profiling.md

Profiling

rocprof

rocprofv3 allows profiling both HSA & HIP API calls.

Let's profile simple copying kernel saved in profile.jl file:

using AMDGPU

function mycopy!(dst, src)
    i = workitemIdx().x + (workgroupIdx().x - 1) * workgroupDim().x
    if i ≤ length(dst)
        @inbounds dst[i] = src[i]
    end
    return
end

function main(N)
    src = ROCArray{Float64}(undef, N)
    dst = ROCArray{Float64}(undef, N)
    groupsize = 256               # nthreads
    gridsize = cld(N, groupsize)  # nblocks

    for i in 1:10
        @roc groupsize=groupsize gridsize=gridsize mycopy!(dst, src)
        AMDGPU.synchronize()
    end

    AMDGPU.unsafe_free!(dst)
    AMDGPU.unsafe_free!(src)
    AMDGPU.synchronize()
    return
end
main(2^24)

Profiling problematic code

ENABLE_JITPROFILING=1 rocprofv3 --output-directory ./profiling --output-format pftrace --hip-trace --hsa-trace --kernel-trace -- julia ./profile.jl

This will produce .pftrace file which can be visualized using Perfetto UI.

Here we can clearly see that host synchronization after each kernel dispatch causes poor device occupancy (empty spaces between kernel dispatches).

Profiling fixed code

We can fix this by moving synchronization outside the loop so that it happens only once.

    ...
    for i in 1:10
        @roc groupsize=groupsize gridsize=gridsize mycopy!(dst, src)
    end
    AMDGPU.synchronize()
    ...

Running profiling again and visualizing results we now see that kernel launches are adjacent to each other and that the average wall duration is lower.

Debugging

Use HIP_LAUNCH_BLOCKING=1 to synchronize immediately after launching GPU kernels. This will allow to pinpoint exact kernel that caused the exception.