Re: Speed regression in the 4.8 kernel?

[Markus Lehtonen <markus.lehtonen@linux.intel.com>]

On Wed, 2016-09-07 at 07:56 -0400, Bruce Ashfield wrote:
> On 2016-09-07 5:27 AM, Richard Purdie wrote:
> > Hi Bruce,
> > 
> > I deliberately spaced out the merges of various things so we could get
> > performance measurements of the system as it happened. Unfortunately
> > the 4.8 kernel appears to regress the kernel build time quite
> > significantly:
> > 
> > The raw data:
> > 
> > ypperf02,master:9428b19a7dd1d265d9f3211004391abe33ea0224,uninative-1.3
> > -414
> > -g9428b19,1:01:32,4:21.16,1:00:32,2:10.86,0:16.19,0:11.21,0:01.20,5:34.
> > 73,26701616,6445160,1477762,5446116
> > ypperf02,master:9428b19a7dd1d265d9f3211004391abe33ea0224,uninative-1.3
> > -414
> > -g9428b19,1:04:14,4:23.82,1:00:40,2:13.70,0:16.18,0:11.28,0:01.22,5:45.
> > 48,26697516,6445724,1478048,5446490
> > ypperf02,master:b9d90ace005597ba35b59adcd8106a1c52e40c1a,uninative-1.3
> > -438
> > -gb9d90ac,1:03:16,7:22.13,1:02:46,2:16.60,0:16.32,0:11.04,0:01.21,5:42.
> > 11,30852876,10550952,1707255,5912282
> > ypperf02,master:f7ca989ddc6d470429b547647d3fbaad83a982d9,uninative-1.3
> > -446
> > -gf7ca989,1:04:42,7:29.05,1:03:04,2:19.71,0:16.21,0:11.05,0:01.24,5:52.
> > 83,30845748,10551316,1707615,5912122
> > 
> > which shows the time for "bitbake virtual/kernel -c cleansstate; time
> > bitbake virtual/kernel" goes from 4:20 to 7:22. The disk footprint of
> > the build went from 26GB to 30GB. The build with rm_work size went from
> > 6.4GB to 10.5GB. The overall build time went up 2-3 minutes which looks
> > like the increased kernel build time. The increased time may well be
> > from the increased data being generated/processed.
> 
> Is it the actual compile itself ? What's the trick to actually get
> individual task
> 
> For the disk footprint, I can check the refs in the tree and purge
> anything that may be dangling. That being said, I can't do that to
> the repository on the git server, so we may need someone that can
> issue a git gc directly on the repository.
> 
> > 
> > We can't ship M3 with this much of a performance degradation and
> > increased space usage :(. Any idea what changed?
> 
> Nope. I can only focus on one thing at a time. I was worried about
> a functionally correct kernel (which I still am) and haven't looked
> at anything in the peripheral yet.
> 
> If I can get individual task timings, I can look at it more.
> 
> I'm seeing significantly faster meta data phases, etc, so I'm interested
> to know if this is purely in the build steps.

In my own test setup I'm seeing similar increase in kernel build time.

Comparing the buildstats of kernel builds from before and after the 4.8 I
got the following numbers (these are cpu times consumed by the tasks):

TASK                      ABSDIFF   RELDIFF  CPUTIME1    CPUTIME2
do_package                 +17.5s   +133.1%     13.1s -> 30.6s
do_deploy                  +19.9s  +1449.4%      1.4s -> 21.3s
do_package_write_rpm       +86.8s   +714.7%     12.1s -> 98.9s
do_compile_kernelmodules  +139.4s    +35.9%    388.2s -> 527.6s
do_compile                +201.1s    +30.0%    670.3s -> 871.4s


I haven't tried to analyze what is causing this yet, though.


Thanks,
  Markus