Re: Speed regression in the 4.8 kernel?

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

On Wed, 2016-09-07 at 08:44 -0400, Bruce Ashfield wrote:
> On 2016-09-07 8:33 AM, Markus Lehtonen wrote:
> > 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
> 
> ok. So as long as the significant increases aren't in do_kernel_metadata
> or do_patch (those two I've measured), we are dealing with something
> in the kernel build itself. I can deal with the footprint by inspecting
> the repo, but Kbuild is a tougher nut to crack.

I took a look at the kernel build directory size and there was a huge
difference (500M vs. 3.5G). Seeing that significant change I took a look at
the kconfigs and found out that CONFIG_DEBUG_KERNEL and CONFIG_DEBUG_INFO
are enabled, among other things. Looking at the kernel metadata I can see
that these are enabled in ktypes/base/base.cfg.

I guess this is not intended?

Thanks,
  Markus


> Out of curiosity, is the 4.4 kernel on master showing a significantly
> shorter build time ? I'm assuming the before is the 4.4 kernel .. but
> I just wanted to be sure, so we can rule out something else that might
> be pathologically reacting to the sheer amount of I/O in a kernel build.
> 
> Bruce
> 
> > 
> > 
> > I haven't tried to analyze what is causing this yet, though.
> > 
> > 
> > Thanks,
> >   Markus
> > 
>