Exploring O3 optimization for Ubuntu

Following our recent work 5 with Ubuntu 24.04 LTS where we enabled frame pointers by default to improve debugging and profiling, we're continuing our performance engineering efforts by evaluating the impact of O3 optimization in Ubuntu.

O3 is a GCC optimization 14 level that applies more aggressive code transformations compared to the default O2 level. These include advanced function and the use of sophisticated algorithms aimed at enhancing execution speed. While O3 can increase binary size and compilation time, it has the potential to improve runtime performance.

Ubuntu Discourse

If these optimisations deliver performance improvements, and the only downside is larger binaries and longer compilation times, it seems like a bit of a no-brainer to enable these, assuming those mentioned downsides are within reason. Are there any downsides they're not mentioning? Browsing around and doing some minor research it seems that -O3 optimisations may break some packages, and can even lead to performance degradation, defeating the purpose altogether.

Looking at a set of benchmarks from Phoronix from a few years ago, in which the Linux kernel was compiled with either O2 and O3 and their performance compared, the results were effectively tied, making it seem not worth it at all. However, during these benchmarks, only the kernel was tested; everything else was compiled normally in both cases. Perhaps compiling the entire system with O3 will yield improvements in other parts of the system that do add up.

For now, you can download unsupported Ubuntu ISOs compiled with O3 optimisations enabled to test them out.