AMD graphics performance regression in 4.15 and later

[Christian König]

Hi Jean,

found the bug reports.

Here is the original bug report from the kernel: 
https://bugzilla.kernel.org/show_bug.cgi?id=198511

And here is an fdo bug report where we tried to investigate the root 
cause, but didn't had time for that yet: 
https://bugs.freedesktop.org/show_bug.cgi?id=105038

Regards,
Christian.

Am 06.04.2018 um 10:03 schrieb Christian König:
> Hi Jean,
>
> yeah, that is a known problem. Using huge pages improves the 
> performance because of better TLB usage, but for the cost of higher 
> allocation overhead.
>
> What we found is that firefox is doing something rather strange by 
> allocating large textures and then just trowing them away again 
> immediately.
>
> We mitigated the problem by avoiding the slow coherent DMA code path 
> on almost all platforms on newer kernels, but essentially somebody 
> needs to figure out why firefox and/or the user space stack is doing 
> this constant allocation/freeing of memory.
>
> There is also a bug tracker on bugs.kernel.org about this, but I can't 
> find it any more of hand.
>
> Regards,
> Christian.
>
> Am 06.04.2018 um 02:30 schrieb Jean-Marc Valin:
>> Hi,
>>
>> I noticed a serious graphics performance regression between 4.14 and
>> 4.15. It is most noticeable with Firefox (tried FF57 through FF60) and
>> causes scrolling to be really choppy/sluggish. I've confirmed that the
>> problem is also there on 4.16, while 4.13 works fine.
>>
>> After a bisection, I've narrowed the regression down to this commit:
>>
>> commit 648bc3574716400acc06f99915815f80d9563783
>> Author: Christian König <christian.koenig at amd.com>
>> Date:   Thu Jul 6 09:59:43 2017 +0200
>>
>>      drm/ttm: add transparent huge page support for DMA allocations v2
>>
>>
>> Some details about my system:
>> Distro: Fedora 27 (up-to-date)
>> Video: MSI Radeon RX 560 AERO
>> CPU: Dual-socket Xeon E5-2640 v4 (20 cores total)
>> RAM: 128 GB ECC
>>
>>
>> As a comparison, when running Firefox with 4.15 on a Lenovo W540 laptop
>> (with Intel graphics only) the responsiveness is much better then what
>> I'm getting on the Xeon machine above with the Radeon card, so this
>> really seems to be an AMD-only issue.
>>
>> Any way to fix the issue?
>>
>> Thanks,
>>
>>     Jean-Marc
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>