[OE-core] Speed regression in the 4.8 kernel?

[Bruce Ashfield]

On 2016-09-07 10:00 AM, Markus Lehtonen wrote:
> 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?

Sure enough, there's a missing series in the 4.8 and master branches
of the kernel meta repo.

I'm going to do a full audit and will send meta data updates once I've
ensured nothing else is missing.

Bruce

>
> 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
>>>
>>
>