# Help Debugging GPU Performance Issue

**URL:** <https://discourse.julialang.org/t/help-debugging-gpu-performance-issue/42274>\
**Category:** GPU\
**Tags:** gpu, debug, cuarrays\
**Created:** [June 29, 2020, 9:46pm UTC](https://discourse.julialang.org/t/help-debugging-gpu-performance-issue/42274 "2020-06-29T21:46:02Z")\
**Posts on this page:** 1\
**Showing post:** 2

<div class="post-metadata">

**Author:** ![Ellipse0934](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/ellipse0934/32/4012_2.png) [@Ellipse0934](https://discourse.julialang.org/u/Ellipse0934)\
**Post date:** [July 1, 2020, 6:46am UTC](https://discourse.julialang.org/t/help-debugging-gpu-performance-issue/42274/2 "2020-07-01T06:46:39Z")

</div>

The way you are measuring time is incorrectly, currently you are measuring CPU time to launch the kernel rather than the kernel itself. To avoid this add `CUDA.@sync` in front the `@time/@btime` or add `CUDA.synchronize()` at the end of your block. Doing so on my machine, I got:

```julia
julia> @benchmark CUDA.@sync begin
           a = CUDA.rand(64)
           findmax(a)
       end
BenchmarkTools.Trial: 
  memory estimate: 8.77 KiB
  allocs estimate: 283
  --------------
  minimum time: 340.618 μs (0.00% GC)
  median time: 28.718 ms (0.00% GC)
  mean time: 29.137 ms (1.16% GC)
  maximum time: 64.510 ms (0.00% GC)
  --------------
  samples: 172
  evals/sample: 1

julia> @benchmark CUDA.@sync begin
           findmax(CUDA.rand(64))
       end
BenchmarkTools.Trial: 
  memory estimate: 2.93 MiB
  allocs estimate: 39756
  --------------
  minimum time: 29.005 ms (0.00% GC)
  median time: 29.461 ms (0.00% GC)
  mean time: 29.906 ms (1.33% GC)
  maximum time: 36.513 ms (17.74% GC)
  --------------
  samples: 168
  evals/sample: 1

```

Now the only thing that pops out to my eyes is the allocations which can be explained by the `unsafe_free!`.

```julia
julia> @benchmark CUDA.@sync begin
           a = CUDA.rand(64)
           findmax(a)
           CUDA.unsafe_free!(a)
       end
BenchmarkTools.Trial: 
  memory estimate: 2.93 MiB
  allocs estimate: 39763
  --------------
  minimum time: 29.770 ms (0.00% GC)
  median time: 30.066 ms (0.00% GC)
  mean time: 30.578 ms (1.34% GC)
  maximum time: 37.684 ms (18.19% GC)
  --------------
  samples: 164
  evals/sample: 1

```

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