kernel_compiler_patch

This patch adds additional optimization/tuning for kernel builds by adding more micro-architectures options accessible under:

 Processor type and features  --->
 Processor family --->

Why a specific patch?

The kernel uses its own set of CFLAGS, KCFLAGS. For example, see:

• arch/x86/Makefile
• arch/x86/Makefile_32.cpu
• arch/x86/Kconfig.cpu

Alternative way to define a -march= option without this patch

As pointed out by codemac in this topic, one can simply export the value/values for the KCFLAGS and KCPPFLAGS before calling make to achieve the same result, see here.

export KCFLAGS=' -march=znver3 -mtune=znver3'
export KCPPFLAGS=' -march=znver3 -mtune=znver3'
make all

Expanded CPUs include

Benchmarks

Intro

Three different machines running a generic x86-64 kernel and an otherwise identical kernel running with the optimized gcc options were tested using a make based endpoint.

Conclusion

There are small but real speed increases to running with this patch as judged by a make endpoint. The increases are on par with the speed increase that the upstream sanctioned core2 option gives users, so not including additional options seems somewhat arbitrary to me.

Details

1. Three test machines: Intel Xeon X3360, Intel i7-2620M, Intel Core i7-3660K.
2. All ran the make benchmark (linked below) 35 times while booted into a 'generic' kernel. Then all ran the same make benchmark 35 times after booting into an optimized kernel. Below are the optimizations chosen for each machine.
   • X3360 = core2
   • i7-2620M = sandybridge
   • i7-3660K = ivybridge
3. Results were analyzed for statistical significance via ANOVA plots that clearly show statistically significant albeit small differences.

Discussion

1. All the assumptions for ANOVA are met:
   • Data are normally distributed as show in the normal quantile plots.
   • The population variances are fairly equal (Levene and Barlett tests).
2. The ANOVA plots clearly show significance.
   • Pair-wise analysis by Tukey-Kramer shows significance at the 0.05 level for all CPUs compared.

Below are the differences in median values:

References

• Bash script that controls the benchmark: https://github.com/graysky2/bin/blob/master/bench
• Log file generated by script: http://repo-ck.com/bench/compile_time_optimization.txt.gz

Credit

• Original author: jeroen AT linuxforge DOT net
• Link to original version: http://www.linuxforge.net/docs/linux/linux-gcc.php

Legacy support

Find support for older version of the linux kernel and of gcc in the outdated_versions directory.

Data

Sandybridge vs. Generic

Ivybridge vs. Generic

Core2 vs. Generic