Re: [PATCH 0/5] kernel hacking: GCC optimization for debug experience (-Og)

[Du, Changbin]

On Wed, May 02, 2018 at 09:33:15AM +0200, Ingo Molnar wrote:
> 
> * changbin.du@xxxxxxxxx <changbin.du@xxxxxxxxx> wrote:
> 
> > Comparison of system performance: a bit drop.
> > 
> >     w/o CONFIG_DEBUG_EXPERIENCE
> >     $ time make -j4
> >     real    6m43.619s
> >     user    19m5.160s
> >     sys     2m20.287s
> > 
> >     w/ CONFIG_DEBUG_EXPERIENCE
> >     $ time make -j4
> >     real    6m55.054s
> >     user    19m11.129s
> >     sys     2m36.345s
> 
> Sorry, that's not a proper kbuild performance measurement - there's no noise 
> estimation at all.
> 
> Below is a description that should produce more reliable numbers.
> 
> Thanks,
> 
> 	Ingo
>
Thanks for your suggestion, I will try your tips to eliminate noise. Since it is
tested in KVM guest, so I just reboot the guest before testing. But in host side
I still need to consider these noises.

> 
> =========================>
> 
> So here's a pretty reliable way to measure kernel build time, which tries to avoid 
> the various pitfalls of caching.
> 
> First I make sure that cpufreq is set to 'performance':
> 
>   for ((cpu=0; cpu<120; cpu++)); do
>     G=/sys/devices/system/cpu/cpu$cpu/cpufreq/scaling_governor
>     [ -f $G ] && echo performance > $G
>   done
> 
> [ ... because it can be *really* annoying to discover that an ostensible 
>   performance regression was a cpufreq artifact ... again. ;-) ]
> 
> Then I copy a kernel tree to /tmp (ramfs) as root:
> 
> 	cd /tmp
> 	rm -rf linux
> 	git clone ~/linux linux
> 	cd linux
> 	make defconfig >/dev/null
> 	
> ... and then we can build the kernel in such a loop (as root again):
> 
>   perf stat --repeat 10 --null --pre			'\
> 	cp -a kernel ../kernel.copy.$(date +%s);	 \
> 	rm -rf *;					 \
> 	git checkout .;					 \
> 	echo 1 > /proc/sys/vm/drop_caches;		 \
> 	find ../kernel* -type f | xargs cat >/dev/null;  \
> 	make -j kernel >/dev/null;			 \
> 	make clean >/dev/null 2>&1;			 \
> 	sync						'\
> 							 \
> 	make -j16 >/dev/null
> 
> ( I have tested these by pasting them into a terminal. Adjust the ~/linux source 
>   git tree and the '-j16' to your system. )
> 
> Notes:
> 
>  - the 'pre' script portion is not timed by 'perf stat', only the raw build times
> 
>  - we flush all caches via drop_caches and re-establish everything again, but:
> 
>  - we also introduce an intentional memory leak by slowly filling up ramfs with 
>    copies of 'kernel/', thus continously changing the layout of free memory, 
>    cached data such as compiler binaries and the source code hierarchy. (Note 
>    that the leak is about 8MB per iteration, so it isn't massive.)
> 
> With 10 iterations this is the statistical stability I get this on a big box:
> 
>  Performance counter stats for 'make -j128 kernel' (10 runs):
> 
>       26.346436425 seconds time elapsed    (+- 0.19%)
> 
> ... which, despite a high iteration count of 10, is still surprisingly noisy, 
> right?
> 
> A 0.2% stddev is probably not enough to call a 0.7% regression with good 
> confidence, so I had to use *30* iterations to make measurement noise to be about 
> an order of magnitude lower than the effect I'm trying to measure:
> 
>  Performance counter stats for 'make -j128' (30 runs):
> 
>       26.334767571 seconds time elapsed    (+- 0.09% )
> 
> i.e. "26.334 +- 0.023" seconds is a number we can have pretty high confidence in, 
> on this system.
> 
> And just to demonstrate that it's all real, I repeated the whole 30-iteration 
> measurement again:
> 
>  Performance counter stats for 'make -j128' (30 runs):
> 
>       26.311166142 seconds time elapsed    (+- 0.07%)
> 

-- 
Thanks,
Changbin Du