composable_kernel/example/ck_tile/01_fmha/README.md

fused multi-head attention

This folder contains example for fmha(fused multi-head attention) using ck_tile tile-programming implementation. It is a good example to demonstrate the usage of tile-programming API, as well as illustrate the new approach to construct a kernel template and instantiate it(them) while keeping compile time fast.

build

# in the root of ck_tile
mkdir build && cd build
sh ../script/cmake-ck-dev.sh  ../ <arch>  # you can replace this <arch> to gfx90a, gfx942...
make tile_example_fmha_fwd -j

This will result in an executable build/bin/tile_example_fmha_fwd

kernel

The kernel template is fmha_fwd_kernel.hpp, this is the grid-wise op in old ck_tile's terminology. We put it here purposely, to demonstrate one can construct a kernel by using various internal component from ck_tile. We may still have an implementation under ck_tile's include path (in the future) for the kernel template.

There are 3 template parameters for this kernel template.

• TilePartitioner is used to map the workgroup to corresponding tile, fmha_fwd_tile_partitioner.hpp in this folder served as this purpose.
• FmhaPipeline is one of the block_tile_pipeline(under include/ck_tile/tile_program/block_tile_pipeline) which is a performance critical component. Indeed, we did a lot of optimization and trials to optimize the pipeline and may still workout more performance pipeline and update into that folder. People only need to replace this pipeline type and would be able to enjoy the benefit of different performant implementations (stay tuned for updated pipeline(s)).
• EpiloguePipeline will modify and store out the result in the last phase. People usually will do lot of post-fusion at this stage, so we also abstract this concept. Currently we didn't do much thing at the epilogue stage but leave the room for future possible support.

codegen

To speed up compile time, we instantiate the kernels into separate file. In this way we can benefit from parallel building from CMake/Make system. This is achieved by generate.py script. Besides, you can look into this script to learn how to instantiate a kernel instance step by step, which is described in FMHA_FWD_KERNEL_BODY variable.

executable

tile_example_fmha_fwd is the example executable, implemented in fmha_fwd.cpp. You can type ./bin/tile_example_fmha_fwd -? to list all supported args. Below is an example of the output (may subject to change)

args:
          -v    weather do CPU validation or not (default:1)
       -mode    kernel mode. 0:batch, 1:group (default:0)
          -b    batch size (default:2)
          -h    num of head, for q (default:8)
        -h_k    num of head, for k/v, 0 means equal to h (default:0)
                 if not equal to h, then this is GQA/MQA case
          -s    seqlen_q. if group-mode, means the average value of seqlen_q (default:3328)
                 total_seqlen_q = seqlen_q * batch, and seqlen_q per batch may vary
        -s_k    seqlen_k, 0 means equal to s (default:0)
          -d    head dim for q, k (default:128)
        -d_v    head dim for v, 0 means equal to d (default:0)
    -scale_s    scale factor of S. 0 means equal to 1/sqrt(hdim). (default:0)
                 note when squant=1, this value will be modified by range_q/k
    -range_q    per-tensor quantization range of q. used if squant=1. (default:2)
    -range_k    per-tensor quantization range of k. used if squant=1. (default:2)
    -range_v    per-tensor quantization range of v. used if squant=1. (default:2)
    -range_p    per-tensor quantization range of p [e^(s-m)]. used if squant=1. (default:1)
    -range_o    per-tensor quantization range of o (p*v). used if squant=1. (default:2)
     -squant    if using static quantization fusion or not. 0: original flow(not prefered) (default:0)
                 1: apply scale_p and scale_o with respect to P and O. calculate scale_s, scale_p,
                 scale_o according to range_q, range_k, range_v, range_p, range_o
      -iperm    permute input (default:1)
                 if true, will be b*h*s*d, else b*s*h*d
      -operm    permute output (default:1)
       -bias    add bias or not (default:0)
       -prec    data type. fp16/bf16/fp8/bf8 (default:fp16)
       -mask    0: no mask, 1: top-left(same as 't'), 2:bottom-right(same as 'b') (default:0)
                 't', top-left causal mask, 'b', bottom-r causal mask
                 't:l,r', top-left sliding window attn(swa) with FA style left right size
                 'b:l,r', bottom-r sliding window attn(swa) with FA style left right size
                 'xt:window_size', xformer style masking from top-left, window_size negative is causal, positive is swa
                 'xb:window_size', xformer style masking from bottom-r, window_size negative is causal, positive is swa
                 'g:y,x', generic attention mask coordinate with y/x size (only debug purpose for now)

    -vlayout    r for row-major(seqlen*hdim), c for col-major(hdim*seqlen) (default:r)
        -lse    0 not store lse, 1 store lse (default:0)
      -kname    if set to 1 will print kernel name (default:0)
       -init    init method. 0:random int, 1:random float, 2:trig float, 3:quantization (default:1)

Example: ./bin/tile_example_fmha_fwd -b=1 -h=16 -s=16384 -d=128 will run a fmha case with batch=1, nhead=16, sequence length=16384, hdim=128, fp16 case.

support features

Currently we are still in rapid development stage, so more features/optimizations will be coming soon.

hdim

Currently we support 32/64/128/256 hdim for fp16/bf16, within which 64/128 is better optimized. hdim should be multiple of 8, while seqlen_s can be arbitrary. For hdim be arbitrary number, it can be support through padding kernel of qr pipeline (we didn't generate this in generate.py by default)

group/batch mode

Currently we support both batch mode and group mode (or varlen, in FA's term), by setting -mode = 0 or 1. In group mode different kind of attention mask is also supported(see below)

MQA/GQA

By setting -h(nhead for q) and -h_k(nhead for k/v) with different number, you can achieve MQA/GQA. Please pay attention that h % h_K == 0 when you set different numbers.

input/output permute, and b*s*3*h*d

If you look at the kernel argument inside fmha_fwd_kernel.hpp, we support providing arbitrary stride for seqlen(stride_q/k/v), nhead, batch of q/k/v matrix, hence it is very flexible to support b*h*s*d or b*s*h*d input/output permute. The -iperm=0/1, -operm=0/1 is a convenient way to achieve this through the executable. We didn't provide a command-line arg to test b*s*3*h*d layout which is by default used by torch/FA, but it's trivial to achieve this if one set the proper stride_q/k/v value as 3*h*d.

attention bias

Attention bias is supported with the layout of 1*1*s*s(similiar to input/output, different layout can be supported by changing the stride value for bias, or even extend to b*h*s*s) and bias value in float number.

lse

For training kernels, "log sum exp" need to store out in forward and used in backward. We support this by setting -lse=1

vlayout

We support v matrix in both row-major(seqlen*hdim) and col-major(hdim*seqlen). Since the accumulate(reduce) dimension for V is along seqlen, for current AMD's mfma layout which expect each thread to have contiguous register holding pixels along reduce dimension, it's easier to support col-major V layout. However, the performance of col-major is not necessarily faster than row-major, there are many factors that may affect the overall performance. We still provide the -vlayout=r/c here to switch/test between different layouts.

attention mask

we support causal mask and sliding window attention(swa) mask in both batch and group mode, either from top-left or bottom-right. Underneath, we unify the mask expression into generic attention mask coordinate, providing an uniformed approach for each batch to locate the corresponding pixel need to be masked out.

Since FA/xformer style with window_size_left/right is more popular, we accept window_size as parameter and convert that internally to our generic coordinate(this coordinate can express more cases). Below shows some example of how to achieve different kind of mask through cmdline.

mask case	cmdline	FA style	xformer style
no mask	-mask=0(default)
causal mask from top-left	-mask=1 or -mask=t	-mask=t:-1,0	-mask=xt:-1
causal mask from bottom-right	-mask=2 or -mask=b	-mask=b:-1,0	-mask=xb:-1
swa from top-left		-mask=t:3,5	-mask=xt:4
swa from bottom-right		-mask=b:10,11	-mask=xb:16

Note FA use bottom-right by default to express swa case, here we require you explicitly specify top-left/bottom-right.

dropout

TBD

FP8 experimental support

As described in this blog, we have an experimental support for fp8 fmha kernels, you can evaluate the performance by setting the arg -prec=fp8 to the tile_example_fmha_fwd, on a gfx940/941/942 machine and ROCm 6.0+.

Currently we only support -vlayout=c( hdim*seqlen for V matrix) and -squant=1(static quantization) with hdim=128 for fp8 now. Full feature support will come later.