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Date:   Sat, 29 Aug 2020 20:06:04 +0200
From:   Thomas Zimmermann <tzimmermann@...e.de>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        kernel test robot <rong.a.chen@...el.com>
Cc:     John Donnelly <John.p.donnelly@...cle.com>,
        Emil Velikov <emil.velikov@...labora.com>,
        LKML <linux-kernel@...r.kernel.org>, lkp@...ts.01.org,
        lkp@...el.com, ying.huang@...el.com, feng.tang@...el.com,
        zhengjun.xing@...el.com
Subject: Re: [drm/mgag200] 913ec479bb: vm-scalability.throughput 26.2%
 improvement

Hi

Am 27.08.20 um 16:56 schrieb Mike Kravetz:
> On 8/27/20 2:16 AM, Thomas Zimmermann wrote:
>> Hi
>>
>> Am 26.08.20 um 10:58 schrieb kernel test robot:
>>> Greeting,
>>>
>>> FYI, we noticed a 26.2% improvement of vm-scalability.throughput due to commit:
>>
>> I guess this resolves the once-measured performance penalty of similar
>> magnitude. But do we really understand these tests? When I sent out
>> patches to resolve the problem, nothing changed. And suddenly the
>> performance is back to normal.
>>
>> Best regards
>> Thomas
>>
>>>
>>>
>>> commit: 913ec479bb5cc27f99f24d5fd111b3ef29a4deb9 ("drm/mgag200: Replace VRAM helpers with SHMEM helpers")
>>> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git master
>>>
>>>
>>> in testcase: vm-scalability
>>> on test machine: 288 threads Intel(R) Xeon Phi(TM) CPU 7295 @ 1.50GHz with 80G memory
>>> with following parameters:
>>>
>>> 	runtime: 300s
>>> 	size: 8T
>>> 	test: anon-cow-seq-hugetlb
>>> 	cpufreq_governor: performance
>>> 	ucode: 0x11
> 
> Hello Thomas,
> 
> Did drm changes really impact anon-cow-seq-hugetlb performance?
> 
> My change c0d0381ade79 ("hugetlbfs: use i_mmap_rwsem for more pmd sharing
> synchronization") caused a -33.4% regression of anon-cow-seq-hugetlb.  A
> recent change 34ae204f185 (hugetlbfs: remove call to huge_pte_alloc without
> i_mmap_rwsem) was tested by Zhengjun Xing and improved performance by 20
> something percent.  That seems in line with this report/improvement.

Some of DRM's memory management might be affected by hugetable changes.
While I cannot really point to a specific location, it's not impossible
that there's a connection.

> 
> Perhaps the tooling is not always accurate in determining the commit which
> causes the performance changes?
> Perhaps I am misreading information in the reports?
> 

From what I remember, some of these tests print to the console, which
has always been slow, and has generally been a bad idea for performance
tests. I guess these tests are not very accurate.

Best regards
Thomas

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer



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