Random short freezes due to TTM buffer migrations

[Felix Kuehling]

Very nice. I'm looking forward to this for KFD as well.

One question: Will it be possible to share these split BOs as dmabufs?

Regards,
  Felix


On 16-08-16 11:27 AM, Christian König wrote:
> Hi Marek,
>
> I'm already working on this.
>
> My current approach is to use a custom BO manager for VRAM with TTM
> and so split allocations into chunks of 4MB.
>
> Large BOs are still swapped out as one, but it makes it much more
> likely to that you can allocate 1/2 of VRAM as one buffer.
>
> Give me till the end of the week to finish this and then we can test
> if that's sufficient or if we need to do more.
>
> Regards,
> Christian.
>
> Am 16.08.2016 um 16:33 schrieb Marek Olšák:
>> Hi,
>>
>> I'm seeing random temporary freezes (up to 2 seconds) under memory
>> pressure. Before I describe the exact circumstances, I'd like to say
>> that this is a serious issue affecting playability of certain AAA
>> Linux games.
>>
>> In order to reproduce this, an application should:
>> - allocate a few very large buffers (256-512 MB per buffer)
>> - allocate more memory than there is available VRAM. The issue also
>> occurs (but at a lower frequency) if the app needs only 80% of VRAM.
>>
>> Example: ttm_bo_validate needs to migrate a 512 MB buffer. The total
>> size of moved memory for that call can be as high as 1.5 GB. This is
>> always followed by a big temporary drop in VRAM usage.
>>
>> The game I'm testing needs 3.4 GB of VRAM.
>>
>> Setups:
>> Tonga - 2 GB: It's nearly unplayable, because freezes occur too often.
>> Fiji - 4 GB: There is one freeze at the beginning (which is annoying
>> too), after that it's smooth.
>>
>> So even 4 GB is not enough.
>>
>> Workarounds:
>> - Split buffers into smaller pieces in the kernel. It's not necessary
>> to manage memory at page granularity (64KB). Splitting buffers into
>> 16MB-large pieces might not be optimal but it would be a significant
>> improvement.
>> - Or do the same in Mesa. This would prevent inter-process and
>> inter-API buffer sharing for split buffers (DRI, OpenCL), but we would
>> at least verify how much the situation improves.
>>
>> Other issues sharing the same cause:
>> - Allocations requesting 1/3 or more VRAM have a high chance of
>> failing. It's generally not possible to allocate 1/2 or more VRAM as
>> one buffer.
>>
>> Comments welcome,
>>
>> Marek
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>
>
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