Zack's Kernel News
Zack's Kernel News

Zack Brown reports on improving spinlock efficiency and adding up a few extra bytes.
Improving Spinlock Efficiency
Alex Kogen, from Oracle, wanted to eke out a little more efficiency from spinlocks. Locks are part of any multitasking operating system – they make sure only one process can access any given system resource at a time. Spinlocks are locks that just loop around and around waiting for one process to let go of a resource so the next process can claim it. There are lots of different kinds of locks, depending on a ton of different special contexts. For example, an MCS lock is a particular type of spinlock named after the people who first described it: Mellor-Crummey and Scott.
In this case, Alex said, "In CNA [compact NUMA-aware], spinning threads are organized in two queues, a primary queue for threads running on the same node as the current lock holder, and a secondary queue for threads running on other nodes. After acquiring the MCS lock and before acquiring the spinlock, the MCS lock holder checks whether the next waiter in the primary queue (if it exists) is running on the same NUMA [non-uniform memory access] node. If it is not, that waiter is detached from the main queue and moved into the tail of the secondary queue. This way, we gradually filter the primary queue, leaving only waiters running on the same preferred NUMA node."
Alex was talking about having spinlocks be aware of the special context of the threads waiting for the resource they guard. Specifically, on systems with more than one CPU (which is pretty much all of them these days), threads on any CPU might be waiting for resources such as RAM on one specific CPU. But whenever a thread uses a resource on a different CPU from its own, access will be slower than a thread accessing that resource on its same CPU. Also, different CPUs may have RAM that's inherently faster or slower than RAM on another CPU. Those are called NUMA systems.
[...]
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.

News
-
System76 Releases COSMIC Alpha 7
With scores of bug fixes and a really cool workspaces feature, COSMIC is looking to soon migrate from alpha to beta.
-
OpenMandriva Lx 6.0 Available for Installation
The latest release of OpenMandriva has arrived with a new kernel, an updated Plasma desktop, and a server edition.
-
TrueNAS 25.04 Arrives with Thousands of Changes
One of the most popular Linux-based NAS solutions has rolled out the latest edition, based on Ubuntu 25.04.
-
Fedora 42 Available with Two New Spins
The latest release from the Fedora Project includes the usual updates, a new kernel, an official KDE Plasma spin, and a new System76 spin.
-
So Long, ArcoLinux
The ArcoLinux distribution is the latest Linux distribution to shut down.
-
What Open Source Pros Look for in a Job Role
Learn what professionals in technical and non-technical roles say is most important when seeking a new position.
-
Asahi Linux Runs into Issues with M4 Support
Due to Apple Silicon changes, the Asahi Linux project is at odds with adding support for the M4 chips.
-
Plasma 6.3.4 Now Available
Although not a major release, Plasma 6.3.4 does fix some bugs and offer a subtle change for the Plasma sidebar.
-
Linux Kernel 6.15 First Release Candidate Now Available
Linux Torvalds has announced that the release candidate for the final release of the Linux 6.15 series is now available.
-
Akamai Will Host kernel.org
The organization dedicated to cloud-based solutions has agreed to host kernel.org to deliver long-term stability for the development team.