[Mesa-dev] [PATCH 4/7] winsys/amdgpu: clean up code around BO VM alignment

[Bas Nieuwenhuizen]

On Mon, Nov 26, 2018 at 8:54 PM Marek Olšák <maraeo at gmail.com> wrote:
>
> On Mon, Nov 26, 2018 at 8:46 AM Bas Nieuwenhuizen <basni at chromium.org> wrote:
>>
>> patches 4-7 are
>>
>> Reviewed-by: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
>>
>> though I agree with Christian that it would be nice to get a case
>> where 7 improves things before we submit it.
>>
>> For patches 1-3 I need to dive into the slab allocator before I have
>> enough knowledge to review.
>
>
> It's actually pretty simple. Each slab buffer contains suballocations of the same size. E.g. one 2M slab can only have 16 128K suballocations. If you want a 64K buffer, you have to allocate a separate 2M slab with 32 64K suballocations. If you need to allocate just 512B, you need another 2MB slab. This wastes memory in 2 ways:
> 1) a 129K buffer will have to be allocated as a 256K buffer, because the allocation size has to be a power of two, wasting space
> 2) there will be too many 2MB slabs, one slab for each size (from 2^9 to 2^18), each having some unused space, wasting space
> 3) having too many 2K and smaller allocations actually decreases memory usage, because the minimum kernel allocation size is 4K
>
> Problem 2 is mitigated by layering slab allocators on top of each other. The top slab allocator can only allocate sizes 2^17 to 2^18, so you only need two 2M slabs to cover all your needs. The middle allocator uses the top allocator and allocates slabs of size 2^17 and uses it to allocate 2^13 to 2^16 sizes. The bottom allocator allocates slabs of size 2^13 and uses it allocate 2^9 to 2^12 sizes.

Sure, just needed to familiarize myself with the code since I don't
want to review just the idea.

1-3 are

Reviewed-by: Bas Nieuwenhuizen <bas at basnieuwenhuizen.nl>
>
> Marek
>
>
>> On Sat, Nov 24, 2018 at 12:41 AM Marek Olšák <maraeo at gmail.com> wrote:
>> >
>> > From: Marek Olšák <marek.olsak at amd.com>
>> >
>> > ---
>> >  src/gallium/winsys/amdgpu/drm/amdgpu_bo.c | 9 +++++++--
>> >  1 file changed, 7 insertions(+), 2 deletions(-)
>> >
>> > diff --git a/src/gallium/winsys/amdgpu/drm/amdgpu_bo.c b/src/gallium/winsys/amdgpu/drm/amdgpu_bo.c
>> > index a9271c33ee9..36e2c4ec0dc 100644
>> > --- a/src/gallium/winsys/amdgpu/drm/amdgpu_bo.c
>> > +++ b/src/gallium/winsys/amdgpu/drm/amdgpu_bo.c
>> > @@ -449,24 +449,29 @@ static struct amdgpu_winsys_bo *amdgpu_create_bo(struct amdgpu_winsys *ws,
>> >     r = amdgpu_bo_alloc(ws->dev, &request, &buf_handle);
>> >     if (r) {
>> >        fprintf(stderr, "amdgpu: Failed to allocate a buffer:\n");
>> >        fprintf(stderr, "amdgpu:    size      : %"PRIu64" bytes\n", size);
>> >        fprintf(stderr, "amdgpu:    alignment : %u bytes\n", alignment);
>> >        fprintf(stderr, "amdgpu:    domains   : %u\n", initial_domain);
>> >        goto error_bo_alloc;
>> >     }
>> >
>> >     va_gap_size = ws->check_vm ? MAX2(4 * alignment, 64 * 1024) : 0;
>> > +
>> > +   unsigned vm_alignment = alignment;
>> > +
>> > +   /* Increase the VM alignment for faster address translation. */
>> >     if (size > ws->info.pte_fragment_size)
>> > -          alignment = MAX2(alignment, ws->info.pte_fragment_size);
>> > +      vm_alignment = MAX2(vm_alignment, ws->info.pte_fragment_size);
>> > +
>> >     r = amdgpu_va_range_alloc(ws->dev, amdgpu_gpu_va_range_general,
>> > -                             size + va_gap_size, alignment, 0, &va, &va_handle,
>> > +                             size + va_gap_size, vm_alignment, 0, &va, &va_handle,
>> >                               (flags & RADEON_FLAG_32BIT ? AMDGPU_VA_RANGE_32_BIT : 0) |
>> >                              AMDGPU_VA_RANGE_HIGH);
>> >     if (r)
>> >        goto error_va_alloc;
>> >
>> >     unsigned vm_flags = AMDGPU_VM_PAGE_READABLE |
>> >                         AMDGPU_VM_PAGE_EXECUTABLE;
>> >
>> >     if (!(flags & RADEON_FLAG_READ_ONLY))
>> >         vm_flags |= AMDGPU_VM_PAGE_WRITEABLE;
>> > --
>> > 2.17.1
>> >
>> > _______________________________________________
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>> > mesa-dev at lists.freedesktop.org
>> > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
>
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