[AMD Official Use Only - General]
> -Original Message-
> From: Paneer Selvam, Arunpravin
> Sent: Saturday, May 27, 2023 2:12 AM
> To: Xiao, Shane ; Kuehling, Felix
> ; amd-gfx@lists.freedesktop.org
> Cc: Deucher, Alexander ; Koenig, Christian
>
> Subject: Re: [PATCH] Revert "drm/amdgpu: remove TOPDOWN flags when
> allocating VRAM in large bar system"
>
>
>
> On 5/22/2023 8:52 AM, Xiao, Shane wrote:
> > [AMD Official Use Only - General]
> >
> >> -Original Message-
> >> From: Kuehling, Felix
> >> Sent: Sunday, May 21, 2023 2:39 AM
> >> To: Paneer Selvam, Arunpravin ;
> amd-
> >> g...@lists.freedesktop.org
> >> Cc: Deucher, Alexander ; Koenig, Christian
> >> ; Xiao, Shane
> >> Subject: Re: [PATCH] Revert "drm/amdgpu: remove TOPDOWN flags when
> >> allocating VRAM in large bar system"
> >>
> >> Am 2023-05-20 um 05:25 schrieb Arunpravin Paneer Selvam:
> >>> This reverts commit c105518679b6e87232874ffc989ec403bee59664.
> >>>
> >>> This patch disables the TOPDOWN flag for APU and few dGPU cards
> >>> which has the VRAM size equal to the BAR size.
> >> With resizable BARs it's not that rare.
> >>
> >>
> >>> When we enable the TOPDOWN flag, we get the free blocks at the
> >>> highest available memory region and we don't split the lower order blocks.
> >>> This change is required to keep off the fragmentation related issues
> >>> particularly in ASIC which has VRAM space <= 500MiB
> > As far as I can understand that, both ways may cause fragmentation issues.
> >
> > As I can understand that remove TOPDOWN flags may get two pros:
> > 1) It can reduce the research time to O(1);
> > 2) Reduce the risk of splitting higher order into lower orders when
> > allocating
> blocks.
> >
> > But as for the second point, Removing TOPDOWN flags still carries the
> > risk of causing fragmentation issue on some scenarios.
> > For example, the apps need allocate (1M + 4K) VRAM size, which is not a
> power of two.
> >
> > Case 1: If not using TOPDOWN flag, we will allocate 1M order size and 4K
> order size from different order.
> > --And then if this 4K buddy and 1M buddy are freed, it may prevent the
> > system
> from merging 4K(or 1M) to 8K(2M) order.
> > --If the app has many such allocation requirements, it may cause
> fragmentation issue under memory stress.
> >
> > Case 2: If using TOPDOWN flag, we will find the highest order which is
> suitable for 4K and 1M needs.
> > --Assuming the highest order size is 8M, then we will split this 8M to lower
> order to meet the needs of 1M and 4K sizes, respectively.
> > --In such case the 8M size will split into different lower orders.
> > --If any other thread or process need allocate 4K or 1M VRAM, and there is
> also a chance to prevent the system from merging 4K(or 1M) to 8K(or 2M)
> order.
> >
> > If there are so many needs to allocate not power of two pages from apps,
> one of the choose is to use suballocator from UMD such as what ROCr does.
> > That means suballocator can allocate power of two pages(for example 2M
> bytes) from driver, and the app can allocate memory from suballocator.
> > In such way, it may reduce the risk of causing fragmentation issues. But I
> > am
> not sure that such an option is feasible.
> >
> > Maybe case 2 occurs less frequently than case one, then we should use
> TOPDOWN flags whether the system is lar-bar or not.
> Yes, It is better to set TOPDOWN flag mainly to avoid splitting down the lower
> order blocks which resolves the fragmentation issues and it doesn't try to
> allocate from visible memory (provided that we have the enough requested
> memory space in the top down region) which improves the graphics
> performances as well.
Hi Arun,
Think it more, the gfx needs get continuous VRAM for display something.
In such case, the TOPDOWN flag can meet this need better than not use TOPDOWN
flag because the drm buddy TOPDOWN flag
is prone to meet the condition in amdgpu_is_vram_mgr_blocks_contiguous, which
means that the blocks are contiguous.
As you mentioned that issue
https://gitlab.freedesktop.org/drm/amd/-/issues/2270 , one of the reasons may
be that TOPDOWN
flag in drm buddy can make it easier to get contiguous VRAM with the size of
not power of two.
This means that we do not need allocate contiguous VRAM again and then do copy
for display something.
In such way, we can get good performance and reduce the risk of allocation
failures.
If so, it's necessary to use TOPDOWN flag for gfx.
Hi Felix & Christian,
However, for compute, there is suballocator on ROCr to allocate power of two
sizes from drm buddy.
If we use the TOPDOWN flag, there is a potential problem that split higher
order into lower order.
It may increase the risk of allocating higher order blocks under memory
pressure if we split too much higher order into lower order.
If so, my question is that do we need to consider using different allocator
strategies on gfx and compute in large bar system?
Best Regards,
Shane
>
> Regards,
> Arun.
> >
> > Best
