[Mesa-dev] [PATCH] gallium/radeon: add a new HUD query for the number of mapped buffers

[Samuel Pitoiset]

On 01/25/2017 03:55 PM, Christian König wrote:
> Am 25.01.2017 um 15:19 schrieb Samuel Pitoiset:
>>
>>
>> On 01/25/2017 03:56 AM, Michel Dänzer wrote:
>>> On 25/01/17 12:05 AM, Marek Olšák wrote:
>>>> On Tue, Jan 24, 2017 at 2:17 PM, Christian König
>>>> <deathsimple at vodafone.de> wrote:
>>>>> Am 24.01.2017 um 11:44 schrieb Samuel Pitoiset:
>>>>>> On 01/24/2017 11:38 AM, Nicolai Hähnle wrote:
>>>>>>> On 24.01.2017 11:34, Samuel Pitoiset wrote:
>>>>>>>> On 01/24/2017 11:31 AM, Nicolai Hähnle wrote:
>>>>>>>>> On 24.01.2017 11:25, Samuel Pitoiset wrote:
>>>>>>>>>> On 01/24/2017 07:39 AM, Michel Dänzer wrote:
>>>>>>>>>>> On 24/01/17 05:44 AM, Samuel Pitoiset wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> Useful when debugging applications which map too much VRAM.
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> Is the number of mapped buffers really useful, as opposed to the
>>>>>>>>>>> total
>>>>>>>>>>> size of buffer mappings? Even if it was the latter though, it
>>>>>>>>>>> doesn't
>>>>>>>>>>> show which mappings are for BOs in VRAM vs GTT, does it?
>>>>>>>>>>> Also, even
>>>>>>>>>>> the
>>>>>>>>>>> total size of mappings of BOs currently in VRAM doesn't directly
>>>>>>>>>>> reflect
>>>>>>>>>>> the pressure on the CPU visible part of VRAM — only the BOs
>>>>>>>>>>> which are
>>>>>>>>>>> actively being accessed by the CPU contribute to that.
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> It's actually useful to know the number of mapped buffers, but
>>>>>>>>>> maybe
>>>>>>>>>> it
>>>>>>>>>> would be better to have two separate counters for GTT and VRAM.
>>>>>>>>>> Although
>>>>>>>>>> the number of mapped buffers in VRAM is most of the time very
>>>>>>>>>> high
>>>>>>>>>> compared to GTT AFAIK.
>>>>>>>>>>
>>>>>>>>>> I will submit in a follow-up patch, something which reduces
>>>>>>>>>> the number
>>>>>>>>>> of mapped buffers in VRAM (when a BO has been mapped only
>>>>>>>>>> once). And
>>>>>>>>>> this new counter helped me.
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> Michel's point probably means that reducing the number/size of
>>>>>>>>> mapped
>>>>>>>>> VRAM buffers isn't actually that important though.
>>>>>>>>
>>>>>>>>
>>>>>>>> It seems useful for apps which map more than 256MB of VRAM.
>>>>>>>
>>>>>>>
>>>>>>> True, if all of that range is actually used by the CPU (which may
>>>>>>> well
>>>>>>> happen, of course). If I understand Michel correctly (and this
>>>>>>> was news
>>>>>>> to me as well), if 1GB of VRAM is mapped, but only 64MB of that are
>>>>>>> regularly accessed by the CPU, then the kernel will migrate all
>>>>>>> of the
>>>>>>> rest into non-visible VRAM.
>>>>>>
>>>>>>
>>>>>> And this can hurt us, for example DXMD maps over 500MB of VRAM. And a
>>>>>> bunch of BOs are only mapped once.
>>>>>
>>>>>
>>>>> But when they are mapped once that won't be a problem.
>>>>>
>>>>> Again as Michel noted when a VRAM buffer is mapped it is migrated
>>>>> into the
>>>>> visible parts of VRAM on access, not on mapping.
>>>>>
>>>>> In other words you can map all your VRAM buffers and keep them
>>>>> mapped and
>>>>> that won't hurt anybody.
>>>>
>>>> Are you saying that I can map 2 GB of VRAM and it will all stay in
>>>> VRAM and I'll get maximum performance if it's not accessed by the CPU
>>>> too much?
>>>
>>> Yes, that's how it's supposed to work.
>>>
>>>
>>>> Are you sure it won't have any adverse effects on anything?
>>>
>>> That's a pretty big statement. :) Bugs happen.
>>>
>>>
>>>> Having useless memory mappings certainly must have some negative
>>>> effect on something. It doesn't seem like a good idea to have a lot of
>>>> mapped memory that doesn't have to be mapped.
>>>
>>> I guess e.g. the bookkeeping overhead might become significant with
>>> large numbers of mappings. Maybe the issue Sam has been looking into is
>>> actually related to something like that, not to VRAM?
>>
>> Well, with some games that new query can report more than 6.8k mapped
>> buffers (both VRAM/GTT) but a bunch are for VRAM. And more than 1GB of
>> mapped VRAM.
>>
>> When I look at the number of bytes moved by TTM, the counter is also
>> very high in these apps and most likely tied to the slowdowns. The
>> kernel memory manager is moving data almost all the time... Presumably
>> it's because of that aperture limit of 256MB.
>
> That is most likely an incorrect assumption. From experience I would
> rather expect that we move buffers in/out of VRAM because we run out of
> it during command submission.
>
> You should take a look at amdgpu_vram_mm and confirm that the visible
> usage is around it's maximum (256MB).

man size:1048576 pages, ram usage:166MB, vis usage:19MB

Presumably, the "vis usage" thing is what should I look?

>
> Regards,
> Christian.
>
>> I would like to approach the problem by reducing the amount of vram
>> needed by the userspace in order to prevent TTM to move lot of data...
>>
>> Anyway, I'm going to push this patch.
>>
>>>
>>>
>