What is the Shortest Caching Time that Keeps Lighthouse / PageSpeed Insights happy?

Answer: 8337601 seconds. One second more than 96.5 days.

As previously mentioned, I have been working on SEO and marketing recently, and it is fascinating what insights one gains into the inner workings of things that power the digital world of today. One such example is figuring out the shortest cache time for Lighthouse / PageSpeed Insights to pass the audit.

To keep your Lighthouse / Google PageSpeed Insights happy, you should set your caching time to at least 8337601 seconds. In Apache config, that would mean adding a line like this:

Header always set Cache-Control "max-age=8337601, public"

Why would you want this to be as short as possible but long enough to keep Lighthouse happy? Well, you want to make sure that your page updates show up as quickly as possible and you don’t want stale content sticking around in the caches, reducing the effectiveness of the optimisations and improvements you make to your site all the time. At the same time, Google has said that page speed is now a ranking factor, and the algorithm they use is related to the one that Lighthouse uses.

So, if you want to set your caching time to a minimum possible time that seems to keep google happy – set it to 8337601 seconds.

Planet MySQL Controversy

Over the last few months, there has been a bit of controversy brewing around Planet MySQL’s apparent censorship of any posts that even mention a competing product, such as MariaDB, PostgreSQL or MongoDB. Given that an awful lot of tooling used in the industry supports multiple databases and not just MySQL, this is a pretty big deal and a number of prominent open source database specialists have voiced their concerns on the subject and withdrawn their posts from the Planet MySQL aggregator.

The good news is that there is now an alternative, open aggregator for open source database news at oursqlcommunity.org. Thank you, Jean-François Gagné.

Warm Reboot on Linux with kexec (Remember QEMM?)

If you are old enough to remember QEMM from back in the ’90s, along with other tools we used to squeeze every last byte of memory under the 640KB limit, you may remember a rather cool feature it had – warm reboot.

What is a Warm Reboot?

Reboot involves the computer doing a Power-On Self Test (POST). This takes time, often as much as a few minutes on some servers and workstations. While you are setting something up and need to test frequently that things come up correctly at boot time, the POST can make progress painfully slow. If only we had something like the warm reboot feature that QEMM had back in the ’90s, which allowed us to reset the RAM and reboot DOS without rebooting the entire machine and suffer the POST time. Well, such a thing does actually exist in modern Linux.

Enter kexec

kexec allows us to do exactly this – load a new kernel, kill all processes, and hand over control to the new kernel as the bootloader does at boot time. What do we need for this magic to work? On a modern distro, not much, it is all already included. Let’s start with a script that I use and explain what each component does:


systemctl isolate multi-user.target

rmmod nvidia_drm nvidia_modeset nvidia_uvm
rmmod nvidia

kexec --load=/boot/vmlinuz-$(uname -r) \
      --initrd=/boot/initramfs-$(uname -r).img \
      --command-line="$(cat /proc/cmdline)"

kexec --exec

Let’s look at the kexec lines first. uname -r returns the current kernel version. $(uname -r) bash syntax allows is to take the output of a command and use it as a string in the invoking command. On recent CentOS 8 here is what we get:

$ uname -r
$ echo $(uname -r)

The kernel and initial ramdisk usually have the kernel version in their names in /boot/:

$ ls /boot/

So in our warm reboot script, vmlinuz-$(uname -r) will expand to vmlinuz-4.18.0-193.6.3.el8_2.centos.plus.x86_64. Similar will happen with the initramfs file name.

Next, what is in /proc/cmdline ? This contains the boot parameters that our currently running kernel was booted with, as provided in our grub conifguration, for example:

$ cat /proc/cmdline
BOOT_IMAGE=(hd0,msdos2)/vmlinuz-4.18.0-193.6.3.el8_2.centos.plus.x86_64 root=ZFS=tank/ROOT quiet elevator=deadline transparent_hugepage=never

This is the minimum needed to boot the kernel. Once we have supplied this information, we initiate the shutdown and process purge, and hand over to the new kernel, using:

kexec --exec.

But what are the systemctl and rmmod lines about? They are mostly to work around finnickiness of Nvidia drivers and GPUs. If you execute kexec immediately, with the Nvidia driver still running, the GPU won’t reset properly and won’t get properly re-initialised by the driver when the kernel warm-boots. So we have to rmmod the nvidia driver. Legacy nvidia driver only includes the nvidia module. Newer versions also include nvidia_drm, nvidia_modeset and nvidia_uvm which depend on the nvidia module, so we have to remove those first. But before we do that, we have to make sure that Xorg isn’t running, otherwise we won’t be able to unload the nvidia driver. To make sure graphical environment isn’t running, we switch the runlevel target to multi-user.target (on a workstation we are probably running graphical.target by default). Once Xorg is no longer running, we can proceed with unloading the nvidia driver modules. And with that done, we can proceed with the warm boot and enjoy a reboot time saving.