VTimeLLM/flash-attention/hopper/benchmark_mla_decode.py

# Copyright (c) 2025, Ted Zadouri, Tri Dao.

# We recommend locking GPU clocks before running the benchmark to ensure consistent results.
# This can be done using the following commands (1830 MHz is the clock for H100):
# sudo nvidia-smi -i 0 -pm 1
# sudo nvidia-smi -i 0 --lock-gpu-clocks 1830,1830
# See more here: https://github.com/triton-lang/triton/blob/d9f10ebdc5da53f73eb852fde73d8d7d80b679d1/python/triton/testing.py#L487

import time
import torch
import torch.nn.functional as F

from triton.testing import do_bench, do_bench_cudagraph

from einops import rearrange

from flash_attn_interface import flash_attn_with_kvcache, get_scheduler_metadata

try:
    from flash_mla import flash_mla_with_kvcache, get_mla_metadata
except ImportError:
    flash_mla_with_kvcache, get_mla_metadata = None, None

try:
    from flash_attn.utils.benchmark import pytorch_profiler
except ImportError:
    pytorch_profiler = None


device = "cuda"
dtype = torch.bfloat16
seqlen = 8192
seqlen_q = 1
# nheads_q = 16
nheads_q = 128

use_bench_cudagraph = False

attn_variants = ["mha", "gqa", "mqa", "mla", "gla"]
# for attn_variant in attn_variants:
for attn_variant in attn_variants[3:5]:
    nheads_kv = nheads_q if attn_variant == "mha" else (max(nheads_q // 8, 1) if attn_variant == "gqa" else (1 if attn_variant == "mla" else 2))
    headdim = 64 if attn_variant in ["mla", "gla"] else 128
    headdim_v = 512 if attn_variant == "mla" else (256 if attn_variant == "gla" else headdim)
    has_qv = headdim == 64 and headdim_v > 64
    # page_size = None
    page_size = 64 if attn_variant in ["mla", "gla"] else 128

    should_run_flashmla = attn_variant == "mla" and page_size == 64 and flash_mla_with_kvcache is not None

    torch.manual_seed(0)

    batch_size = 128
    cache_seqlens = None
    # cache_seqlens = torch.tensor([seqlen] * batch_size, device=device, dtype=torch.int)
    # cache_seqlens = torch.tensor([seqlen - 1, 1024, 1024, 1024], device=device, dtype=torch.int32)
    # cache_seqlens = torch.tensor([1024] * batch_size, device=device, dtype=torch.int)
    # cache_seqlens = torch.tensor([4500, 45000, 1800, 1800], dtype=torch.int32, device=device)

    print(f"\n{attn_variant.upper()}, nheads_q = {nheads_q}, nheads_kv = {nheads_kv}, headdim = {headdim}, headdim_v = {headdim_v}, page_size = {page_size}")

    for seqlen in [s * 1024 for s in [1, 2, 4, 8, 16, 32, 64]]:
    # for seqlen in [s * 1024 for s in [8]]:
        cache_seqlens = torch.tensor([seqlen] * batch_size, device=device, dtype=torch.int)
        num_splits = 0
        q = torch.randn(batch_size, seqlen_q, nheads_q, headdim, dtype=dtype, device=device)
        try:
            v_cache = torch.randn(batch_size, seqlen, nheads_kv, headdim_v, dtype=dtype, device=device)
            k_cache = torch.randn(batch_size, seqlen, nheads_kv, headdim, dtype=dtype, device=device)
            if page_size is not None:
                assert seqlen % page_size == 0
                k_cache, v_cache = [rearrange(x, "b (n p) h d -> (b n) p h d", p=page_size) for x in [k_cache, v_cache]]
                page_table = rearrange(torch.arange(batch_size * seqlen // page_size, device=device, dtype=torch.int32),
                                    "(b s) -> b s", s=seqlen // page_size)
            else:
                page_table = None
        except torch.OutOfMemoryError:
            continue
        qv = torch.randn(batch_size, seqlen_q, nheads_q, headdim_v, dtype=dtype, device=device) if has_qv else None

        # Precomputing this saves ~2us
        scheduler_metadata = get_scheduler_metadata(
            batch_size, seqlen_q, seqlen, nheads_q, nheads_kv, headdim,
            cache_seqlens, q.dtype, headdim_v=headdim_v, page_size=page_size, causal=True
        )
        # scheduler_metadata = None
        # breakpoint()
        fn0 = lambda: flash_attn_with_kvcache(q, k_cache, v_cache, cache_seqlens=cache_seqlens, num_splits=num_splits, qv=qv, page_table=page_table, causal=True, scheduler_metadata=scheduler_metadata)
        time.sleep(1)  # to avoid power throttling
        # Time in ms
        if not use_bench_cudagraph:
            t0 = do_bench(fn0, warmup=1, rep=10)
        else:
            torch.cuda.synchronize()  # Gotta wait, otherwise e.g. k_cache might not be ready
            with torch.cuda.stream(torch.cuda.Stream()):
                t0 = do_bench_cudagraph(fn0, rep=10)
        # exit(0)
        if should_run_flashmla:
            # Separate out the preprocessing since this can be done once and reused for all layers
            mla_metadata = get_mla_metadata(cache_seqlens, seqlen_q * nheads_q // nheads_kv, nheads_kv)
            q_concat = torch.concat([q, qv], dim=-1) if has_qv else q
            kv_cache_concat = torch.concat([v_cache, k_cache], dim=-1)
            fn1 = lambda: flash_mla_with_kvcache(q_concat, kv_cache_concat, page_table, cache_seqlens, headdim_v, *mla_metadata, causal=True)
            time.sleep(1)  # to avoid power throttling
            if not use_bench_cudagraph:
                t1 = do_bench(fn1, warmup=1, rep=10)
            else:
                torch.cuda.synchronize()  # Gotta wait, otherwise e.g. k_cache might not be ready
                with torch.cuda.stream(torch.cuda.Stream()):
                    t1 = do_bench_cudagraph(fn1, rep=10)

        total_seqlen = seqlen * batch_size if cache_seqlens is None else cache_seqlens.sum().item()
        mem_io = total_seqlen * nheads_kv * (headdim + headdim_v) * 2 + q.numel() * 2 + (qv.numel() * 2 if has_qv else 0) + q.numel() * headdim_v // headdim * 2  # last term is for the output
        flops = seqlen_q * total_seqlen * nheads_q * (headdim + headdim_v * (2 if has_qv else 1)) * 2
        ideal_h100_time_mem = mem_io / 3.35e12 * 1e6
        ideal_h100_time_flop = flops / 989e12 * 1e6
        ideal_h100_time = max(ideal_h100_time_mem, ideal_h100_time_flop)
        print(f"Seqlen = {seqlen}, FA3 time{'' if not use_bench_cudagraph else ' w CUDA Graph'}: {t0 * 1e3:.1f} us, {mem_io * 1e-9 / (t0 * 1e-3):.0f} GB/s, {flops * 1e-12 / (t0 * 1e-3):.0f} TFLOPS/s")
        if should_run_flashmla:
            print(f"Seqlen = {seqlen}, FlashMLA time{'' if not use_bench_cudagraph else ' w CUDA Graph'}: {t1 * 1e3:.1f} us, {mem_io * 1e-9 / (t1 * 1e-3):.0f} GB/s, {flops * 1e-12 / (t1 * 1e-3):.0f} TFLOPS/s")
        print(f"Arithmetic intensity: {flops / mem_io:.1f}")
        print(f"Ideal time: {ideal_h100_time:.0f} us")

        # if pytorch_profiler is not None:
        #     time.sleep(1)  # to avoid power throttling
        #     pytorch_profiler(fn0)
        #     if should_run_flashmla:
        #         time.sleep(1)  # to avoid power throttling
        #         pytorch_profiler(fn1)