The seven second kernel compile
16 comments
·October 27, 2025metanonsense
I remember back in 2000 or so when I declined the invitation to a party because I wanted to compile a new kernel in the evening.
ok123456
Did it compile?
bicolao
It's 2000. Build failure was pretty much expected for any software. Probably a good idea to stay home and work through any problem. Nowadays you'll just fire up a build and go. And the build is probably finished before you're out of the door.
svara
The way I remember it the Linux kernel compiled really reliably back then. It would take a few hours though.
MisterTea
I just measured and built the latest 9front AMD64 kernel in 15.4 seconds using a Celeron J1900 with a SATA SSD. I also posted this from 9front.
systemswizard
Please append the date on old posts like this
ok123456
I remember starting a 1.2 kernel compile on my 486 with 4 MB of RAM, going to bed, then going to school, and finding that it had finished when I came back home.
null
emil-lp
One the one hand, we have Moore's law. On the other hand, kernel compilation time. Since compilation time is monotonically increasing, do we observe exponential compilation complexity in the kernel?
btilly
Yes. See https://accu.org/journals/overload/14/71/miller_2004/ for one reason why compilation time can easily become exponential.
In many organizations, compilation time tends to hover around a benchmark of "this is acceptable." If it is below that benchmark, nobody pays attention to performance. If it is above, someone fixes something.
In multiple interviews Linus Torvalds has said that this benchmark is about 10 minutes for him. But considering that his personal hardware gets better faster than Moore's law alone, that means that compiles get slower for the rest of us.
panki27
What are compile times like right now, with modern hardware?
zamadatix
Phoronix includes a "Timed Linux Kernel Compilation" test as part of their reviews using the default build config.
Here is one comparing some modern high end server CPUs: https://www.phoronix.com/benchmark/result/amd-5th-gen-epyc-9... (2P = dual socket)
Here is one comparing some modern consumer CPUs: https://www.phoronix.com/benchmark/result/amd-ryzen-9-9900x-...
Searching "Phoronix ${cpuModel}" will take you to the full review for that model, along with the rest of the build specs.
With the default build in a standard build environment the clock speed tends to matter more. With tuning one could probably squeeze more out of the higher core count systems.
zrm
Note that those two links are using different configs. Here's the link for Threadripper 9995WX:
https://www.phoronix.com/review/amd-threadripper-9995wx-trx5...
That's using the same config as the server systems (allmodconfig) but it has the 9950X listed there and on that config it takes 547.23 seconds instead 47.27. That puts all of the consumer CPUs as slower than any of the server systems on the list. You can also see the five year old 2.9GHz Zen2 Threadripper 3990X in front of the brand new top of the range 4.3GHz Zen5 9950X3D because it has more cores.
You can get a pretty good idea of how kernel compiles scale with threads by comparing the results for the 1P and 2P EPYC systems that use the same CPU model. It's generally getting ~75% faster by doubling the number of cores, and that's including the cost of introducing cross-socket latency when you go from 1P to 2P systems.
dripton
It varies a lot depending on how much you have enabled. The distro kernels that are designed to support as much hardware as possible take a long time to build. If you make a custom kernel where you winnow down the config to only support the hardware that's actually in your computer, there's much less code to compile so it's much faster.
I recently built a 6.17 kernel using a full Debian config, and it took about an hour on a fast machine. (Sorry, I didn't save the exact time, but the exact time would only be relevant if you had the exact same hardware and config.) I was surprised how slow it still was. It appears the benefits of faster hardware have been canceled by the amount of new code added.
Back in pre-module days, Slackware shipped with "big" kernel with lots of drivers compiled in. The advantage was that this way the kernel could boot on a wide range of hardware. But it was very bloated (for the time) and the users were expected to recompile the kernel with unnecessary drivers removed. I remember compiling it on Pentium 60 with 16MB of RAM. Took 1-2 hours or so.