Building OPAL firmware for POWER9

Recently, we merged into the op-build project (the build scripts for OpenPOWER Firmware) a defconfig for building OPAL for (certain) POWER9 simulators. I won’t bother linking over to articles on the POWER9 chip or schedule (there’s search engines for that), but with this commit – if you happen to be able to get your hands on a POWER9 simulator, you can now boot to the petitboot bootloader on it!

We’re using upstream Linux 4.7.0-rc3 and upstream skiboot (master), so all of this code is already upstream!

Now, by no means is this complete. There’s some fairly fundamental things that are missing (e.g. PCI) – but how many other platforms can you build open source firmware for before you can even get your hands on a simulator?

The sad state of MySQL and NUMA

Way back in 2010, MySQL Bug 57241 was filed, pointing out that the “swap insanity” problem was getting serious on x86 systems – with NUMA being more and more common back then.

The swapping problem is due to running out of memory on a NUMA node and having to swap things to other nodes (see Jeremy Cole‘s blog entry also from 2010 on the topic of swap insanity). This was back when 64GB and dual quad core CPUs was big – in the past five years big systems have gotten bigger.

Back then there were two things you could do to have your system be usable: 1) numa=off as kernel boot parameter (this likely has other implications though) and 2) “numactl –interleave all” in mysqld_safe script (I think MariaDB currently has this built in if you set an option but I don’t think MySQL does, otherwise perhaps the bug would have been closed).

Anyway, it’s now about 5 years since this bug was opened and even when there’s been a patch in the Twitter MySQL branch for a while (years?) and my Oracle Contributor Agreement signed patch attached to bug 72811 since May 2014 (over a year) we still haven’t seen any action.

My patch takes the approach of you want things allocated at server startup to be interleaved across nodes (e.g. buffer pool) while runtime allocations are probably per connection and are thus fine (in fact, better) to do node local allocations.

Without a patch like this, or without running mysqld with the right numactl incantation, you end up either having all your memory on one NUMA node (potentially not utilising full memory bandwidth of the hardware), or you end up with swap insanity, or you end up with some other not exactly what you’d expect situation.

While we could have MySQL be more NUMA aware and perhaps do a buffer pool instance per NUMA node or some such thing, it’s kind of disappointing that for dedicated database servers bought in the past 7+ years (according to one comment on one of the bugs) this crippling issue hasn’t been addressed upstream.

Just to make it even more annoying, on certain workloads you end up with a lot of mutex contention, which can end up meaning that binding MySQL to fewer NUMA nodes (memory and CPU) ends up increasing performance (cachelines don’t have as far to travel) – this is a different problem than swap insanity though, and one that is being addressed.

Update: My patch as part of has been merged! MySQL on NUMA machines just got a whole lot better. I just hope it’s enabled by default…

skiboot/OPAL versioning

skiboot is boot and runtime firmware for OpenPower systems. There are other components that make up all the firmware you need, but if you’re, say, a Linux kernel, you’re going to be interacting with skiboot.

I recently committed doc/versioning.txt to skiboot to try and explain our current thoughts on versioning releases.

It turns out that picking version numbers is a bit harder than you’d expect, especially when you want to construct a version string to display in places that has semantic meaning. In fact, the writing on Semantic Versioning influenced us heavily.

Since we’re firmware, making incompatible API changes is something we should basically never, ever do. Old kernels should must boot and work on new firmware and new kernels should boot and function on old firmware (and if they don’t, it plainly be a kernel bug). So, ignore the Major version parts of Semantic Versioning for us :)

For each new release, we plan to bump the minor version for mostly bug fix releases, while bump the major version for added functionality. Any additional information is to describe the version on that particular platform – as everybody shipping OPAL is likely to build it themselves with possibly some customizations (e.g. YOUR COMPANY NAME HERE, support for some on board RAID card or on-board automated coffee maker). See doc/versioning.txt for details.

You may wonder why we started at 4.0 for our first real version number. Well… this is purely a cunning plan to avoid confusion with other things, the details of which will only be extracted out of my when plied with a suitable amount of excellent craft beer (because if I’m going to tell a boring story, I may as well have awesome craft beer).