (the GNU Compiler Collection (GCC), the GNU Debugger (GDB) and the GNU binary utilities) and was the largest ever such meeting, with approximately 150 attendees from Universities and companies around the world.

The sessions were inherently practical in their nature – it being a meeting for working engineers – and talks represented the leading edge in compiler design.

Thanks to sponsorship from several companies and organizations, the event was free to attend, making it an attractive opportunity for students and hobbyists, as well as the professionals who made up the bulk of the participants.

The event was organized by Embecosm (a Hall of Fame company) and Linaro. Dr David Chisnall hosted the event on behalf of the Computer Laboratory.

As well as the support from the Computer Laboratory, the sponsors included Mentor Graphics, IBM, ARM (another Hall of Fame company), Google, Red Hat and the Knowledge Transfer Network (KTN). The support of the UK Technology Strategy Board through the KTN underlines the leading role the UK plays in compiler development.

The event at the Computer Laboratory was the second major compiler meeting to be held in the UK this year.

All the talks were videoed, and will shortly be available, along with the slide presentations at

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