• Linux

    Linux 5.0 Netfilter bug

    On two desktop systems running Debian Buster with Linux kernel version 5.0.7, I was experiencing a problem when Shorewall6 was stopping or restarting. This kernel backtrace appeared in the logs: On one of the two systems, this would result in the system failing to shut down properly: the kernel would hang completely when trying to shut down. The problem is known, and can be fixed by this patch, which has been queued in the stable 5.0 tree. It will hopefully be included in the 5.0.8 version.

  • Linux

    Linux security hardening recommendations

    In a previous blog post, I wrote how to secure OpenSSH against brute force attacks. However, what if someone manages to get a shell on your system, despite all your efforts? You want to protect your system from your users doing nasty things? It is important to harden your system further according to the principle of defense in depth in order. Software updates Make sure you are running a supported distribution, and by preference the most recent version one. For example, Debian Jessie is still supported, however upgrading to Debian Stretch is strongly recommended, because it offers various security improvements (more recent kernel with new security hardening, PHP 7 with…

  • Linux

    Debian Stretch on AMD EPYC (ZEN) with an NVIDIA GPU for HPC

    Recently at work we bought a new Dell PowerEdge R7425 server for our HPC cluster. These are some of the specifications: 2 AMD EPYC 7351 16-Core Processors 128 GB RAM (16 DIMMs of 8 GB) Tesla V100 GPU Our FAI configuration automatically installed Debian stretch on it without any problem. All hardware was recognized and working. The installation of the basic operating system took less than 20 minutes. FAI also sets up Puppet on the machine. After booting the system, Puppet continues setting up the system: installing all needed software, setting up the Slurm daemon (part of the job scheduler), mounting the NFS4 shared directories, etc. Everything together, the system…

  • Linux

    Leap second causing ksoftirqd and java to use lots of cpu time

    Today there was a leap second at 23:59:60 UTC. On one of my systems, this caused a high CPU load starting from around 02h00 GMT+2 (which corresponds with the time of the leap second). ksoftirqd and some java (glassfish) process where using lots of CPU time. This system was running Debian Squeeze with kernel 2.6.32-45. The problem is very easy to fix: just run # date -s "`date`" and everything will be fine again. I found this solution on the Linux Kernel Mailing List: http://marc.info/?l=linux-kernel&m=134113389621450&w=2. Apparently a similar problem can happen with Firefox, Thunderbird, Chrome/Chromium, Java, Mysql, Virtualbox and probably other processes. I was a bit suprised that this problem…

  • Linux

    Linux performance improvements

    Two years ago I wrote an article presenting some Linux performance improvements. These performance improvements are still valid, but it is time to talk about some new improvements available. As I am using Debian now, I will focus on that distribution, but you should be able to easily implement these things on other distributions too. Some of these improvements are best suited for desktop systems, other for server systems and some are useful for both.

  • Linux,  Uncategorized

    Linux kernel: The battle of the CPU schedulers

    Since some time already, different patches are being written for the Linux kernel, which improve the CPU scheduler. The CPU scheduler, is that part of the kernel, that’s responsible for assigning CPU time to the different task running on your system. If you sometimes experience problems with sound stuttering or your mouse becoming jerky while running other CPU intensive tasks, then this is definitely a problem caused by the task scheduler. Con Kolivas has been maintaining an alternative scheduler for some time. His Staircase scheduler was designed with interactivity in mind, especially for desktop systems, where people want their system to be quickly responsive under all kinds of workloads. This…

  • Linux,  Mandriva

    Struggling with Linux’ OOM killer when building RPMs

    Last two weeks, I have created a lot of updated packages for Mandriva 2007.1. I packaged Gnome 2.18.1, and also updated subversion snaphots of kdepim and kdegraphics. Kdepim, because it has received a lot of bug fixing love the last two months, and kdegraphics, because it contains kpdf using new xpdf code, which should be compatible with PDF 1.6 and 1.7 specifications. In kdepim, they also removed the kitchensynk tool, which offered synchronization options with external devices. Apparently it was too buggy to be really useful. I have the impression that Kmail is indeed also more stable than in 3.5.6. I could not reproduce yet the hangs I sometimes experienced…