On Thu, Oct 07, 2021 at 03:59:08PM +0200, Greg Kroah-Hartman wrote: > On Thu, Oct 07, 2021 at 02:51:15PM +0300, Andy Shevchenko wrote: > > On Thu, Oct 7, 2021 at 1:34 PM Greg Kroah-Hartman > > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: ... > > Meanwhile, Thorsten, can you have a look at my approach and tell if it > > makes sense? > > No, do not use ccache when trying to benchmark the speed of kernel > builds, that tests the speed of your disk subsystem... First rule of the measurement is to be sure WHAT we are measuring. And I'm pretty much explained WHAT and HOW. On the other hand, the kcbench can't answer to the question about C preprocessing speed without help of ccache or something similar. Measuring complete build is exactly not what we want because of O(compilation) vs. o(C preprocessing) meaning that any fluctuation in the former makes silly to measure anything from the latter. You see, my theory is proved by practical experiment: $ kcbench -i 3 -j 64 -o $O -s $PWD --no-download -m Processor: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz [88 CPUs] Cpufreq; Memory: powersave [intel_pstate]; 128823 MiB Linux running: 5.6.0-2-amd64 [x86_64] Compiler: gcc (Debian 10.3.0-11) 10.3.0 Linux compiled: 5.15.0-rc4 Config; Environment: allmodconfig; CCACHE_DISABLE="1" Build command: make vmlinux modules Filling caches: This might take a while... Done Run 1 (-j 64): 464.07 seconds / 7.76 kernels/hour [P:6001%] Run 2 (-j 64): 464.64 seconds / 7.75 kernels/hour [P:6000%] Run 3 (-j 64): 486.41 seconds / 7.40 kernels/hour [P:5727%] $ kcbench -i 3 -j 64 -o $O -s $PWD --no-download -m Processor: Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz [88 CPUs] Cpufreq; Memory: powersave [intel_pstate]; 128823 MiB Linux running: 5.6.0-2-amd64 [x86_64] Compiler: gcc (Debian 10.3.0-11) 10.3.0 Linux compiled: 5.15.0-rc4 Config; Environment: allmodconfig; CCACHE_DISABLE="1" Build command: make vmlinux modules Filling caches: This might take a while... Done Run 1 (-j 64): 462.32 seconds / 7.79 kernels/hour [P:6009%] Run 2 (-j 64): 462.33 seconds / 7.79 kernels/hour [P:6006%] Run 3 (-j 64): 465.45 seconds / 7.73 kernels/hour [P:5999%] In [41]: numpy.median(y1) Out[41]: 464.64 In [42]: numpy.median(y2) Out[42]: 462.33 Speedup: +0.5% -- With Best Regards, Andy Shevchenko
