Re: Speed regression in the 4.8 kernel?

[Bruce Ashfield <bruce.ashfield@windriver.com>]

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.

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
>